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UPC , currently referred to as BarcelonaTECH and commonly named just as UPC, is the largest engineering university in Catalonia, Spain - albeit encompassing other disciplines such as mathematics and architecture.BarcelonaTECH's objectives are based on internationalization, as it is Spain's technical university with the highest number of international PhD students and Spain's university with the highest number of international master's degree students. BarcelonaTECH is a university aiming at achieving the highest degree of engineering/technical excellence and has bilateral agreements with several top-ranked European universities.The Polytechnic University of Catalonia is a member of the Top Industrial Managers for Europe network, which allows for student exchanges between leading European engineering schools. It is also a member of several university federations, including the Conference of European Schools for Advanced Engineering Education and Research and UNITECH.The university was founded in March 1971 as the Universitat Politècnica de Barcelona through the merger of engineering and architecture schools originally founded during the 19th century. As of 2007 it has 25 schools in Catalonia located in the cities of Barcelona, Castelldefels, Manresa, Sant Cugat del Vallès, Terrassa, Igualada, Vilanova i la Geltrú and Mataró. UPC has about 30,000 students and 2,500 professors and researchers.Template:When? Wikipedia.


Pereira J.,Polytechnic University of Catalonia | Averbakh I.,University of Toronto
Computers and Operations Research | Year: 2011

We consider the Assignment Problem with interval data, where it is assumed that only upper and lower bounds are known for each cost coefficient. It is required to find a minmax regret assignment. The problem is known to be strongly NP-hard. We present and compare computationally several exact and heuristic methods, including Benders decomposition, using CPLEX, a variable depth neighborhood local search, and two hybrid population-based heuristics. We report results of extensive computational experiments. © 2010 Elsevier Ltd.


Kolodziej J.,University of Bielsko Biala | Xhafa F.,Polytechnic University of Catalonia
International Journal of Applied Mathematics and Computer Science | Year: 2011

Tasks scheduling and resource allocation are among crucial issues in any large scale distributed system, including Computational Grids (CGs). These issues are commonly investigated using traditional computational models and resolution methods that yield near-optimal scheduling strategies. One drawback of such approaches is that they cannot effectively tackle the complex nature of CGs. On the one hand, such systems account for many administrative domains with their own access policies, user privileges, etc. On the other, CGs have hierarchical nature and therefore any computational model should be able to effectively express the hierarchical architecture in the optimization model. Recently, researchers have been investigating the use of game theory for modeling user requirements regarding task and resource allocation in grid scheduling problems. In this paper we present two general non-cooperative game approaches, namely, the symmetric non-zero sum game and the asymmetric Stackelberg game for modeling grid user behavior defined as user requirements. In our game-theoretic approaches we are able to cast new requirements arising in allocation problems, such as asymmetric users relations, security and reliability restrictions in CGs. For solving the games, we designed and implemented GA-based hybrid schedulers for approximating the equilibrium points for both games. The proposed hybrid resolution methods are experimentally evaluated through the grid simulator under heterogeneity, and large-scale and dynamics conditions. The relative performance of the schedulers is measured in terms of the makespan and flowtime metrics. The experimental analysis showed high efficiency of meta-heuristics in solving the game-based models, especially in the case of an additional cost of secure task scheduling to be paid by the users.


Herranz J.,Polytechnic University of Catalonia
Theoretical Computer Science | Year: 2011

In this work we consider the following primitive, that we call restricted adaptive oblivious transfer. On the one hand, the owner of a database wants to restrict the access of users to this data according to some policy, in such a way that a user can only obtain information satisfying the restrictions imposed by the owner. On the other hand, a legitimate user wants to privately retrieve allowed parts of the data, in a sequential and adaptive way, without letting the owner know which part of the data is being obtained. After having formally described the components and required properties of a protocol for restricted adaptive oblivious transfer, we propose two generic ways to realize this primitive. The first one uses a cryptographic tool which has received a lot of attention from the literature in recent years: cryptosystems which are both multiplicatively and additively homomorphic. Our second generic construction is based on secret sharing schemes. © 2011 Elsevier B.V. All rights reserved.


Senovilla J.M.M.,University of the Basque Country | Torres R.,Polytechnic University of Catalonia
Classical and Quantum Gravity | Year: 2015

We provide a general formalism that allows us to analyze the phenomenon of tunneling in arbitrary spacetimes. We show that a flux of particles produced by tunneling through general marginally trapped surfaces (MTSs) may be perceived by some privileged observers. We discuss how this particle perception can be related to Hawking/Unruh radiation in specific cases. Our approach naturally leads to an expression for the effective surface gravity of MTSs. The procedure is applicable to general astrophysical and cosmological dynamical situations. Some practical examples for known and new cases are provided. © 2015 IOP Publishing Ltd.


Slater M.,University College London | Spanlang B.,Polytechnic University of Catalonia | Corominas D.,University of Barcelona
ACM Transactions on Graphics | Year: 2010

A new definition of immersion with respect to virtual environment (VE) systems has been proposed in earlier work, based on the concept of simulation. One system (A) is said to be more immersive than another (B) if A can be used to simulate an application as if it were running on B. Here we show how this concept can be used as the basis for a psychophysics of presence in VEs, the sensation of being in the place depicted by the virtual environment displays (Place Illusion, PI), and also the illusion that events occurring in the virtual environment are real (Plausibility Illusion, Psi). The new methodology involves matching experiments akin to those in color science. Twenty participants first experienced PI or Psi in the initial highest level immersive system, and then in 5 different trials chose transitions from lower to higher order systems and declared a match whenever they felt the same level of PI or Psi as they had in the initial system. In each transition they could change the type of illumination model used, or the field-of-view, or the display type (powerwall or HMD) or the extent of self-representation by an avatar. The results showed that the 10 participants instructed to choose transitions to attain a level of PI corresponding to that in the initial system tended to first choose a wide field-of-view and headmounted display, and then ensure that they had a virtual body that moved as they did. The other 10 in the Psi group concentrated far more on achieving a higher level of illumination realism, although having a virtual body representation was important for both groups. This methodology is offered as a way forward in the evaluation of the responses of people to immersive virtual environments, a unified theory and methodology for psychophysical measurement. © 2010 ACM.


Reig M.,University of Barcelona | Puigdollers J.,Polytechnic University of Catalonia | Velasco D.,University of Barcelona
Journal of Materials Chemistry C | Year: 2015

Charge transport in organic devices depends strongly on the molecular order and morphology of the organic semiconductor thin films. In the design of new organic semiconductors, the selection of the appropriate core plays a key role in the molecular packing and charge transport characteristics of the organic device. Four derivatives of carbazole that mainly differ in the extension of the π-conjugated core, including indolo[3,2-b]carbazole and triindole derivatives, exhibited hole mobilities ranging from 10-5 to 10-2 cm2 V-1 s-1 as active layers in organic thin-film transistors (OTFTs). X-ray analyses of the single crystals and evaporated thin films gave insights into the molecular packing of the compounds that justified their OTFTs characteristics. © The Royal Society of Chemistry 2015.


Prieto G.,Polytechnic University of Valencia | Concepcion P.,Polytechnic University of Valencia | Martinez A.,Polytechnic University of Valencia | Mendoza E.,Polytechnic University of Catalonia
Journal of Catalysis | Year: 2011

A series of 2.5% Rh/M@Al2O3 model catalysts were prepared by supporting Rh on high-area γ-Al2O3, resulting in a surface covered by a monolayer (4.5-7 atoms/nm2) of MOx promoter oxides (M = Fe, V, Nb, Ta, Ti, Y, Pr, Nd, Sm). The catalysts were extensively characterized and evaluated for the conversion of synthesis gas to oxygenates at 553 K, 5.0 MPa, H2/CO = 1, and space velocity adjusted to attain CO conversion around 15%. The broad range of products formed depending on the specific promoter were, for the first time, quantitatively described using the selectivity parameter (Φ) defined here, which indicates, for a given reaction product, the contribution of carbon atoms derived from dissociative (Cdis) and nondissociative (C ins) activation of CO. Both the catalytic activity and, more interestingly, the selectivity pattern given by the Φ parameter were correlated with the electronic properties of the MOx promoters (i.e., electron-donating/electron-withdrawing capacity) for an extensive series of catalysts. Low-temperature and at-work CO-FTIR experiments suggested that the high activity and hydrocarbon selectivity displayed by catalysts promoted by more electron-withdrawing (acidic) oxide promoters (e.g., TaOx) were related to a higher proportion of bridged Rh2(CO)B adsorption sites and to a higher electron density (i.e., a higher electron back-donation ability) of the Rh0 surface sites, both factors promoting CO dissociation events. In contrast, linear CO adsorption on Rh 0 sites displaying decreased electron back-donation in catalysts promoted by electron-donating (basic) oxides (e.g., PrOx, SmO x) was likely related to nondissociative CO activation and thus to the selective formation of oxygenates. TEM, XPS, and CO-FTIR results pointed to differences in morphology, rather than size or partial electronic charge, of the nanosized Rh0 crystallites as the likely cause for the different proportions of CO adsorption sites. The Rh0 NP morphology, both as-reduced and at-work, is a function of the electronic properties of the underlying promoter oxide. © 2011 Elsevier Inc. All rights reserved.


Pigolotti S.,Polytechnic University of Catalonia | Benzi R.,University of Rome Tor Vergata
Physical Review Letters | Year: 2014

We study a stochastic spatial model of biological competition in which two species have the same birth and death rates, but different diffusion constants. In the absence of this difference, the model can be considered as an off-lattice version of the voter model and presents similar coarsening properties. We show that even a relative difference in diffusivity on the order of a few percent may lead to a strong bias in the coarsening process favoring the more agile species. We theoretically quantify this selective advantage and present analytical formulas for the average growth of the fastest species and its fixation probability. © 2014 American Physical Society.


Riba J.-R.,Polytechnic University of Catalonia
European Journal of Physics | Year: 2015

This paper analyzes the skin and proximity effects in different conductive nonmagnetic straight conductor configurations subjected to applied alternating currents and voltages. These effects have important consequences, including a rise of the ac resistance, which in turn increases power loss, thus limiting the rating for the conductor. Alternating current (ac) resistance is important in power conductors and bus bars for line frequency applications, as well as in smaller conductors for high frequency applications. Despite the importance of this topic, it is not usually analyzed in detail in undergraduate and even in graduate studies. To address this, this paper compares the results provided by available exact formulas for simple geometries with those obtained by means of two-dimensional finite element method (FEM) simulations and experimental results. The paper also shows that FEM results are very accurate and more general than those provided by the formulas, since FEM models can be applied in a wide range of electrical frequencies and configurations. © 2015 IOP Publishing Ltd.


Casciati F.,University of Pavia | Rodellar J.,Polytechnic University of Catalonia | Yildirim U.,University of Pavia
Journal of Intelligent Material Systems and Structures | Year: 2012

It is internationally recognized that structural control was introduced in civil engineering through a pioneering article by Yao and through the implementations promoted by Kobori. The concepts of active and semi-active structural control in civil and infrastructure engineering date back 40 years and much progress has been recorded during these four decades. Periodically, state-of-the-art manuscripts have been published and technical books were also printed to testify the maturation of the topic. This article only covers the period from the second semester of 2009 to the first semester of 2011, emphasizing the developments in terms of theoretical, numerical and experimental studies, as well as the use of control algorithms and devices in actual implementations. It is observed that there are still several operational limitations to prevent from the expected growth of the applications in standard design. Nevertheless, some innovative concepts help to foresee future developments within special sectors of applications. ©The Author(s) 2012.


Nene N.R.,Imperial College London | Garca-Ojalvo J.,Polytechnic University of Catalonia | Zaikin A.,University College London
PLoS ONE | Year: 2012

Despite being governed by the principles of nonequilibrium transitions, gene expression dynamics underlying cell fate decision is poorly understood. In particular, the effect of signaling speed on cellular decision making is still unclear. Here we show that the decision between alternative cell fates, in a structurally symmetric circuit, can be biased depending on the speed at which the system is forced to go through the decision point. The circuit consists of two mutually inhibiting and self-activating genes, forced by two external signals with identical stationary values but different transient times. Under these conditions, slow passage through the decision point leads to a consistently biased decision due to the transient signaling asymmetry, whereas fast passage reduces and eventually eliminates the switch imbalance. The effect is robust to noise and shows that dynamic bifurcations, well known in nonequilibrium physics, are important for the control of genetic circuits. © 2012 Nené et al.


Graffelman J.,Polytechnic University of Catalonia | Moreno V.,University of Barcelona
Statistical Applications in Genetics and Molecular Biology | Year: 2013

Objective: Exact tests for Hardy-Weinberg equilibrium are widely used in genetic association studies. We evaluate the mid p-value, unknown in the genetics literature, as an alternative for the standard p-value in the exact test. Method: The type 1 error rate and the power of the exact test are calculated for different sample sizes, sigificance levels, minor allele counts and degrees of deviation from equilibrium. Three different p-value are considered: the standard two-sided p-value, the doubled one-sided p-value and the mid p-value. Practical implications of using the mid p-value are discussed with HapMap datasets and a data set on colon cancer. Results: The mid p-value is shown to have a type 1 error rate that is always closer to the nominal level, and to have better power. Differences between the standard p-value and the mid p-value can be large for insignificant results, and are smaller for significant results. The analysis of empirical databases shows that the mid p-value uncovers more significant markers, and that the equilibrium null distribution is not tenable for both databases. Conclusion: The standard exact p-value is overly conservative, in particular for small minor allele frequencies. The mid p-value ameliorates this problem by bringing the rejection rate closer to the nominal level, at the price of ocasionally exceeding the nominal level. © 2013 Walter de Gruyter GmbH, Berlin/Boston.


Torras J.,Polytechnic University of Catalonia
Physical Chemistry Chemical Physics | Year: 2015

A new QM/MM molecular dynamics approach that can deal with the dynamics of large real systems involving several simultaneous active zones is presented. Multiple, unconnected but interacting quantum regions are treated independently in an ordinary QM/MM approach but in a manner which converges to a unique simulation. The multiple active zones in the hybrid QM/MM molecular dynamics methodology (maz-QM/MM MD) involve molecular dynamics that is driving the whole simulation with several parallel executions of energy gradients within the QM/MM approach that merge into each MD step. The Ewald-summation method is used to incorporate long-range electrostatic interactions among the active zones in conjunction with periodic boundary conditions. To illustrate and ascertain capabilities and limitations, we present several benchmark calculations using this approach. Our results show that the maz-QM/MM MD method is able to provide simultaneous treatment of several active zones of very large proteins such as the Cu-4His-ΔC∗ cage, a self-assembly of a 24-mer cage-like protein ferritin. This journal is © the Owner Societies 2015.


Nescolarde L.,Polytechnic University of Catalonia
European Journal of Clinical Nutrition | Year: 2016

Background/Objectives:Bioelectrical impedance vector analysis (BIVA) is increasingly used in clinical research to assess soft tissue hydration. It is known that physical characteristics of electrodes, such as low intrinsic impedance, low electrode/skin contact impedance and type of gel, affect the reliability of noninvasive bioimpedance assessments. The aim of this study was to determine the effect of intrinsic impedance of electrode on the bioimpedance vector displacement in RXc graph.Subjects/Methods:The intrinsic impedance is measured in nine pregelled disposable Ag/AgCl electrodes usually used for bioimpedance measures. The BIVA method is performed on 35 healthy volunteers using a 50 kHz phase-sensitive bioimpedance analyzer (BIA 101 Anniversary) with the lowest intrinsic impedance electrode and highest. The individual bioimpedance vector is plotted on the bivariate normal interval of reference population. The differences in the mean bioimpedance vectors obtained with each electrode are plotted, with their 95% confidence ellipses, on the dRXc graph. The paired one-sample Hotelling's T2-test is used to compare the differences of the mean bioimpedance vectors.Results:We found large variability in intrinsic resistance (11–665 Ω) and reactance (0.25–2.5 Ω) values of the electrodes analyzed and significant displacement (P<0.05) of bioimpedance vector positions in healthy adults according to the paired one-sample Hotelling's T2-test.Conclusions:A robust study of all physical characteristics of commercial Ag/AgCl electrodes is necessary to reach consensus on pregelled Ag/AgCl electrodes valid for bioimpedance measurement. This information will enable BIVA users to avoid systemic errors when performing BIVA assessments, specifically when these measurements are used for clinical interpretations.European Journal of Clinical Nutrition advance online publication, 6 July 2016; doi:10.1038/ejcn.2016.121. © 2016 Macmillan Publishers Limited


Kartashov Y.V.,Polytechnic University of Catalonia | Kartashov Y.V.,Russian Academy of Sciences | Konotop V.V.,University of Lisbon | Abdullaev F.K.,Sao Paulo State University
Physical Review Letters | Year: 2013

We report a diversity of stable gap solitons in a spin-orbit-coupled Bose-Einstein condensate subject to a spatially periodic Zeeman field. It is shown that the solitons can be classified by the main physical symmetries they obey, i.e., symmetries with respect to parity (P), time (T), and internal degree of freedom, i.e., spin (C), inversions. The conventional gap and gap-stripe solitons are obtained in lattices with different parameters. It is shown that solitons of the same type but obeying different symmetries can exist in the same lattice at different spatial locations. PT and CPT symmetric solitons have antiferromagnetic structure and are characterized, respectively, by nonzero and zero total magnetizations. © 2013 American Physical Society.


Astrakharchik G.E.,Polytechnic University of Catalonia | Girardeau M.D.,University of Arizona
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

The ground-state properties of a single-component one-dimensional Coulomb gas are investigated. We use Bose-Fermi mapping for the ground-state wave function which permits solution of the Fermi sign problem in the following respects: (i) the nodal surface is known, permitting exact calculations; and (ii) evaluation of determinants is avoided, reducing the numerical complexity to that of a bosonic system and, thus, allowing simulation of a large number of fermions. Due to the mapping, the energy and local properties in one-dimensional Coulomb systems are exactly the same for Bose-Einstein and Fermi-Dirac statistics. The exact ground-state energy is calculated in homogeneous and trapped geometries using the diffusion Monte Carlo method. We show that in the low-density Wigner crystal limit an elementary low-lying excitation is a plasmon, which is to be contrasted with the high-density ideal Fermi gas/Tonks-Girardeau limit, where low-lying excitations are phonons. Exact density profiles are compared to the ones calculated within the local density approximation, which predicts a change from a semicircular to an inverted parabolic shape of the density profile as the value of the charge is increased. © 2011 American Physical Society.


Faundez-Zanuy M.,Polytechnic University of Catalonia | Vivaracho C.,University of Valladolid
Engineering Applications of Artificial Intelligence | Year: 2011

This paper proposes a multi-section vector quantization approach for on-line signature recognition. We have used the MCYT database, which consists of 330 users and 25 skilled forgeries per person performed by 5 different impostors. This database is larger than those typically used in the literature. Nevertheless, we also provide results from the SVC database. Our proposed system outperformsthe winner of SVC with a reduced computational requirement, which is around 47 times lower than DTW. In addition, our system improves the database storage requirements due to vector compression, and is more privacy-friendly as it is not possible to recover the original signature using the codebooks. Experimental results with MCYT provide a 99.76% identification rate and 2.46% EER (skilled forgeries and individual threshold). Experimental results with SVC are 100% of identification rate and 0% (individual threshold) and 0.31% (general threshold) when using a two-section VQ approach. © 2010 Elsevier Ltd. All rights reserved.


Chen H.,University Pompeu Fabra | Gimenez O.,Polytechnic University of Catalonia
Journal of Computer and System Sciences | Year: 2010

The causal graph is a directed graph that describes the variable dependencies present in a planning instance. A number of papers have studied the causal graph in both practical and theoretical settings. In this work, we systematically study the complexity of planning restricted by the causal graph. In particular, any set of causal graphs gives rise to a subcase of the planning problem. We give a complete classification theorem on causal graphs, showing that a set of graphs is either polynomial-time tractable, or is not polynomial-time tractable unless an established complexity-theoretic assumption fails; our theorem describes which graph sets correspond to each of the two cases. We also give a classification theorem for the case of reversible planning, and discuss the general direction of structurally restricted planning. © 2009 Elsevier Inc. All rights reserved.


Marti-Farre J.,Polytechnic University of Catalonia
Electronic Journal of Combinatorics | Year: 2014

This paper deals with the question of completing a monotone increasing family of subsets Γ of a finite set Ω to obtain the dependent sets of a matroid. Specifically, we provide several natural processes for transforming the clutter Λ of the inclusionminimal subsets of the family Γ into the set of circuits C(M) of a matroid M with ground set Ω. In addition, by combining these processes, we prove that all the minimal matroidal completions of the family can be obtained.


Alonso-Meijide J.M.,University of Santiago de Compostela | Carreras F.,Polytechnic University of Catalonia
Expert Systems with Applications | Year: 2011

We propose a modification of the Shapley value for monotonic games with a coalition structure. The resulting coalitional value is a twofold extension of the Shapley value in the following sense: (1) the amount obtained by any union coincides with the Shapley value of the union in the quotient game; and (2) the players of the union share this amount proportionally to their Shapley value in the original game (i.e.; without unions). We provide axiomatic characterizations of this value close to those existing in the literature for the Owen value and include applications to coalition formation in bankruptcy and voting problems. © 2010 Elsevier Ltd. All rights reserved.


Lopez-Benitez M.,Polytechnic University of Catalonia | Gozalvez J.,University Miguel Hernandez
IEEE Transactions on Mobile Computing | Year: 2011

Heterogeneous wireless systems are envisaged as the integration and joint cooperative management of diverse radio access networks and technologies through which network providers can satisfy the wide variety of user/service demands in a more efficient manner by exploiting their varying characteristics and properties. To achieve this objective, a key tool is common radio resource management technique designed to jointly manage the radio resources from different radio access technologies. In this context, this work proposes and optimizes new common radio resource management techniques designed to efficiently distribute traffic among the available radio access technologies while providing adequate quality of service levels under heterogeneous traffic scenarios. The obtained results demonstrate the ability of the proposed solutions to provide high user/service satisfaction levels while adequately exploiting the overall system resources. © 2011 IEEE.


Pierobon M.,Georgia Institute of Technology | Akyildiz I.F.,Georgia Institute of Technology | Akyildiz I.F.,Polytechnic University of Catalonia
IEEE Transactions on Signal Processing | Year: 2011

Molecular communication (MC) will enable the exchange of information among nanoscale devices. In this novel bio-inspired communication paradigm, molecules are employed to encode, transmit and receive information. In the most general case, these molecules are propagated in the medium by means of free diffusion. An information theoretical analysis of diffusion-based MC is required to better understand the potential of this novel communication mechanism. The study and the modeling of the noise sources is of utmost importance for this analysis. The objective of this paper is to provide a mathematical study of the noise at the reception of the molecular information in a diffusion-based MC system when the ligand-binding reception is employed. The reference diffusion-based MC system for this analysis is the physical end-to-end model introduced in a previous work by the same authors, where the reception process is realized through ligand-binding chemical receptors. The reception noise is modeled in this paper by following two different approaches, namely, through the ligand-receptor kinetics and through the stochastic chemical kinetics. The ligand-receptor kinetics allows to simulate the random perturbations in the chemical processes of the reception, while the stochastic chemical kinetics provides the tools to derive a closed-form solution to the modeling of the reception noise. The ligand-receptor kinetics model is expressed through a block scheme, while the stochastic chemical kinetics results in the characterization of the reception noise using stochastic differential equations. Numerical results are provided to demonstrate that the analytical formulation of the reception noise in terms of stochastic chemical kinetics is compliant with the reception noise behavior resulting from the ligand-receptor kinetics simulations. © 2011 IEEE.


Domingo M.C.,Polytechnic University of Catalonia
ETRI Journal | Year: 2011

Energy consumption is a key issue in body sensor networks (BSNs) since energy-constrained sensors monitor the vital signs of human beings in healthcare applications. In this paper, packet size optimization for BSNs has been analyzed to improve the efficiency of energy consumption. Existing studies on packet size optimization in wireless sensor networks cannot be applied to BSNs because the different operational characteristics of nodes and the channel effects of in-body and on-body propagation cannot be captured. In this paper, automatic repeat request (ARQ), forward error correction (FEC) block codes, and FEC convolutional codes have been analyzed regarding their energy efficiency. The hop-length extension technique has been applied to improve this metric with FEC block codes. The theoretical analysis and the numerical evaluations reveal that exploiting FEC schemes improves the energy efficiency, increases the optimal payload packet size, and extends the hop length for all scenarios for in-body and on-body propagation. © 2011 ETRI.


Jornet J.M.,Georgia Institute of Technology | Akyildiz I.F.,Georgia Institute of Technology | Akyildiz I.F.,Polytechnic University of Catalonia
IEEE Transactions on Wireless Communications | Year: 2011

Nanotechnologies promise new solutions for several applications in the biomedical, industrial and military fields. At the nanoscale, a nanomachine is considered as the most basic functional unit which is able to perform very simple tasks. Communication among nanomachines will allow them to accomplish more complex functions in a distributed manner. In this paper, the state of the art in molecular electronics is reviewed to motivate the study of the Terahertz Band (0.1-10.0 THz) for electromagnetic (EM) communication among nano-devices. A new propagation model for EM communications in the Terahertz Band is developed based on radiative transfer theory and in light of molecular absorption. This model accounts for the total path loss and the molecular absorption noise that a wave in the Terahertz Band suffers when propagating over very short distances. Finally, the channel capacity of the Terahertz Band is investigated by using this model for different power allocation schemes, including a scheme based on the transmission of femtosecond-long pulses. The results show that for very short transmission distances, in the order of several tens of millimeters, the Terahertz channel supports very large bit-rates, up to few terabits per second, which enables a radically different communication paradigm for nanonetworks. © 2011 IEEE.


Rockhill A.A.,American Superconductor Corporation | Liserre M.,Polytechnic of Bari | Teodorescu R.,University of Aalborg | Rodriguez P.,Polytechnic University of Catalonia | Rodriguez P.,University of Aalborg
IEEE Transactions on Industrial Electronics | Year: 2011

This paper describes the design procedure and performance of an LCL grid filter for a medium-voltage neutral-point clamped converter to be adopted for a multimegawatt (multi-MW) wind turbine. The unique filter design challenges in this application are driven by a combination of the medium-voltage converter, a limited allowable switching frequency, component physical size and weight concerns, and the stringent limits for allowable injected current harmonics. Traditional design procedures of grid filters for lower power and higher switching frequency converters are not valid for a multi-MW filter connecting a medium-voltage converter switching at low frequency to the electric grid. This paper demonstrates a frequency-domain-model-based approach to determine the optimum filter parameters that provide the necessary performance under all operating conditions given the necessary design constraints. To achieve this goal, new concepts, such as virtual-harmonic content and virtual filter losses are introduced. Moreover, a new passive-damping technique that provides the necessary damping with low losses and very little degradation of the high-frequency attenuation is proposed. © 2011 IEEE.


Gomez G.,Polytechnic University of Catalonia | Lagakos S.W.,Harvard University
Statistics in Medicine | Year: 2013

When comparing two treatment groups in a time-to-event analysis, it is common to use a composite event consisting of two or more distinct outcomes. The goal of this paper is to develop a statistical methodology to derive efficiency guidelines for deciding whether to expand a study primary endpoint from E1 (for example, non-fatal myocardial infarction and cardiovascular death) to the composite of E1 and E2 (for example, non-fatal myocardial infarction, cardiovascular death or revascularisation). We investigate this problem by considering the asymptotic relative efficiency of a log-rank test for comparing treatment groups with respect to a primary relevant endpoint E1 versus the composite primary endpoint, say Ed{hooktop}, of E1 and E2, where E2 is some additional endpoint. © 2012 John Wiley & Sons, Ltd.


Gibert K.,Polytechnic University of Catalonia
International Journal of Computer Mathematics | Year: 2014

In real applications, important rates of missing data are often found and have to be preprocessed before the analysis. The literature for missing imputation is abundant. However, the most precise imputation methods require long time, and sometimes specific software; this implies a significant delay to get final results. The Mixed Intelligent-Multivariate Missing Imputation (MIMMI) method is proposed as a hybrid missing imputation methodology based on clustering. The MIMMI is a non-parametric method that combines the prior expert knowledge with multivariate analysis without requiring assumptions on the probabilistic models of the variables (normality, exponentiality, etc.). The proposed imputation values implicitly take into account the joint distribution of all variables and can be determined in a relatively short time. The MIMMI uses the conditional mean according to the self-underlying structure of the data set. It provides a good trade-off between accuracy and both simplicity and required time to data preparation. The mechanics of the method is illustrated with some case-studies, both synthetic and real applications related with human behaviour. In both cases, acceptable quality results were obtained in short time. © 2013 © 2013 Taylor & Francis.


Gordillo M.C.,Pablo De Olavide University | Cazorla C.,CSIC - Institute of Materials Science | Boronat J.,Polytechnic University of Catalonia
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

Using quantum Monte Carlo we have studied the superfluid density of the first layer of He4 and H2 adsorbed on graphene and graphite. Our main focus has been on the equilibrium ground state of the system, which corresponds to a registered √3×√3 phase. The perfect solid phase of H2 shows no superfluid signal, whereas He4 has a finite but small superfluid fraction (0.67%). The introduction of vacancies in the crystal makes the superfluidity increase, showing values as large as 14% in He4 without destroying the spatial solid order. © 2011 American Physical Society.


Serra A.,Polytechnic University of Catalonia | Bacon D.J.,University of Liverpool | Pond R.C.,University of Exeter
Physical Review Letters | Year: 2010

A Comment on the Letter by B. Li and E. Ma, Phys. Rev. Lett.PRLTAO0031-9007 103, 035503 (2009)10.1103/PhysRevLett.103.035503. The authors of the Letter offer a Reply. © 2010 The American Physical Society.


De Valcarcel G.J.,University of Valencia | Staliunas K.,Polytechnic University of Catalonia
Physical Review Letters | Year: 2010

We propose a novel forcing technique of spatially extended self-oscillatory systems able to excite phase bistability and the dissipative structures associated with it. The forcing is time periodic at a frequency close to the oscillators' frequency and is spatially modulated. The effects of this type of forcing are demonstrated analytically and numerically in a directly driven complex Ginzburg-Landau equation. Both spatially periodic and spatially random drives prove to be effective. © 2010 The American Physical Society.


Geyer A.,Polytechnic University of Catalonia | Bindeman I.,University of Oregon
Journal of Volcanology and Geothermal Research | Year: 2011

It has been suggested that deglaciations have influenced volcanism in several areas around the world increasing productivity of mantle melting and eruptions from crustal magma chambers. However, the connection between glaciations and increased volcanism is not straightforward. Investigation of Ar-Ar, U-Pb, and 14C ages of caldera-forming eruptions for the past million years in the glaciated arc of Kamchatka has lead to the observation that the majority of large-volume ignimbrites, which are associated with the morphologically preserved calderas, correspond in time with "maximum glacial" conditions for the past several glacial cycles. In the field, the main proof is related to the fact that glaciated multi-caldera volcanoes hosted thick glacial ice caps. Additional evidence comes from clustering Kamchatka-derived marine ash layers with glacial moraines in DSDP cores. Here we present a set of new results from numerical modelling using the Finite Element Method that investigate how the glacial load dynamic may affect the conditions for ring-fault formation in such glaciated multi-caldera volcanoes. Different scenarios were simulated by varying: (1) the thickness and asymmetric distribution of the existing ice cap, (2) the depth and size of the magmatic reservoir responsible for the subsequent collapse event, (3) the thickness and mechanical properties of the roof rock due to the alteration by hydrothermal fluids, (4) the existence of a deeper and wider magmatic reservoir and (5) possible gravitational failure triggered, in part, by subglacial rock mass build up and hydrothermal alteration. The results obtained indicate that: (1) Any ice cap plays against ring fault formation; (2) Asymmetric distribution of ice may favour the initiation of trap-door type collapse calderas; (3) Glacial erosion of part of volcanic edifice or interglacial edifice failure may facilitate subsequent ring fault formation; (4) hydrothermal system under an ice cap may lead to a quite effective hydrothermal rotting of the intracaldera roof rocks and hence to variations of their mechanical properties and inhibit/deflect ring fracture propagation; and (5) rock surface topography/load influenced by glacial erosion and ice volume change during the interstadials. Although, the analysis of the stress field may inform us about the possibility of ring-fracture initiation, it does not ensure its complete propagation. Parameters controlling this phenomenon are also discussed here. Overall, the maximal glacial time represent the most dynamic time in a multi-caldera volcano life (as compared to more quiet interglacial) promoting physical and chemical feedbacks. We consider that brief interstadial periods during maximal glacial creates most favourable conditions for initiation of caldera-forming eruption, largely through its influence on surface topography by glacial action, mass wasting, and influencing magma vesiculation/discharge as a function of rapidly changing overload. © 2011 Elsevier B.V.


Vysloukh V.A.,Universidad de Las Americas Puebla | Kartashov Y.V.,Polytechnic University of Catalonia | Kartashov Y.V.,Russian Academy of Sciences
Optics Letters | Year: 2014

We study resonant mode conversion in parity time (PT)-symmetric multimode waveguides, where symmetry breaking manifests itself in the sequential destabilization (i.e., the appearance of complex eigenvalues) of the pairs of adjacent guided modes. We show that efficient mode conversion is possible even in the presence of the resonant longitudinal modulation of the complex refractive index. The distinguishing feature of the resonant mode conversion in the PT-symmetric structure is a drastic growth of the width of the resonance curve when the gain/losses coefficient approaches a critical value, at which symmetry breaking occurs. We found that in the system with broken symmetry, the resonant coupling between the exponentially growing mode and the stable higher-order one effectively stabilizes dynamically coupled pairs of modes and remarkably diminishes the average rate of the total power growth. © 2014 Optical Society of America


Khater H.A.,Assiut University | Serra A.,Polytechnic University of Catalonia | Pond R.C.,University of Exeter
Philosophical Magazine | Year: 2013

Disconnection motion along () and () twins in Zr is investigated using atomic-scale simulation. In particular, the high mobility of glissile disconnections is studied in terms of the atomic shears and shuffles involved. Using a quasi-static simulation procedure, the displacements of individual atoms are followed as they transit from matrix sites, through interfacial sites, and hence to twin sites by repeated passages of disconnections along the interface. It is found that the overall displacements for the cases studied are those predicted by the Bilby and Crocker (1965) theory which invokes homogeneous shear deformation. However, the present work enables atomic tracks to be followed through the cores of moving disconnections. The combinations of shears and shuffles in the two twinning systems are found to be quite distinct. In addition to tracking their coordinates, the variation of hydrostatic pressure experienced by the atoms is also quantified. © 2013 Taylor & Francis.


Lopez-Martinez C.,Polytechnic University of Catalonia | Papathanassiou K.P.,German Aerospace Center
IEEE Transactions on Geoscience and Remote Sensing | Year: 2013

This paper investigates the polarimetric dependence of the interferometric complex correlation and proposes a methodology for cancelling individual scattering mechanisms, in terms of the complex correlation coefficient phase, under the assumption of the random volume over ground model. This allows the estimation of the ground topography on forested and vegetated areas. The first part of the analysis considers the separation of the volume from the ground (including the double-bounce scattering mechanism). This process identifies the polarization states, without constraining them to be equal in both polarimetric acquisitions, which allow to cancel either the volume scattering contribution or the ground contribution. In order to have access to the interferometric phase of the remaining or isolated scattering mechanism, the polarimetric phase contribution of this scattering mechanism has to be removed in a second step. In the case of forested areas, the previous methodology is considered from two different point of views. For the estimation of the underlying ground topography, the cancellation of the volume scattering contribution makes possible to access the interferometric phase associated to the ground contribution. In addition, the interferometric information associated to the volume scattering contribution is estimated based on the cancellation of the ground contribution. The proposed techniques are analyzed on the basis of simulated and experimental polarimetric interferometric synthetic aperture radar data, demonstrating that the ground topography, as well as the height associated to the volume contribution, are asymptotically nonbiased and dependent on the shape of the particles of the random volume. In case of spheres (η = 0) , the ground-to-volume ratio presents large values favoring the accurate estimation of the topographic phase. For the case of dipole like particles (η = 0.5), the ground-to-volume ration decreases producing a coherence |p| in the order of 0.1, making necessary a large speckle filtering to obtain a reliable estimation of the topographic phase. © 1980-2012 IEEE.


Castro J.,Polytechnic University of Catalonia | Cuesta J.,Rovira i Virgili University
Mathematical Programming | Year: 2011

One of the most efficient interior-point methods for some classes of primal block-angular problems solves the normal equations by a combination of Cholesky factorizations and preconditioned conjugate gradient for, respectively, the block and linking constraints. Its efficiency depends on the spectral radius-in [0,1)- of a certain matrix in the definition of the preconditioner. Spectral radius close to 1 degrade the performance of the approach. The purpose of this work is twofold. First, to show that a separable quadratic regularization term in the objective reduces the spectral radius, significantly improving the overall performance in some classes of instances. Second, to consider a regularization term which decreases with the barrier function, thus with no need for an extra parameter. Computational experience with some primal block-angular problems confirms the efficiency of the regularized approach. In particular, for some difficult problems, the solution time is reduced by a factor of two to ten by the regularization term, outperforming state-of-the-art commercial solvers. © 2010 Springer and Mathematical Programming Society.


Longland R.,Polytechnic University of Catalonia | Longland R.,Institute Destudis Espacials Of Catalonia Ieec
Astronomy and Astrophysics | Year: 2012

Context. Recent reaction rate evaluations include reaction rate uncertainties that have been determined in a statistically meaningful manner. Furthermore, reaction rate probability density distributions have been determined and published in the form of lognormal parameters with the specific goal of pursuing Monte Carlo nucleosynthesis studies. Aims. A variety of methods is available for randomly sampling over reaction rate probability densities. The aim of this work is to investigate these methods and determine the most accurate method for estimating elemental abundance uncertainties. Methods. Experimental Monte Carlo reaction rates are first computed for the 22Ne + α, 20Ne(p, γ)21Na, 25Mg(p, γ)26Al, and 18F(p, α) 15O reactions, which are used to calculate reference nucleosynthesis yields for 16 nuclei affected by nucleosynthesis in massive stars and classical novae. Five different methods of randomly sampling over these reaction rate probability distributions are then developed, tested, and compared with the reference nucleosynthesis yields. Results. Given that the reaction rate probability density distributions can be described accurately with a lognormal distribution, Monte Carlo nucleosynthesis variations arising from the parametrised estimates for the reaction rate variations agree remarkably well with those obtained from the true rate samples. Most significantly, the most simple parametrisation agrees within just a few percent, meaning that Monte Carlo nucleosynthesis studies can be performed reliably using lognormal parametrisations of reaction rate probability density functions. © 2012 ESO.


Quintanilla R.,Polytechnic University of Catalonia
Journal of Thermal Stresses | Year: 2011

This paper is concerned with the linear theory of thermoelasticity with microtemperatures. Our main aim is to show how the logarithmic convexity arguments can be used in this theory. Results concerning uniqueness, instability and structural stability are consequences of the use of this method in the proposed thermomechanical theory. The main difficulty is the fact that the thermal and microthermal parts of the system of field equations do not define a self-adjoin problem. However the use of suitable weights permits to save the difficulties. Copyright © Taylor & Francis Group, LLC.


Quintanilla R.,Polytechnic University of Catalonia
Zeitschrift fur Angewandte Mathematik und Physik | Year: 2010

In this note, we investigate the spatial behaviour of the solutions for a theory for the heat conduction with a delay term. We obtain an alternative of the Phragmen-Lindelof type. That is the solutions either decay in a exponential way or blow-up at infinity in a exponential way. We also describe how to obtain an upper bound for the amplitude term. It is worth noting that this is the first contribution on spatial behaviour for partial differential equations involving a delay term. We use the energy arguments to obtain our main results. The main point of the contribution is the use of a suitable weighted energy function. © 2009 Birkháuser Verlag Basel/Switzerland.


Kozyreff G.,Free University of Colombia | Dominguez-Juarez J.L.,ICFO - Institute of Photonic Sciences | Martorell J.,ICFO - Institute of Photonic Sciences | Martorell J.,Polytechnic University of Catalonia
Laser and Photonics Reviews | Year: 2011

Over the last fifteen years, a series of theoretical and experimental investigations have demonstrated the usefulness of circular geometries to tailor second-order nonlinear optical effects. However, until recently, such effects have remained rather weak, calling for their enhancement. In parallel, developments in the field of high quality factor spherical or ring resonators have shown that many different types of light-matter interactions can be dramatically amplified when light is coupled in the whispering gallery modes of such resonators. In high-quality spherical micro-resonators, close to one million interactions can occur between a nonlinear molecule and a circulating light pulse. Recent research on nonlinear optics in spherical geometry is reviewed, from micrometer-size spheres to whispering gallery mode resonators. Theoretical and experimental investigations have demonstrated the usefulness of circular geometries to tailor second-order nonlinear optical effects. However, until recently, such effects have remained rather weak. Developments in the field of high quality factor spherical or ring resonators have shown that many different types of light-matter interactions can be dramatically amplified when light is coupled in the whispering gallery modes of such resonators. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


Ramos G.A.,National University of Colombia | Costa-Castello R.,Polytechnic University of Catalonia
IET Control Theory and Applications | Year: 2012

Repetitive control has proven to be an efficient control technique in power factor correction by active filtering. Unfortunately, this technique shows a dramatic performance decay when the network frequency is not exactly known or it varies with time. In order to overcome the varying/uncertain frequency problem, a robust high-order repetitive control strategy can be used; however, most internal models obtained by these approaches are unstable. Although this fact does not compromise the closed-loop stability, practical problems can arise during the implementation. This study proposes and studies a stable second-order odd-harmonic repetitive control system, presents a stability analysis of high-order internal models and describes the performance degradation of the standard repetitive control in terms of the active filter (AF) application. In this way, an experimental validation has been carried out implementing the proposed internal model in a shunt AF current controller. As a result, this high-order controller allows dealing with the grid frequency variations without using adaptive schemes. © 2012 The Institution of Engineering and Technology.


Moretti M.,University of Pisa | Perez-Neira A.I.,Polytechnic University of Catalonia
IEEE Transactions on Wireless Communications | Year: 2013

In this paper we address the problem of margin adaptive scheduling in the downlink of an orthogonal frequency division multiple access (OFDMA) multiple-input multiple-output (MIMO) system. Optimal resource allocation in MIMO systems requires the joint optimization of: a) linear transmit and receive spatial filters, b) channel assignment and c) power allocation. This problem is not convex and its complexity becomes thus intractable already for small sets of users and subcarriers. To reduce the complexity of the problem at hand, we propose a novel heuristic strategy that partitions the users in different groups according to their average channel quality and addresses the original problem by solving a succession of lower-complexity allocation problems. The spatial dimension is employed to prevent multiple access interference from hindering the performance of the sequential allocation. To further reduce the complexity burden we introduce a linear programming formulation in combination with a waterfilling-based strategy to allocate channels and power to the groups of users. Numerical results and evaluation of the computational complexity show that, though suboptimal, in most cases the proposed algorithm manages to exploit in an original way the inherent multi-user diversity of multi-carrier systems to ease the task of resource allocation with a very limited performance loss from the theoretic optimum. © 2013 IEEE.


Castro J.,Polytechnic University of Catalonia
International Journal of Uncertainty, Fuzziness and Knowlege-Based Systems | Year: 2012

Minimum distance controlled tabular adjustment is a recent perturbative approach for statistical disclosure control in tabular data. Given a table to be protected, it looks for the closest safe table, using some particular distance. Controlled adjustment is known to provide high data utility. However, the disclosure risk has only been partially analyzed using theoretical results from optimization. This work extends these previous results, providing both a more detailed theoretical analysis, and an extensive empirical assessment of the disclosure risk of the method. A set of 25 instances from the literature and four different attacker scenarios are considered, with several random replications for each scenario, both for L1 and L2 distances. This amounts to the solution of more than 2000 optimization problems. The analysis of the results shows that the approach has low disclosure risk when the attacker has no good information on the bounds of the optimization problem. On the other hand, when the attacker has good estimates of the bounds, and the only uncertainty is in the objective function (which is a very strong assumption), the disclosure risk of controlled adjustment is high and it should be avoided. © World Scientific Publishing Company.


Closas P.,Catalonia Technology Center of Telecomunications | Fernandez-Prades C.,Catalonia Technology Center of Telecomunications | Vila-Valls J.,Polytechnic University of Catalonia
IEEE Transactions on Signal Processing | Year: 2012

Bayesian filtering is a statistical approach that naturally appears in many signal processing problems. Ranging from Kalman filter to particle filters, there is a plethora of alternatives depending on model assumptions. With the exception of very few tractable cases, one has to resort to suboptimal methods due to the inability to analytically compute the Bayesian recursion in general dynamical systems. This is why it has attracted the attention of many researchers in order to develop efficient algorithms to implement it. We focus our interest into a recently developed algorithm known as the Quadrature Kalman filter (QKF). Under the Gaussian assumption, the QKF can tackle arbitrary nonlinearities by resorting to the Gauss-Hermite quadrature rules. However, its complexity increases exponentially with the state-space dimension. In this paper we study a complexity reduction technique for the QKF based on the partitioning of the state-space, referred to as the Multiple QKF. We prove that partitioning schemes can effectively be used to reduce the curse of dimensionality in the QKF. Simulation results are also provided to show that a nearly-optimal performance can be attained, while drastically reducing the computational complexity with respect to state-of-the-art algorithms that are able to deal with such nonlinear filtering problems. © 2012 IEEE.


Jornet J.M.,Georgia Institute of Technology | Akyildiz I.F.,Georgia Institute of Technology | Akyildiz I.F.,Polytechnic University of Catalonia
IEEE Transactions on Nanotechnology | Year: 2012

Wireless nanosensor networks (WNSNs) consist of nanosized communicating devices, which can detect and measure new types of events at the nanoscale. WNSNs are the enabling technology for unique applications such as intrabody drug delivery systems or surveillance networks for chemical attack prevention. One of the major bottlenecks in WNSNs is posed by the very limited energy that can be stored in a nanosensor mote in contrast to the energy that is required by the device to communicate. Recently, novel energy harvesting mechanisms have been proposed to replenish the energy stored in nanodevices. With these mechanisms, WNSNs can overcome their energy bottleneck and even have infinite lifetime (perpetual WNSNs), provided that the energy harvesting and consumption processes are jointly designed. In this paper, an energy model for self-powered nanosensor motes is developed, which successfully captures the correlation between the energy harvesting and the energy consumption processes. The energy harvesting process is realized by means of a piezoelectric nanogenerator, for which a new circuital model is developed that can accurately reproduce existing experimental data. The energy consumption process is due to the communication among nanosensor motes in the terahertz band (0.1-10 THz). The proposed energy model captures the dynamic network behavior by means of a probabilistic analysis of the total network traffic and the multiuser interference. A mathematical framework is developed to obtain the probability distribution of the nanosensor mote energy and to investigate the end-to-end successful packet delivery probability, the end-to-end packet delay, and the achievable throughput of WNSNs. Nanosensor motes have not been built yet and, thus, the development of an analytical energy model is a fundamental step toward the design of WNSNs architectures and protocols. © 2002-2012 IEEE.


Torres J.P.,ICFO - Institute of Photonic Sciences | Torres J.P.,Polytechnic University of Catalonia | Banaszek K.,University of Warsaw | Walmsley I.A.,University of Oxford
Progress in Optics | Year: 2011

Nonlinear optics is a very convenient way to generate photonic entanglement, that is, ensembles of photons that show correlations in certain degrees of freedom that go beyond anything possible with classical light beams.The convenience is both technical - the sources can be operated in the laboratory with commonly available lasers and optical materials and components, such as fibers and nonlinear crystals - and fundamental - the quantum correlations in various degrees of freedom of the field can be controlled.In this article, we describe the means by which this control can be exercised, enabling one to engineer the entanglement, or the lack thereof, for various applications. A quantum description of propagating-wave nonlinear optics is reviewed, and the formalism is applied to the common nonlinear optical processes that are used in quantum optics: spontaneous parametric downconversion and four-wave-mixing.The tailoring of photonic entanglement is crucial for a number of applications in quantum-enhanced technologies, including metrology and sensing, communications and cryptography, and computation. Examples of elementary protocols associated with each of these are given, and the role of intentional and extraneous entanglement discussed. © 2011 Elsevier B.V.


Muhammad S.,Technical University of Delft | Doria-Cerezo A.,Polytechnic University of Catalonia
IET Control Theory and Applications | Year: 2012

A family of passivity-based controllers for dynamic positioning of ships is presented. The authors exploit the idea of shaping the energy function of the closed-loop system to obtain different formulations of the passivity-based control law using the interconnection and damping assignment-passivity-based control (IDA-PBC) methodology. A salient feature of this study is that the proposed control laws are output feedback controllers and the relative velocity measurement is not required. First, we design and analyse two static controllers which can be seen as a non-linear version of the conventional proportional-derivative (PD) controllers. In presence of unknown disturbances, these controllers do not provide the desired regulation properties. To remove this discrepancy we propose, also in the context of the IDA-PBC technique, a dynamic extension of the system and obtain two new controllers that have the desired regulation properties. These new control laws can be seen as a non-linear version of the conventional proportional-integral-derivative (PID) controllers. Simulations are included to validate the theoretical results. © 2012 The Institution of Engineering and Technology.


Tang Z.,Nanjing University of Aeronautics and Astronautics | Periaux J.,Polytechnic University of Catalonia
Computer Methods in Applied Mechanics and Engineering | Year: 2012

A new robust optimization method is introduced to extend single point design to more realistic problems in aerodynamics taking into account uncertainties. It is well known that single point design techniques produce solutions that perform well for the selected design point but have poor off-design performance. Following ideas of Taguchi's robust control theory, a design with uncertainties is replaced by an optimization problem with two objectives which are mean performance and variance. Here, this two-objective optimization problem is solved by Pareto and Nash game strategies combined with the adjoint method, in the sense that solutions are less sensitive to uncertainties of input parameters. A constrained Nash strategy is implemented for performing multi-criteria optimization problems with constraints. Starting from a statistical definition of stability, the method simultaneously captures, Pareto and Nash equilibrium solutions ensuring performance and stability. © 2012 Elsevier B.V.


Castro J.,Polytechnic University of Catalonia
Optimization Methods and Software | Year: 2016

Constraints matrices with block-angular structures are pervasive in optimization. Interior-point methods have shown to be competitive for these structured problems by exploiting the linear algebra. One of these approaches solves the normal equations using sparse Cholesky factorizations for the block constraints, and a preconditioned conjugate gradient (PCG) for the linking constraints. The preconditioner is based on a power series expansion which approximates the inverse of the matrix of the linking constraints system. In this work, we present an efficient solver based on this algorithm. Some of its features are as follows: it solves linearly constrained convex separable problems (linear, quadratic or nonlinear); both Newton and second-order predictor-corrector directions can be used, either with the Cholesky+PCG scheme or with a Cholesky factorization of normal equations; the preconditioner may include any number of terms of the power series; for any number of these terms, it estimates the spectral radius of the matrix in the power series (which is instrumental for the quality of the preconditioner). The solver has been hooked to the structure-conveying modelling language (SML) based on the popular AMPL modeling language. Computational results are reported for some large and/or difficult instances in the literature: (1) multicommodity flow problems; (2) minimum congestion problems; (3) statistical data protection problems using l1 and l2 distances (which are linear and quadratic problems, respectively), and the pseudo-Huber function, a nonlinear approximation to l1 which improves the preconditioner. In the largest instances, of up to 25 millions of variables and 300,000 constraints, this approach is from 2 to 3 orders of magnitude faster than state-of-the-art linear and quadratic optimization solvers. © 2015 Taylor & Francis.


Castellano C.,CNR Institute for Complex Systems | Pastor-Satorras R.,Polytechnic University of Catalonia
Physical Review Letters | Year: 2010

We study the threshold of epidemic models in quenched networks with degree distribution given by a power-law. For the susceptible-infected-susceptible model the activity threshold λc vanishes in the large size limit on any network whose maximum degree kmax∼ diverges with the system size, at odds with heterogeneous mean-field (HMF) theory. The vanishing of the threshold has nothing to do with the scale-free nature of the network but stems instead from the largest hub in the system being active for any spreading rate λ>1/√kmax∼ and playing the role of a self-sustained source that spreads the infection to the rest of the system. The susceptible-infected-removed model displays instead agreement with HMF theory and a finite threshold for scale-rich networks. We conjecture that on quenched scale-rich networks the threshold of generic epidemic models is vanishing or finite depending on the presence or absence of a steady state. © 2010 The American Physical Society.


Raspall F.,Polytechnic University of Catalonia
Computer Networks | Year: 2012

Packet sampling is needed to measure network traffic scalably at high speeds. While many sampled-based measurement techniques have been developed in the recent past, most approaches select packets uniformly, without regard to their size. We discuss that this behavior can negatively impact the performance of tools, and present a sampling scheme that, taking into account packet sizes, overcomes several weaknesses of classic sampling methods. While the idea behind of our approach is conceptually not new, the way we propose to implement it yields a cost-effective solution that is suitable at very high speeds. To see the advantages of the approach, we study the problem of estimating traffic volumes. Our analysis and experimental evaluation with real traffic traces show that sampling considering packet sizes can improve the quality of measurements and make their accuracy less dependant on the properties of the traffic, at a small additional overhead compared to traditional methods. © 2012 Elsevier B.V. All rights reserved.


Dias E.O.,Federal University of Pernambuco | Alvarez-Lacalle E.,Polytechnic University of Catalonia | Carvalho M.S.,Pontifical Catholic University of Rio de Janeiro | Miranda J.A.,Federal University of Pernambuco
Physical Review Letters | Year: 2012

Conventional viscous fingering flow in radial Hele-Shaw cells employs a constant injection rate, resulting in the emergence of branched interfacial shapes. The search for mechanisms to prevent the development of these bifurcated morphologies is relevant to a number of areas in science and technology. A challenging problem is how best to choose the pumping rate in order to restrain the growth of interfacial amplitudes. We use an analytical variational scheme to look for the precise functional form of such an optimal flow rate. We find it increases linearly with time in a specific manner so that interface disturbances are minimized. Experiments and nonlinear numerical simulations support the effectiveness of this particularly simple, but nontrivial, pattern controlling process. © 2012 American Physical Society.


Vincenti M.A.,Aegis USA | De Ceglia D.,Aegis USA | Roppo V.,Polytechnic University of Catalonia | Scalora M.,AMR Research
Optics Express | Year: 2011

We have conducted a theoretical study of harmonic generation from a silver grating having slits filled with GaAs. By working in the enhanced transmission regime, and by exploiting phase-locking between the pump and its harmonics, we guarantee strong field localization and enhanced harmonic generation under conditions of high absorption at visible and UV wavelengths. Silver is treated using the hydrodynamic model, which includes Coulomb and Lorentz forces, convection, electron gas pressure, plus bulk X(3) contributions. For GaAs we use nonlinear Lorentz oscillators, with characteristic X(2) and X(3) and nonlinear sources that arise from symmetry breaking and Lorentz forces. We find that: (i) electron pressure in the metal contributes to linear and nonlinear processes by shifting/reshaping the band structure; (ii) TE- and TM-polarized harmonics can be generated efficiently; (iii) the X (2) tensor of GaAs couples TE- and TM-polarized harmonics that create phase-locked pump photons having polarization orthogonal compared to incident pump photons; (iv) Fabry-Perot resonances yield more efficient harmonic generation compared to plasmonic transmission peaks, where most of the light propagates along external metal surfaces with little penetration inside its volume. We predict conversion efficiencies that range from 10-6 for second harmonic generation to 10-3 for the third harmonic signal, when pump power is 2GW/cm2. © 2011 Optical Society of America.


Staliunas K.,Polytechnic University of Catalonia
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2011

We propose that periodically in space- and time-modulated potentials (dynamic lattices) can efficiently remove the excited (the high-energy and large momentum) components of the trapped Bose-Einstein condensates (BECs) and, consequently, can result in efficient cleaning of the BECs. We prove the idea by numerically solving the mean-field models (the Schrödinger equation for noninteracting condensates and the Gross-Pitaevskii equation for interacting condensates of repulsive atoms), and we evaluate parameters and conditions for the efficient removal of excitations. © 2011 American Physical Society.


Faundez-Zanuy M.,Polytechnic University of Catalonia
Pattern Analysis and Applications | Year: 2011

This paper proposes a multi-section vector quantization approach for on-line signature recognition. We have used a database of 330 users which includes 25 skilled forgeries performed by 5 different impostors. This database is larger than those typically used in the literature. Nevertheless, we also provide results from the SVC database. Our proposed system obtains similar results as the state-of-the-art online signature recognition algorithm, Dynamic Time Warping, with a reduced computational requirement, around 47 times lower. In addition, our system improves the database storage requirements due to vector compression, and is more privacy-friendly because it is not possible to recover the original signature using the codebooks. Experimental results reveal that our proposed multi-section vector quantization achieves a 98% identification rate, minimum Detection Cost Function value equal to 2.29% for random forgeries and 7.75% for skilled forgeries. © 2010 Springer-Verlag London Limited.


Dellen B.,Polytechnic University of Catalonia | Worgotter F.,University of Gottingen
International Journal of Computer Vision | Year: 2011

A novel Fourier-based technique for local motion detection from image sequences is proposed. In this method, the instantaneous velocities of local image points are inferred directly from the global 3D Fourier components of the image sequence. This is done by selecting those velocities for which the superposition of the corresponding Fourier gratings leads to constructive interference at the image point. Hence, image velocities can be assigned locally even though position is computed from the phases and amplitudes of global Fourier components (spanning the whole image sequence) that have been filtered based on the motion-constraint equation, reducing certain aperture effects typically arising from windowing in other methods. Regularization is introduced for sequences having smooth flow fields. Aperture effects and their effect on optic-flow regularization are investigated in this context. The algorithm is tested on both synthetic and real image sequences and the results are compared to those of other local methods. Finally, we show that other motion features, i.e. motion direction, can be computed using the same algorithmic framework without requiring an intermediate representation of local velocity, which is an important characteristic of the proposed method. © The Author(s) 2010. This article is published with open access at Springerlink.com.


Araujo-Pardo G.,National Autonomous University of Mexico | Balbuena C.,Polytechnic University of Catalonia
Networks | Year: 2011

In this article, some structures in the projective plane of order q are found which allow us to construct small k-regular balanced bipartite graphs of girth 6 for all k ≤ q. When $k=q$, the order of these q-regular graphs is 2(q2-1); and when k ≤ q-1, the order of these k-regular graphs is 2(qk-2) . Moreover, the incidence matrix of a k-regular balanced bipartite graph of girth 6 having 2(qk-2) vertices, where k is an integer and q is a prime power with 3 ≤ k ≤ q-1, is provided. These graphs improve upon the best known upper bounds for the number of vertices in regular graphs of girth 6. © 2010 Wiley Periodicals, Inc.


Arias M.,Polytechnic University of Catalonia | Balcazar J.L.,University of Cantabria
Machine Learning | Year: 2011

We describe an alternative construction of an existing canonical representation for definite Horn theories, the Guigues-Duquenne basis (or GD basis), which minimizes a natural notion of implicational size. We extend the canonical representation to general Horn, by providing a reduction from definite to general Horn CNF. Using these tools, we provide a new, simpler validation of the classic Horn query learning algorithm of Angluin, Frazier, and Pitt, and we prove that this algorithm always outputs the GD basis regardless of the counterexamples it receives. © The Author(s) 2011.


Ruiz B.,Technology Center | Flotats X.,Polytechnic University of Catalonia
Waste Management | Year: 2014

Citrus waste accounts for more than half of the whole fruit when processed for juice extraction. Among valorisation possibilities, anaerobic digestion for methane generation appears to be the most technically feasible and environmentally friendly alternative. However, citrus essential oils can inhibit this biological process. In this paper, the characteristics of citrus essential oils, as well as the mechanisms of their antimicrobial effects and potential adaptation mechanisms are reviewed. Previous studies of anaerobic digestion of citrus waste under different conditions are presented; however, some controversy exists regarding the limiting dosage of limonene for a stable process (24-192. mg of citrus essential oil per liter of digester and day). Successful strategies to avoid process inhibition by citrus essential oils are based either on recovery or removal of the limonene, by extraction or fungal pre-treatment respectively. © 2014 Elsevier Ltd.


Serrat C.,Polytechnic University of Catalonia
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2011

Quantum control in high-order-harmonic generation is considered theoretically by using a spatial distribution of static electric fields along the propagation direction of the driving field. It is shown that the trajectories of the electrons during its acceleration by the laser field in the high-harmonics-generation process can be controlled by periodically distributed static electric fields, which conveniently shape the driving laser field during propagation. Applying this mechanism, a quasi-phase-matching scheme that leads to filtered enhanced high harmonics is achieved. The harmonics in the plateau region are enhanced due to periodical phase variations in the long quantum trajectories as a consequence of the faster change experienced by the intensity-dependent phase along the longer electron trajectories. This effect should be observed in all quasi-phase-matching schemes based on perturbation of the microscopic quantum phase. The richness of adding a static-electric-field spatial pattern in the interaction region suggests a general scheme for feedback loop control in high-order-harmonic generation. © 2011 American Physical Society.


Sacristan-Murga D.,Catalonia Technology Center of Telecomunications | Pascual-Iserte A.,Polytechnic University of Catalonia
IEEE Transactions on Wireless Communications | Year: 2010

This paper proposes a differential quantization strategy to be used in the feedback link of a multiple-input multiple-output (MIMO) communication system. This algorithm is applied to the channel Gram matrix using geodesic curves and exploiting the intrinsic geometry of positive definite Hermitian matrices. It also exploits the temporal correlation of the channel, and follows on average the gradient of the cost function associated to the transmitter design criterion. A full description of the algorithm, including the computational cost and a numerical analysis of the effect of delays and errors in the feedback link is presented. Simulation results show that the proposed algorithm improves other techniques based on the direct quantization of the channel response matrix or the quantization of the subspace spanned by the strongest eigenmodes of the MIMO channel, i.e., Grassmannian based techniques. The main drawback of Grassmannian based algorithms is that the transmitter is constrained to apply a uniform power allocation among spatial transmission modes, which is not forced in the algorithm proposed in this paper. © 2006 IEEE.


Van Spengen W.M.,Technical University of Delft | Roca J.B.,Polytechnic University of Catalonia
Journal of Raman Spectroscopy | Year: 2013

We report for the first time on the thorough experimental and theoretical assessment of the noise limit of mechanical stress and temperature measurements with micro-Raman spectroscopy. A comprehensive study has been performed in which, for different incident laser light intensities and acquisition times, 1000 Raman spectra of mono-crystalline silicon were acquired per setting. Curve fitting was employed to obtain the peak positions of all the spectra, from which the standard deviations of the measured peak positions were obtained versus the total accumulated amount of laser light incident on the sample during one measurement. It has been found that the noise in the obtained peak position decreases as 1/sqrt(n) over more than three decades of the incident amount of laser light. At very low light conditions, the noise decreases as 1/n. By comparing the experimental results obtained to recent theoretical work, we show that the acquisition is limited by photon shot noise over most of the range and is limited by electronic detector noise at very low light conditions only. Pixelation errors do not play a role. It is concluded that the low electronic noise of typical Raman spectroscope detectors is overkill for the investigation of mechanical stress and temperature in silicon and other materials with comparable peaks, as it has absolutely no influence on the noise level of such an experiment. Maximum Raman signal intensity on the detector and high quantum efficiency detection are more important. © 2013 John Wiley & Sons, Ltd.


Ceballos Carrascosa P.,Polytechnic University of Catalonia | Stojanovic M.,Northeastern University
IEEE Journal of Oceanic Engineering | Year: 2010

In this paper, frequency and time correlation of the underwater channel are exploited to obtain a low-complexity adaptive channel estimation algorithm for multiple-inputmultiple-output (MIMO) spatial multiplexing of independent data streams. The algorithm is coupled with nonuniform Doppler prediction and tracking, which enable decision-directed operation and reduces the overhead. Performance is demonstrated on experimental data recorded in several shallow-water channels over distances on the order of 1 km. Nearly error-free performance is observed for two and four transmitters with BCH(64,10) encoded quadrature phase-shift keying (QPSK) signals. With a 24-kHz bandwidth, overall data rates of up to 23 kb/s after coding were achieved with 2048 carriers. Good results have also been observed in two other experiments with varying MIMO-OFDM (orthogonal frequency-division multiplexing) configurations. © 2005 IEEE.


Garcia-Ojalvo J.,University Pompeu Fabra | Garcia-Ojalvo J.,Polytechnic University of Catalonia | Martinez Arias A.,University of Cambridge
Current Opinion in Genetics and Development | Year: 2012

The spatiotemporal organization of a developing organism requires carefully orchestrated sequences of cellular differentiation events. These events are triggered by decisions made by individual cells about their fate, which are in turn controlled by gene and protein regulation processes. While these cell fate decisions are subject to stochasticity and are not reproducible at the single-cell level, they result in highly consistent, almost deterministic patterns at the level of the whole cell population. The question of how this macroscopic order arises from a disordered microscopic behaviour is still outstanding, and is reminiscent of problems in physical systems that are readily addressed by statistical mechanics. Here we review recent studies that are beginning to provide the data needed to address this question and discuss conceptual ideas that might be used in a theoretical understanding of cell fate decision processes, emphasizing the challenges that biology poses to the application of statistical mechanics approaches to developmental biology. © 2012 Elsevier Ltd.


Yaroshchuk A.,Polytechnic University of Catalonia | Luxbacher T.,Anton Paar GmbH
Langmuir | Year: 2010

It is shown that in tangential electrokinetic measurements with porous films the porous structure makes contribution not only to the cell electric conductance (as demonstrated previously) but also to the observed streaming current. Both of these contributions give rise to dependences of streaming-potential and streaming-current coefficients on the channel height. However, due to the combined contribution of two phenomena, the dependence of streaming-potential coefficient on the channel height may be rather complicated and not allow for simple extrapolation. At the same time, the dependences of streaming-current coefficient and cell electric conductance on the channel height turn out linear and can be easily extrapolated to zero channel heights. This enables one to determine separately the contributions of external surface of porous film and of its porous structure to the streaming current and of the channel and porous structure to the cell electric conductance. This procedure is illustrated by the measurements of tangential electrokinetic phenomena and electric conductance with Millipore mixed-cellulose membrane filters of various average pore sizes (from 0.025 to 5 μm) in the so-called adjustable-gap cell of SurPASS electrokinetic instrument (Anton Paar GmbH). The design of this cell allows for easy and quasi-continuous variation of channel height as well as accurate determination of cell electric conductance, streaming-current coefficient, and channel height (from the cell hydraulic permeability). The quality of linear fits of experimental data has been found to be very good, and thus, the extrapolation procedures were quite reliable and accurate. Zeta-potentials could be determined of both external film and internal pore surfaces. It is demonstrated that the porous structures make considerable contributions to both streaming-current coefficient and cell electric conductance especially in the case of filters with larger pores. It is also found that, rather surprisingly, in filters with smaller pores the reduction in the filter electric conductivity turns out essentially stronger than could be expected proceeding from the filter porosity. © 2010 American Chemical Society.


Fernandez-Francos X.,Rovira i Virgili University | Ramis X.,Polytechnic University of Catalonia | Serra A.,Rovira i Virgili University
Journal of Polymer Science, Part A: Polymer Chemistry | Year: 2014

The curing kinetics and network buildup during curing of epoxy-anhydride formulations using tertiary amines as initiators are reviewed in this work. A mechanism-based kinetic and structural model has been defined, showing better prediction capabilities than previous living polymerization and simple regeneration models. The curing kinetics have been analyzed using differential scanning calorimetry (DSC), and the gelation during curing has been determined by combined thermomechanical analysis and DSC. The effect of initiator content and epoxy equivalent weight are taken into account. The network buildup has been modeled using a stochastic network buildup model based on the random combination of primary chains generated by the kinetic model. © 2013 Wiley Periodicals, Inc.


Castanos F.,CINVESTAV | Kunusch C.,Polytechnic University of Catalonia
IEEE Transactions on Industrial Electronics | Year: 2015

This paper presents a nonsmooth adaptive extremum seeker that minimizes the hydrogen consumption in a fuel-cell system. The extremum seeker operates by estimating the gradient of the objective function but, unlike other seekers, it does not require a dither signal to produce such estimate. The absence of a dither signal simplifies the choice of parameter values for the seeker, and more importantly, it allows it to converge to the optimal value exactly, not only to a small neighborhood. The proper functioning of the proposed scheme is proved using nonsmooth Lyapunov analysis. The strategy is tested on the input-output map of a real polymer electrolyte fuel cell. © 2015 IEEE.


Jeltsema D.,Technical University of Delft | Doria-Cerezo A.,Polytechnic University of Catalonia
Proceedings of the IEEE | Year: 2012

In this paper, we consider memristors, meminductors, and memcapacitors and their properties as port-Hamiltonian systems. The port-Hamiltonian formalism naturally arises from network modeling of physical systems in a variety of domains. Exposing the relation between the energy storage, dissipation, and interconnection structure, this framework underscores the physics of the system. One of the strong aspects of the port-Hamiltonian formalism is that a power-preserving interconnection between port-Hamiltonian systems results in another port-Hamiltonian system with composite energy, dissipation, and interconnection structure. This feature can advantageously be used to model, analyze, and simulate networks consisting of complex interconnections of both conventional and memory circuit elements. Furthermore, the port-Hamiltonian formalism naturally extends the fundamental properties of the memory elements beyond the realm of electrical circuits. © 1963-2012 IEEE.


Carrion-Fite F.J.,Polytechnic University of Catalonia
Tenside, Surfactants, Detergents | Year: 2013

The objective of this research was to study the effect of the copolymer polyvinylpyrrolidone/vinylimidazole (PVP/VI) for use as a dye transfer inhibitor (DTI) during the washing of dyed polyester fabric, depending on the detergent components. The DTI was used to find its efficiency during repeated washings of polyester fabric dyed with disperse dyes. At the same time, its efficiency was tested for preventing dye migration to other fabrics, such as polyester, cellulose diacetate, acrylic, polyamide, cotton and wool. The influence of anionic (SDBS) and non-ionic (alkyl ethoxylates with 6 EO-groups) surfactants in different proportions and with zeolite as a builder and the aforementioned DTI was tested. Results of CIELAB colour differences were obtained for the dyed and undyed polyester fabrics after five washings.


Rosales-Guzman C.,ICFO - Institute of Photonic Sciences | Volke-Sepulveda K.,National Autonomous University of Mexico | Torres J.P.,ICFO - Institute of Photonic Sciences | Torres J.P.,Polytechnic University of Catalonia
Optics Letters | Year: 2012

Tang and Cohen [Phys. Rev. Lett. 104, 163901 (2010)] recently demonstrated a scheme to enhance the chiral response of molecules, which relies on the use of circularly polarized light in a standing wave configuration. Here we show a new type of light that possesses orbital angular momentum and enhanced chiral response. In the locations where the beams show enhanced optical chirality, only the longitudinal components of the electric and magnetic fields survive, which has unexpectedly shown what we believe is a new way to yield an enhanced optical chiral response. © 2012 Optical Society of America.


Laage D.,CNRS PASTEUR Laboratory | Stirnemann G.,CNRS PASTEUR Laboratory | Sterpone F.,CNRS PASTEUR Laboratory | Rey R.,Polytechnic University of Catalonia | And 2 more authors.
Annual Review of Physical Chemistry | Year: 2011

The reorientation of a water molecule is important for a host of phenomena, ranging overin an only partial listingthe key dynamic hydrogen-bond network restructuring of water itself, aqueous solution chemical reaction mechanisms and rates, ion transport in aqueous solution and membranes, protein folding, and enzymatic activity. This review focuses on water reorientation and related dynamics in pure water, and for aqueous solutes with hydrophobic, hydrophilic, and amphiphilic character, ranging from tetra-methylurea to halide ions and amino acids. Attention is given to the application of theory, simulation, and experiment in the probing of these dynamics, in usefully describing them, and in assessing the description. Special emphasis is placed on a novel sudden, large-amplitude jump mechanism for water reorientation, which contrasts with the commonly assumed Debye rotational diffusion mechanism, characterized by small-amplitude angular motion. Some open questions and directions for further research are also discussed. © 2011 by Annual Reviews. All rights reserved.


Cecati C.,University of LAquila | Citro C.,Polytechnic University of Catalonia | Siano P.,University of Salerno
IEEE Transactions on Sustainable Energy | Year: 2011

The integration of renewable energy systems (RESs) in smart grids (SGs) is a challenging task, mainly due to the intermittent and unpredictable nature of the sources, typically wind or sun. Another issue concerns the way to support the consumers' participation in the electricity market aiming at minimizing the costs of the global energy consumption. This paper proposes an energy management system (EMS) aiming at optimizing the SG's operation. The EMS behaves as a sort of aggregator of distributed energy resources allowing the SG to participate in the open market. By integrating demand side management (DSM) and active management schemes (AMS), it allows a better exploitation of renewable energy sources and a reduction of the customers' energy consumption costs with both economic and environmental benefits. It can also improve the grid resilience and flexibility through the active participation of distribution system operators (DSOs) and electricity supply/demand that, according to their preferences and costs, respond to real-time price signals using market processes. The efficiency of the proposed EMS is verified on a 23-bus 11-kV distribution network. © 2011 IEEE.


Golomedov A.E.,Institute of Spectroscopy | Astrakharchik G.E.,Polytechnic University of Catalonia | Lozovik Y.E.,Moscow Institute of Physics and Technology
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2011

A mesoscopic system of dipolar bosons trapped by a harmonic potential is considered. The system has a number of physical realizations including dipole excitons, atoms with large dipolar moment, polar molecules, and Rydberg atoms in inhomogeneous electric field. We carry out a diffusion Monte Carlo simulation to define the quantum properties of a two-dimensional system of trapped dipoles at zero temperature. In dimensionless units the system is described by two control parameters, namely, the number of particles and the strength of the interparticle interaction. We have shown that when the interparticle interaction is strong enough a mesoscopic crystal is formed. As the strength of interactions is decreased a multistage melting takes place. Off-diagonal order in the system is tested using natural-orbitals analysis. We have found that the system might be Bose condensed even in the case of strong interparticle interactions. There is a set of parameters for which a spatially ordered structure is formed while simultaneously the fraction of Bose-condensed particles is nonzero. This might be considered as a realization of a mesoscopic supersolid. © 2011 American Physical Society.


De Haro J.,Polytechnic University of Catalonia | Elizalde E.,CSIC - Institute of Marine Sciences
Physical Review Letters | Year: 2012

Particle production from vacuum fluctuations during inflation is briefly revisited. The moduli problem occurring with light particles produced at the end of inflation is addressed, namely, the fact that some results are in disagreement with nucleosynthesis constrains. A universal solution to this problem is found which leads to reasonable reheating temperatures in all cases. It invokes the assumption that, immediately after inflation, the moduli evolve like nonrelativistic matter. The assumption is justified in the context of massive chaotic inflation where, at the end of inflation, the Universe evolves as if it were matter dominated. © 2012 American Physical Society.


Torrent-Burgues J.,Polytechnic University of Catalonia
Colloids and Surfaces A: Physicochemical and Engineering Aspects | Year: 2012

Mixed Langmuir and Langmuir-Blodgett films of dissimilar components, arachidic acid and a phthalocyanine of zinc, have been investigated. These compounds form mixed films with phase separation, with domains at the sub-micro scale which are detected by AFM in LB films but not observed by BAM. These domains present different contrast in the surface potential images obtained with SP-AFM and in the friction images. The analysis of the isotherms and of the AFM images, at several compositions, reveals that the components are immiscible. The difference in height between the arachidic acid and the phthalocyanine of zinc phases is in accordance with the heights of the molecules, and points to the presence of a monolayer in the zinc phthalocyanine phase. AFM has revealed as a necessary technique to elucidate the miscibility in mixed films at the nanometric scale. UV-Vis spectroscopy shows changes in the spectra bands of LB films in respect to those in solution, due to the organization of the molecules in the LB film. © 2011 Elsevier B.V.


Here, we introduce bimodal atomic force microscopy operated with sub-nm and ultra-small, i.e., sub-angstrom, first and second mode amplitudes in ambient conditions. We show how the tip can be made to oscillate in the proximity of the surface and in perpetual contact with the adsorbed water layers while the second mode amplitude and phase provide enhanced contrast and sensitivity. Nonlinear and nonmonotonic behavior of the experimental observables is discussed theoretically with a view to high resolution, enhanced contrast, and minimally invasive mapping. Fractions of meV of energy dissipation are shown to provide contrast above the noise level. © 2013 AIP Publishing LLC.


Brazhnyi V.A.,University of Porto | Malomed B.A.,Polytechnic University of Catalonia | Malomed B.A.,Tel Aviv University
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2011

We introduce discrete systems in the form of straight (infinite) and ring-shaped chains, with two symmetrically placed nonlinear sites. The systems can be implemented in nonlinear optics (as waveguiding arrays) and Bose-Einstein condensates (by means of an optical lattice). A full set of exact analytical solutions for symmetric, asymmetric, and antisymmetric localized modes is found, and their stability is investigated in a numerical form. The symmetry-breaking bifurcation, through which the asymmetric modes emerge from the symmetric ones, is found to be of the subcritical type. It is transformed into a supercritical bifurcation if the nonlinearity is localized in relatively broad domains around two central sites, and also in the ring of a small size, i.e., in effectively nonlocal settings. The family of antisymmetric modes does not undergo bifurcations and features both stable and unstable portions. The evolution of unstable localized modes is investigated by means of direct simulations. In particular, unstable asymmetric states, which exist in the case of the subcritical bifurcation, give rise to breathers oscillating between the nonlinear sites, thus restoring an effective dynamical symmetry between them. © 2011 American Physical Society.


Abdullaev F.K.,University of Lisbon | Kartashov Y.V.,Polytechnic University of Catalonia | Konotop V.V.,University of Lisbon | Zezyulin D.A.,University of Lisbon
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2011

The existence of localized modes supported by the PT-symmetric nonlinear lattices is reported. The system considered reveals unusual properties: unlike other typical dissipative systems, it possesses families (branches) of solutions, which can be parametrized by the propagation constant; relatively narrow localized modes appear to be stable, even when the conservative nonlinear lattice potential is absent; and finally, the system supports stable multipole solutions. © 2011 American Physical Society.


Riba J.-R.,Polytechnic University of Catalonia
Electric Power Systems Research | Year: 2015

Abstract Skin and proximity effects in single- or multi-conductor systems can notoriously affect the AC resistance in conductors intended for electrical power transmission and distribution systems and for electronic devices. This increase of the AC resistance raises power loss and limits the conductors' current-carrying capacity, being an important design parameter. There are some internationally recognized exact and approximated formulas to calculate the AC resistance of conductors, whose accuracy and applicability is evaluated in this paper. However, since these formulas can be applied under a wide range of configurations and operating conditions, it is necessary to evaluate the applicability of these models. This is done by comparing the results that they provide with experimental data and finite element method (FEM) simulation results. The results provided show that FEM results are very accurate and more general than those provided by the formulas, since FEM models can be applied to a wide range of conductors' configurations and electrical frequencies. © Elsevier B.V.


Blum C.,Polytechnic University of Catalonia | Puchinger J.,AIT Austrian Institute of Technology | Raidl G.R.,Vienna University of Technology | Roli A.,University of Bologna
Applied Soft Computing Journal | Year: 2011

Research in metaheuristics for combinatorial optimization problems has lately experienced a noteworthy shift towards the hybridization of metaheuristics with other techniques for optimization. At the same time, the focus of research has changed from being rather algorithm-oriented to being more problem-oriented. Nowadays the focus is on solving the problem at hand in the best way possible, rather than promoting a certain metaheuristic. This has led to an enormously fruitful cross-fertilization of different areas of optimization. This cross-fertilization is documented by a multitude of powerful hybrid algorithms that were obtained by combining components from several different optimization techniques. Hereby, hybridization is not restricted to the combination of different metaheuristics but includes, for example, the combination of exact algorithms and metaheuristics. In this work we provide a survey of some of the most important lines of hybridization. The literature review is accompanied by the presentation of illustrative examples. © 2010 Elsevier B.V. All rights reserved.


Graffelman J.,Polytechnic University of Catalonia
Journal of Statistical Software | Year: 2015

Testing genetic markers for Hardy-Weinberg equilibrium is an important issue in genetic association studies. The Hardy Weinberg package offers the classical tests for equilibrium, functions for power computation and for the simulation of marker data under equilibrium and disequilibrium. Functions for testing equilibrium in the presence of missing data by using multiple imputation are provided. The package also supplies various graphical tools such as ternary plots with acceptance regions, log-ratio plots and Q-Q plots for exploring the equilibrium status of a large set of diallelic markers. Classical tests for equilibrium and graphical representations for diallelic marker data are reviewed. Several data sets illustrate the use of the package. © 2015, American Statistical Association. All rights reserved.


Girardeau M.D.,University of Arizona | Astrakharchik G.E.,Polytechnic University of Catalonia
Physical Review Letters | Year: 2012

The ground state of a one-dimensional (1D) quantum gas of dipoles oriented perpendicular to the longitudinal axis, with a strong 1/x3 repulsive potential, is studied at low 1D densities n. Near contact the dependence of the many-body wave function on the separation x of two particles reduces to a two-body wave function Ψrel(x). Immediately after a sudden rotation of the dipoles so that they are parallel to the longitudinal axis, this wave function will still be that of the repulsive potential, but since the potential is now that of the attractive potential, it will not be stationary. It is shown that as nd2→0 the rate of change of this wave function approaches zero. It follows that for small values of nd2, this state is metastable and is an analog of the super Tonks-Girardeau state of bosons with a strong zero-range attraction. The dipolar system is equivalent to a spinor Fermi gas with spin z components σ= (perpendicular to the longitudinal axis) and σ= (parallel to the longitudinal axis). A Fermi-Fermi mapping from spinor to spinless Fermi gas followed by the standard 1960 Fermi-Bose mapping reduces the Fermi system to a Bose gas. Potential experiments realizing the sudden spin rotation with ultracold dipolar gases are discussed, and a few salient properties of these states are accurately evaluated by a MonteCarlo method. © 2012 American Physical Society.


Averbakh I.,University of Toronto | Pereira J.,Polytechnic University of Catalonia
IIE Transactions (Institute of Industrial Engineers) | Year: 2012

Given a network whose edges need to be constructed, the problem is to find a construction schedule that minimizes the total recovery time of the vertices, where the recovery time of a vertex is the time when the vertex becomes connected to a special vertex (depot) that is the initial location of the construction crew. The construction speed is constant and is assumed to be incomparably slower than the travel speed of the construction crew in the already constructed part of the network. In this article, this new problem is introduced, its complexity is discussed, mixed-integer linear programming formulations are developed, fast and simple heuristics are proposed, and an exact branch-and-bound algorithm is presented which is based on specially designed lower bounds and dominance tests that exploit the problem's combinatorial structure. The results of extensive computational experiments are also presented. Connections between the problem and the Traveling Repairman Problem, also known as the Delivery Man Problem, and applications in emergency restoration operations are discussed. © 2012 "IIE".


Astrakharchik G.E.,Polytechnic University of Catalonia | Brouzos I.,University of Ulm
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2013

Ground-state properties of a single impurity in a one-dimensional Fermi gas are investigated in uniform and trapped geometries. The energy of a trapped system is obtained (i) by generalizing the McGuire expression from a uniform to trapped system (ii) within the local density approximation (iii) using the perturbative approach in the case of a weakly interacting impurity and (iv) diffusion Monte Carlo method. We demonstrate that there is a closed formula based on the exact solution of the homogeneous case which provides a precise estimation for the energy of a trapped system even for a small number of fermions and arbitrary coupling constant of the impurity. Using this expression, we analyze energy contributions from kinetic, interaction, and potential components, as well as spatial properties such as the system size and the pair-correlation function. Finally, we calculate the frequency of the breathing mode. Our analysis is directly connected and applicable to the recent experiments in microtraps. © 2013 American Physical Society.


Avila M.,Friedrich - Alexander - University, Erlangen - Nuremberg | Mellibovsky F.,Polytechnic University of Catalonia | Roland N.,Max Planck Institute for Dynamics and Self-Organization | Hof B.,Max Planck Institute for Dynamics and Self-Organization | Hof B.,Institute of Science and Technology Austria
Physical Review Letters | Year: 2013

Although the equations governing fluid flow are well known, there are no analytical expressions that describe the complexity of turbulent motion. A recent proposition is that in analogy to low dimensional chaotic systems, turbulence is organized around unstable solutions of the governing equations which provide the building blocks of the disordered dynamics. We report the discovery of periodic solutions which just like intermittent turbulence are spatially localized and show that turbulent transients arise from one such solution branch. © 2013 American Physical Society.


Revilla-Lopez G.,Polytechnic University of Catalonia
Biopolymers | Year: 2012

The structural consequences derived from the incorporation of either a methyl or a phenyl group at the α carbon of proline were recently investigated by quantum mechanical calculations (J Org Chem 2008, 73, 3418). In this work, the effect produced by contraction of the pyrrolidine ring on such α-substituted proline analogs has been explored using the same computational methods. Specifically, the intrinsic conformational preferences of the N-acetyl-N'-methylamide derivatives of the lower proline homolog L-azetidine-2-carboxylic acid (Aze), characterized by a four- instead of a five-membered ring, and its α-methyl (αMeAze) and α-phenyl (αPhAze) derivatives have been determined using quantum mechanical calculations and compared to those observed before for the proline counterparts. Replacement of the pyrrolidine ring by an azetidine cycle leads to a reduction of the conformational flexibility, especially for the Aze and αMeAze derivatives, which should be attributed to the quasi-planar geometry of the four-membered ring. Furthermore, the azetidine nitrogen shows pyramidalization, which depending on the peptide backbone conformation favors the formation of an attractive N-H···N interaction or alleviates a severe steric hindrance. Calculations on different environments predict that the tendency of αMeAze to adopt γ-turns is higher than that of unsubstituted Aze and α-methylproline, this feature being in full agreement with the experimental observations available. Copyright © 2011 Wiley Periodicals, Inc.


Serra T.,Polytechnic University of Catalonia
Energy Economics | Year: 2011

Previous literature on volatility links between food and energy prices is scarce and mainly based on parametric approaches. This article examines these links by using a semiparametric GARCH model recently proposed by Long et al. (2011), which is essentially a nonparametric correction of the parametric conditional covariance function. The analysis focuses on price links between crude oil, ethanol and sugar prices in Brazil. Results suggest strong volatility links between the prices studied. Parametric approximations of the conditional covariance matrix may lead to misleading results that can be improved upon by using nonparametric techniques. © 2011 Elsevier B.V.


Perez G.,University of Lleida | Rincon L.,University of Lleida | Vila A.,University of Lleida | Gonzalez J.M.,Polytechnic University of Catalonia | Cabeza L.F.,University of Lleida
Applied Energy | Year: 2011

This paper presents a classification of green vertical systems for buildings. The aim of this classification is to facilitate the identification and differentiation between systems. This classification is also essential to compare future research results relating to their operation. In addition, the mechanisms by which green facades can be used as passive energy savings systems are reviewed: shadow produced by the vegetation, insulation provided by vegetation and substrate, evaporative cooling by evapotranspiration, and the barrier effect to the wind. Finally, the paper describes the first results about the behaviour of a double-skin green facade or green curtain in Dry Mediterranean Continental conditions. It is verified that a microclimate between the wall of the building and the green curtain is created, and it is characterized by slightly lower temperatures and higher relative humidity. This means that the green screen acts as a wind barrier and confirms the evapotranspiration effect of the plants. © 2011 Elsevier Ltd.


Lopez-Dekker P.,German Aerospace Center | Mallorqui J.J.,Polytechnic University of Catalonia
IEEE Transactions on Geoscience and Remote Sensing | Year: 2010

This paper discusses the use of Capon's minimum-variance method (MVM) and Amplitude and Phase EStimation (APES) spectral-estimation algorithms to synthetic aperture radar rangeazimuth focusing. The rationale of the algorithms is discussed. An implementation of a Capon or APES processing chain is explained, and processing parameters such as chip-image size, resampling factor, and diagonal loading are discussed. For multichannel cases, a joint-processing approach is presented. A set of Monte Carlo simulations are described and used to benchmark Capon- and APES-based processing against conventional matched-filter-based approaches. Both methods improve the resolution and reduce sidelobes. APES yields generally better estimates of amplitude and phase than Capon but with worse resolution. Results with RADARSAT-2 quad-polarization data over Barcelona are used to qualitatively study the real-life performance of these algorithms. © 2010 IEEE.


Martinez-Velasco J.A.,Polytechnic University of Catalonia
Electric Power Systems Research | Year: 2011

This paper presents a methodology aimed at obtaining the equivalent circuit of regulating transformers. This aspect is insufficiently treated in most current textbooks in which the equivalent circuit of a tapped transformer is not well justified. The document shows how to obtain the equivalent circuit of a regulating transformer with control of voltage magnitude and phase angle, and how to apply it using either physical quantities or per unit (pu) quantities. The resulting equivalent circuit is adequate for steady-state calculations under balanced conditions and neglects transformer core parameters. The usage of per unit values can be made by selecting the base quantities in an arbitrary manner, which is an obvious advantage when the turns ratio must be estimated. © 2011 Elsevier B.V. All rights reserved.


Garmendia E.,Basque Center For Climate Change 3 | Garmendia E.,University of the Basque Country | Gamboa G.,Polytechnic University of Catalonia
Ecological Economics | Year: 2012

The use of multi-criteria evaluation tools in combination with participatory approaches provides a promising framework for integrating multiple interests and perspectives in the effort to provide sustainability. However, the inclusion of diverse viewpoints requires the "compression" of complex issues, a process that is controversial. Ensuring the quality of the compression process is a major challenge, especially with regards to retaining the essential elements of the various perspectives. Based on the lessons learned during a case study that assessed sustainable management options for the Urdaibai Estuary (Basque Country-Southern Europe), we propose a process in which the explicit elicitation of weights (the prioritisation of criteria) within a participatory multi-criteria evaluation serves as a quality assurance mechanism to check the robustness of the evaluation process. The results demonstrate that diverse individual priorities can be grouped in a reduced set of social preferences by means of cluster analysis reinforced with a deliberative appraisal among a wide variety of social actors. The approach presented retains relevant information regarding extreme and sometimes irreconcilable positions, allows an explicit social sensitivity analysis of the MCE process, and enables participants to learn from and reflect upon diverse social preferences without forcing their consensus. © 2012 Elsevier B.V.


Puentes G.,ICFO - Institute of Photonic Sciences | Hermosa N.,ICFO - Institute of Photonic Sciences | Torres J.P.,ICFO - Institute of Photonic Sciences | Torres J.P.,Polytechnic University of Catalonia
Physical Review Letters | Year: 2012

Weak measurements are a unique tool for accessing information about weakly interacting quantum systems with minimal back action. Joint weak measurements of single-particle operators with pointer states characterized by a two-dimensional Gaussian distribution can provide, in turn, key information about quantum correlations that can be relevant for quantum information applications. Here we demonstrate that by employing two-dimensional pointer states endowed with orbital angular momentum (OAM), it is possible to extract weak values of the higher order moments of single-particle operators, an inaccessible quantity with Gaussian pointer states only. We provide a specific example that illustrates the advantages of our method both in terms of signal enhancement and information retrieval. © 2012 American Physical Society.


Carmona J.,Polytechnic University of Catalonia
Data Mining and Knowledge Discovery | Year: 2012

Traces are everywhere from information systems that store their continuous executions, to any type of health care applications that record each patient's history. The transformation of a set of traces into a mathematical model that can be used for a formal reasoning is therefore of great value. The discovery of process models out of traces is an interesting problem that has received significant attention in the last years. This is a central problem in Process Mining, a novel area which tries to close the cycle between system design and validation, by resorting on methods for the automated discovery, analysis and extension of process models. In this work, algorithms for the derivation of a Petri net from a set of traces are presented. The methods are grounded on the theory of regions, which maps a model in the state-based domain (e.g., an automata) into a model in the event-based domain (e.g., a Petri net). When dealing with large examples, a direct application of the theory of regions will suffer from two problems: one is the state-explosion problem, i.e., the resources required by algorithms that work at the state-level are sometimes prohibitive. This paper introduces decomposition and projection techniques to alleviate the complexity of the region-based algorithms for Petri net discovery, thus extending its applicability to handle large inputs. A second problem is known as the overfitting problem for region-based approaches, which informally means that, in order to represent with high accuracy the trace set, the models obtained are often spaghetti-like. By focusing on special type of processes called conservative and for which an elegant theory and efficient algorithms can be devised, the techniques presented in this paper alleviate the overfitting problem and moreover incorporate structure into the models generated. © 2011 The Author(s).


Ferrer-I-Cancho R.,Polytechnic University of Catalonia | Elvevag B.,U.S. National Institutes of Health
PLoS ONE | Year: 2010

Background: Zipf's law states that the relationship between the frequency of a word in a text and its rank (the most frequent word has rank 1, the 2nd most frequent word has rank 2,...) is approximately linear when plotted on a double logarithmic scale. It has been argued that the law is not a relevant or useful property of language because simple random texts - constructed by concatenating random characters including blanks behaving as word delimiters - exhibit a Zipf's lawlike word rank distribution. Methodology/Principal Findings: In this article, we examine the flaws of such putative good fits of random texts. We demonstrate - by means of three different statistical tests - that ranks derived from random texts and ranks derived from real texts are statistically inconsistent with the parameters employed to argue for such a good fit, even when the parameters are inferred from the target real text. Our findings are valid for both the simplest random texts composed of equally likely characters as well as more elaborate and realistic versions where character probabilities are borrowed from a real text. Conclusions/Significance: The good fit of random texts to real Zipf's law-like rank distributions has not yet been established. Therefore, we suggest that Zipf's law might in fact be a fundamental law in natural languages.


Garcia J.E.,Polytechnic University of Catalonia
Materials | Year: 2015

Piezoceramic materials generally exhibit a notable instability of their functional properties when they work under real external conditions. This undesirable effect, known as nonlinear behavior, is mostly associated with the extrinsic contribution to material response. In this article, the role of the ferroelectric domain walls' motion in the nonlinear response in the most workable lead-based and lead-free piezoceramics is reviewed. Initially, the extrinsic origin of the nonlinear response is discussed in terms of the temperature dependence of material response. The influence of the crystallographic phase and of the phase boundaries on the material response are then reviewed. Subsequently, the impact of the defects created by doping in order to control the extrinsic contribution is discussed as a way of tuning material properties. Finally, some aspects related to the grain-size effect on the nonlinear response of piezoceramics are surveyed. © 2015 by the authors.


de Llorens Duran J.I.,Polytechnic University of Catalonia
Proceedings of Institution of Civil Engineers: Construction Materials | Year: 2013

The 'wine cathedrals' were wine cellars built for agricultural cooperatives in southern Catalonia in the early twentieth century. They were developed by Cesar Martinell (1888-1973). As the cooperatives could not afford to build with steel or concrete, he used traditional masonry and drew on the achievements of contemporary modern architects such as Antoni Gaudí, Lluis Domènech and Josep Puig. Martinell took advantage of the constructive and architectural possibilities of brick by using it not only for walls and pillars but also to replace main beams, joist beams, floor slabs and roofs. This gave rise to a type of building in which the constructive system becomes the architectural form and satisfies the technical and functional requirements. Some of the resources used by Martinell are still valid in low-cost and sustainable construction today when labour and local resources are more readily available than energy and imported products.


Reverter F.,Polytechnic University of Catalonia
Journal of Low Power Electronics and Applications | Year: 2012

This paper reviews the direct connection of sensors to microcontrollers without using any analogue circuit (such as an amplifier or analogue-to-digital converter) in the signal path, thus resulting in a low-cost, lower-power sensor electronic interface. It first discusses the operating principle and explains how resistive and capacitive sensors with different topologies (i.e., single, differential and bridge type) can be directly connected to a microcontroller to build the so-called direct interface circuit. It then shows some applications of the proposed circuits using commercial devices and discusses their performance. Finally, it deals with the power consumption and proposes some design guidelines to reduce the current consumption of such circuits in active mode. © 2012 by the authors; licensee MDPI, Basel, Switzerland.


Tolosana-Delgado R.,Polytechnic University of Catalonia
Sedimentary Geology | Year: 2012

This paper serves two goals. The first part shows how mass evolution processes of different nature become undistinguishable once we take a size-limited, noisy sample of its compositional fingerprint: processes of exponential decay, mass mixture and complementary accumulation are simulated, and then samples contaminated with noise are extracted. The aim of this exercise is to illustrate the limitations of typical graphical representations and statistical methods when dealing with compositional data, i.e. data in percentages, concentrations or proportions. The second part presents a series of concepts, tools and methods to represent and statistically treat a compositional data set attending to these limitations. The aim of this second part is to offer a state-of-the-art Compositional Data Analysis. This includes: descriptive statistics and graphics (the biplot); ternary diagrams with confidence regions for the mean; regression and ANalysis-Of-VAriance models to explain compositional variability; and the use of compositional information to predict environmental covariables or discriminate between groups. All these tools share a four-step algorithm: (1) transform compositions with an invertible log-ratio transformation; (2) apply a statistical method to the transformed scores; (3) back-transform the results to compositions; and (4) interpret results in relative terms. Using these techniques, a data set of sand petrographic composition has been analyzed, highlighting that: finer sands are richer in single-crystal grains in relation to polycrystalline grains, and that grain-size accounts for almost all compositional variability; a stronger water flow (river discharge) favors mica grains against quartz or rock fragment grains, possibly due to hydrodynamic sorting effects; a higher relief ratio implies shorter residence times, which may favor survival of micas and rock fragments, relatively more labile grains. © 2012 Elsevier B.V.


Millan M.S.,Polytechnic University of Catalonia
Journal of Optics (United Kingdom) | Year: 2012

On the verge of the 50th anniversary of VanderLugts formulation for pattern matching based on matched filtering and optical correlation, we acknowledge the very intense research activity developed in the field of correlation-based pattern recognition during this period of time. The paper reviews some domains that appeared as emerging fields in the last years of the 20th century and have been developed later on in the 21st century. Such is the case of three-dimensional (3D) object recognition, biometric pattern matching, optical security and hybrid optical-digital processors. 3D object recognition is a challenging case of multidimensional image recognition because of its implications in the recognition of real-world objects independent of their perspective. Biometric recognition is essentially pattern recognition for which the personal identification is based on the authentication of a specific physiological characteristic possessed by the subject (e.g.fingerprint, face, iris, retina, and multifactor combinations). Biometric recognition often appears combined with encryption-decryption processes to secure information. The optical implementations of correlation-based pattern recognition processes still rely on the 4f-correlator, the joint transform correlator, or some of their variants. But the many applications developed in the field have been pushing the systems for a continuous improvement of their architectures and algorithms, thus leading towards merged optical-digital solutions. © 2012 IOP Publishing Ltd.


Heltai L.,International School for Advanced Studies | Arroyo M.,Polytechnic University of Catalonia | DeSimone A.,International School for Advanced Studies
Computer Methods in Applied Mechanics and Engineering | Year: 2014

Isogeometric analysis (IGA) is emerging as a technology bridging computer aided geometric design (CAGD), most commonly based on Non-Uniform Rational B-Splines (NURBS) surfaces, and engineering analysis. In finite element and boundary element isogeometric methods (FE-IGA and IGA-BEM), the NURBS basis functions that describe the geometry define also the approximation spaces. In the FE-IGA approach, the surfaces generated by the CAGD tools need to be extended to volumetric descriptions, a major open problem in 3D. This additional passage can be avoided in principle when the partial differential equations to be solved admit a formulation in terms of boundary integral equations, leading to boundary element isogeometric analysis (IGA-BEM). The main advantages of such an approach are given by the dimensionality reduction of the problem (from volumetric-based to surface-based), by the fact that the interface with CAGD tools is direct, and by the possibility to treat exterior problems, where the computational domain is infinite. By contrast, these methods produce system matrices which are full, and require the integration of singular kernels. In this paper we address the second point and propose a nonsingular formulation of IGA-BEM for 3D Stokes flows, whose convergence is carefully tested numerically. Standard Gaussian quadrature rules suffice to integrate the boundary integral equations, and carefully chosen known exact solutions of the interior Stokes problem are used to correct the resulting matrices, extending the work by Klaseboer et al. (2012) [27] to IGA-BEM. © 2013 Elsevier B.V.


De Valcarcel G.J.,University of Valencia | Staliunas K.,Polytechnic University of Catalonia
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2013

We study pattern formation in a passive nonlinear optical cavity on the basis of the classic Lugiato-Lefever model with a periodically modulated injection. When the injection amplitude sign alternates, e.g., following a sinusoidal modulation in time or in space, a phase-bistable response emerges, which is at the root of the spatial pattern formation in the system. An asymptotic description is given in terms of a damped nonlinear Schrödinger equation with parametric amplification, which allows gaining insight into the basic spatiotemporal dynamics of the system. One- and two-dimensional phase-bistable spatial patterns, such as bright and dark-ring cavity solitons and labyrinths, are demonstrated. © 2013 American Physical Society.


Pastor-Satorras R.,Polytechnic University of Catalonia | Castellano C.,CNR Institute for Complex Systems | Castellano C.,University of Rome La Sapienza | Van Mieghem P.,Technical University of Delft | And 2 more authors.
Reviews of Modern Physics | Year: 2015

In recent years the research community has accumulated overwhelming evidence for the emergence of complex and heterogeneous connectivity patterns in a wide range of biological and sociotechnical systems. The complex properties of real-world networks have a profound impact on the behavior of equilibrium and nonequilibrium phenomena occurring in various systems, and the study of epidemic spreading is central to our understanding of the unfolding of dynamical processes in complex networks. The theoretical analysis of epidemic spreading in heterogeneous networks requires the development of novel analytical frameworks, and it has produced results of conceptual and practical relevance. A coherent and comprehensive review of the vast research activity concerning epidemic processes is presented, detailing the successful theoretical approaches as well as making their limits and assumptions clear. Physicists, mathematicians, epidemiologists, computer, and social scientists share a common interest in studying epidemic spreading and rely on similar models for the description of the diffusion of pathogens, knowledge, and innovation. For this reason, while focusing on the main results and the paradigmatic models in infectious disease modeling, the major results concerning generalized social contagion processes are also presented. Finally, the research activity at the forefront in the study of epidemic spreading in coevolving, coupled, and time-varying networks is reported. © 2015 American Physical Society. © 2015 American Physical Society.


Blum C.,Polytechnic University of Catalonia | Miralles C.,Polytechnic University of Valencia
Computers and Operations Research | Year: 2010

Certain types of manufacturing processes can be modelled by assembly line balancing problems. In this work we deal with a specific assembly line balancing problem that is known as the assembly line worker assignment and balancing problem (ALWABP). This problem appears in settings where tasks must be assigned to workers, and workers to work stations. Task processing times are worker specific, and workers might even be incompatible with certain tasks. The ALWABP was introduced to model assembly lines typical for sheltered work centers for the Disabled. In this paper we introduce an algorithm based on beam search for solving the ALWABP with the objective of minimizing the cycle time when given a fixed number of work stations, respectively, workers. This problem version is denoted as ALWABP-2. The experimental results show that our algorithm is currently a state-of-the-art method for the ALWABP-2. In comparison to results from the literature, our algorithm obtains better or equal results in all cases. Moreover, the algorithm is very robust for what concerns the application to problem instances of different characteristics. © 2010 Elsevier Ltd. All rights reserved.


Sun K.,Tsinghua University | Zhang L.,Nanjing University of Aeronautics and Astronautics | Xing Y.,Nanjing University of Aeronautics and Astronautics | Guerrero J.M.,Polytechnic University of Catalonia | Guerrero J.M.,University of Aalborg
IEEE Transactions on Power Electronics | Year: 2011

Modular generation system, which consists of modular power conditioning converters, is an effective solution to integrate renewable energy sources with conventional utility grid to improve reliability and efficiency, especially for photovoltaic generation. A distributed control strategy based on improved dc bus signaling is proposed for a modular photovoltaic (PV) generation system with battery energy storage elements. In this paper, the modular PV generation system is composed of three modular dc/dc converters for PV arrays, two grid-connected dc/ac converters, and one dc/dc converter for battery charging/discharging and local loads, which is available of either grid-connected operation or islanding operation. By using the proposed control strategy, the operations of a modular PV generation system are categorized into four modes: islanding with battery discharging, grid-connected rectification, grid-connected inversion, and islanding with constant voltage (CV) generation. The power balance of the system under extreme conditions such as the islanding operation with a full-charged battery is taken into account in this control strategy. The dc bus voltage level is employed as an information carrier to distinguish different modes and determine mode switching. Control methods of modular dc/dc converters, battery converter, and grid-connected converter are addressed. An autonomous control method for modular dc/dc converters is proposed to realize smooth switching between CV operation and maximum power point tracking operation, which enables the dc bus voltage regulation capability of modular dc/dc converters. Seamless switching of a battery converter between charging and discharging and that of a grid-connected converter between rectification and inversion are ensured by the proposed control methods. Experiments verify the practical feasibility and the effectiveness of the proposed control strategies. © 2006 IEEE.


Antonopoulos A.,Catalonia Technology Center of Telecomunications | Kartsakli E.,Polytechnic University of Catalonia | Verikoukis C.,Catalonia Technology Center of Telecomunications
IEEE Communications Magazine | Year: 2014

The widespread proliferation of mobile devices has motivated Device-to-Device (D2D) communications as a means of cell offloading toward better Quality of Service (QoS) and higher energy efficiency. Although Wi-Fi networks have the lion's share regarding the D2D communications in the unlicensed spectrum, it is uncertain whether they constitute the best option as technology evolves. In particular, the increasing transmission data rates, the novel interference cancelation techniques, as well as the potential of centralized network support, stress the need for new Medium Access Control (MAC) protocols, especially in content dissemination scenarios where all nodes share the same goal. In this article we study the suitability of Wi-Fi technology in content dissemination scenarios with multiple available source nodes, and we propose two energy-aware game theoretic MAC strategies (a distributed and a network-assisted) as possible alternatives. Our simulation results show the effectiveness and the flexibility of our proposed solutions, highlighting the necessity for new MAC designs. © 1979-2012 IEEE.


Gonzalez S.,Water Technology Center | Lopez-Roldan R.,Water Technology Center | Cortina J.-L.,Water Technology Center | Cortina J.-L.,Polytechnic University of Catalonia
Environmental Pollution | Year: 2012

Llobregat River (North-East Spain) is the most important drinking water source for Barcelona and its surrounding area. As one of the only water sources in the area the river water have been overexploited and effluents from more than 30 urban wastewater treatment plants, industries and agriculture runoffs have been discharged into the river. This article reviews the presence of emerging contaminants published during the last decades, emphasizing on the observed effects on ecosystems caused by the contamination. Pesticides, surfactants, estrogens, pharmaceuticals and personal care products and even abuse drugs are the main groups detected in different studies, reporting alterations in species composition, abundance or biomass and endocrine disruption measured by alterations in enzymatic activity or specific protein production. The information available provides an overview of the river status according to the Water Framework Directive. © 2011 Elsevier Ltd. All rights reserved.


Cubarsi R.,Polytechnic University of Catalonia
Astronomy and Astrophysics | Year: 2010

The closure problem for the stellar hydrodynamic equations is studied by describing the family of phase space density functions, for which the collisionless Boltzmann equation is strictly equivalent to a finite subset of moment equations. It is proven that the redundancy of the higher-order moment equations and the recurrence of the velocity moments are of similar nature. The method is based on the use of maximum entropy distributions, which are afterwards generalised to phase space density functions depending on any isolating integral of motion in terms of a polynomial function of degree n in the velocities. The equivalence between the moment equations up to an order n+1 and the collisionless Boltzmann equation is proven. It is then possible to associate the complexity of a stellar system, i.e., the minimum set of velocity moments needed to describe its main kinematic features, with the number of moment equations required to model it. © 2010 ESO.


Torres J.P.,ICFO - Institute of Photonic Sciences | Torres J.P.,Polytechnic University of Catalonia | Hendrych M.,ICFO - Institute of Photonic Sciences | Valencia A.,ICFO - Institute of Photonic Sciences
Advances in Optics and Photonics | Year: 2010

The dispersive properties of materials, i.e., their frequency-dependent response to the interaction with light, in most situations determines whether an optical process can be observed. Although one can always search for a specific material with the sought-after properties, this material might be far from optimum or might not even exist. Therefore, it is of great interest to develop methods that could tune the dispersive properties of a medium independently of the working frequency band. Pulses with angular dispersion, or pulse-front tilt, precisely allow us to achieve this goal. In this tutorial, we show the basics of how angular dispersion can manage to tune the dispersion parameters that characterize the propagation of light in a medium, thus permitting the observation and application of various optical processes in nonlinear and quantum optics that could not be realized otherwise. To keep the focus on first principles, the list of topics addressed is not exhaustive. More specifically, we consider the role of angular dispersion for pulse stretching and compression, broadband second-harmonic generation, the generation of temporal solitons in nonlinear χ(2) media, the tunable generation of terahertz waves by means of optical rectification of femtosecond pulses, and the tuning of the frequency correlations and of the bandwidth of entangled paired photons. © 2010 Optical Society of America.


Serra A.,Polytechnic University of Catalonia | Bacon D.J.,University of Liverpool
Philosophical Magazine | Year: 2010

Atomic-scale computer simulation is used to investigate the interaction of a moving {101̄2} twin boundary in a hcp metal with either a straight 1/3 〈11̄2〉(0001) dislocation lying perpendicular to the direction of twinning shear or a periodic row of perfect dislocation loops. The screw dislocation does not decompose in the moving interface and has no effect on its motion. The 60-mixed dislocation is attracted by the boundary and decomposes into twinning dislocations and a disconnection (an interfacial defect with both step and dislocation character): the sign of the crystal dislocation determines the form of the disconnection and, thus, its effect on twin boundary motion. Boundary reactions with crystal dislocations are likely to be important for assisting the twinning process. Loops with Burgers vector, b, parallel to the interface are reformed in the other crystal after the twin boundary has passed through. The boundary attracts both interstitial and vacancy dislocation loops with inclined b, but is not transparent to them because the complete loop is swept along its glide prism by the moving interface. Depending on its nature, a loop either retains its structure in its parent crystal or is absorbed in the interface. The decomposition product in the latter case is consistent with the reactions of straight dislocations. The results indicate that twinning is efficient at sweeping loops from the microstructure when their density is low and is suppressed by loops when their density is high. © 2010 Taylor & Francis.


Cubarsi R.,Polytechnic University of Catalonia
Astronomy and Astrophysics | Year: 2014

Under a common potential, a finite mixture of ellipsoidal velocity distributions satisfying the Boltzmann collisionless equation provides a set of integrability conditions that may constrain the population kinematics. They are referred to as conditions of consistency and were discussed in a previous paper on mixtures of axisymmetric populations. As a corollary, these conditions are now extended to point-axial symmetry, that is, point symmetry around the rotation axis or bisymmetry, by determining which potentials are connected with a more flexible superposition of stellar populations. Under point-axial symmetry, the potential is still axisymmetric, but the velocity and mass distributions are not necessarily. A point-axial stellar system is, in a natural way, consistent with a flat velocity distribution of a disc population. Therefore, no additional integrability conditions are required to solve the Boltzmann collisionless equation for such a population. For other populations, if the potential is additively separable in cylindrical coordinates, the populations are not kinematically constrained, although under point-axial symmetry, the potential is reduced to the harmonic function, which, for the Galaxy, is proven to be non-realistic. In contrast, a non-separable potential provides additional conditions of consistency. When mean velocities for the populations are unconstrained, the potential becomes quasi-stationary, being a particular case of the axisymmetric model. Then, the radial and vertical mean velocities of the populations can differ and produce an apparent vertex deviation of the whole velocity distribution. However, single population velocity ellipsoids still have no vertex deviation in the Galactic plane and no tilt in their intersection with a meridional Galactic plane. If the thick disc and halo ellipsoids actually have non-vanishing tilt, as the surveys of the solar neighbourhood that include RAdial Velocity Experiment (RAVE) data seem to show, the point-axial model is unable to fit the local velocity distribution. Conversely, the axisymmetric model is capable of making a better approach. If, in the end, more accurate data confirm a negligible tilt of the populations, then the point-axisymmetric model will be able to describe non-axisymmetric mass and velocity distributions, although in the Galactic plane the velocity distribution will still be axisymmetric. © ESO, 2014.


Masdemont J.J.,Polytechnic University of Catalonia
Journal of Computational Finance | Year: 2014

Measuring the contribution of individual transactions inside the total risk of a credit portfolio is a major issue in financial institutions. Value-at-risk (VaR) contributions and expected shortfall (ES) contributions have become two popular ways of quantifying these risks. However, the usual Monte Carlo approach is known to be a very time-consuming method for computing the risk contributions. In this paper, we accurately calculate the ES and we decompose the VaR and the ES into a sum of risk contributions from individual obligors representing the marginal impact on the total portfolio risk. We take the Vasicek one-factor model as the model framework. © Incisive Media Ltd. All rights reserved.


Sandoval C.,University of Santiago de Chile | Roca P.,Polytechnic University of Catalonia
Construction and Building Materials | Year: 2013

Three alternative equations to predict the carrying capacity of brick masonry walls subjected to concentric and eccentric vertical loading are presented. The proposals introduced, which are based on the reduction factors for slenderness and eccentricity commonly used in structural design of masonry walls, are derived from a large amount of numerical data generated on-purpose by the systematic use of a simplified micro-model. Wall slenderness, masonry stiffness and load eccentricity are all taken into consideration as the input parameters for the methods proposed. The research also includes an expression for the evaluation of the contribution of the tensile strength of the unit-mortar interface on the carrying capacity of the masonry walls. The paper describes the methodology utilized to derive the equations and a discussion of the equations' limits and applicability. Finally, the performance of the methods proposed is analyzed by comparison with a selection of experimental results and the method provided by Eurocode-6. © 2013 Elsevier B.V. All rights reserved.


Gavara R.,New University of Lisbon | Llorca J.,Polytechnic University of Catalonia | Lima J.C.,New University of Lisbon | Rodriguez L.,University of Barcelona
Chemical Communications | Year: 2013

The reaction of the water soluble phosphine 1,3,5-triaza-7- phosphaadamantane (PTA) with [Au(CC-C5H4N)]n yields the highly luminescent water soluble [(PTA)Au(4-pyridylethynyl)] complex. A detailed analysis of the compound shows the formation of gel structure giving rise to very long fibers, being the first example reported with such a simple structure. © 2012 The Royal Society of Chemistry.


Serratosa F.,Rovira i Virgili University | Alquezar R.,Polytechnic University of Catalonia | Amezquita N.,Rovira i Virgili University
Expert Systems with Applications | Year: 2012

This paper describes a probabilistic integrated object recognition and tracking framework called PIORT, together with two specific methods derived from it, which are evaluated experimentally in several test video sequences. The first step in the proposed framework is a static recognition module that provides class probabilities for each pixel of the image from a set of local features. These probabilities are updated dynamically and supplied to a tracking decision module capable of handling full and partial occlusions. The two specific methods presented use RGB color features and differ in the classifier implemented: one is a Bayesian method based on maximum likelihood and the other one is based on a neural network. The experimental results obtained have shown that, on one hand, the neural net based approach performs similarly and sometimes better than the Bayesian approach when they are integrated within the tracking framework. And on the other hand, our PIORT methods have achieved better results when compared to other published tracking methods in video sequences taken with a moving camera and including full and partial occlusions of the tracked object. © 2011 Elsevier Ltd. All rights reserved.


Nowak W.,University of Stuttgart | Rubin Y.,University of California at Berkeley | De Barros F.P.J.,Polytechnic University of Catalonia
Water Resources Research | Year: 2012

Most field campaigns aim at helping in specified scientific or practical tasks, such as modeling, prediction, optimization, or management. Often these tasks involve binary decisions or seek answers to yes/no questions under uncertainty, e.g., Is a model adequate? Will contamination exceed a critical level? In this context, the information needs of hydro(geo)logical modeling should be satisfied with efficient and rational field campaigns, e.g., because budgets are limited. We propose a new framework to optimize field campaigns that defines the quest for defensible decisions as the ultimate goal. The key steps are to formulate yes/no questions under uncertainty as Bayesian hypothesis tests, and then use the expected failure probability of hypothesis testing as objective function. Our formalism is unique in that it optimizes field campaigns for maximum confidence in decisions on model choice, binary engineering or management decisions, or questions concerning compliance with environmental performance metrics. It is goal oriented, recognizing that different models, questions, or metrics deserve different treatment. We use a formal Bayesian scheme called PreDIA, which is free of linearization, and can handle arbitrary data types, scientific tasks, and sources of uncertainty (e.g., conceptual, physical, (geo)statistical, measurement errors). This reduces the bias due to possibly subjective assumptions prior to data collection and improves the chances of successful field campaigns even under conditions of model uncertainty. We illustrate our approach on two instructive examples from stochastic hydrogeology with increasing complexity. © 2012 American Geophysical Union. All Rights Reserved.


Sartori P.,Max Planck Institute for the Physics of Complex Systems | Pigolotti S.,Polytechnic University of Catalonia
Physical Review Letters | Year: 2013

We study stochastic copying schemes in which discrimination between a right and a wrong match is achieved via different kinetic barriers or different binding energies of the two matches. We demonstrate that, in single-step reactions, the two discrimination mechanisms are strictly alternative and cannot be mixed to further reduce the error fraction. Close to the lowest error limit, kinetic discrimination results in a diverging copying velocity and dissipation per copied bit. On the other hand, energetic discrimination reaches its lowest error limit in an adiabatic regime where dissipation and velocity vanish. By analyzing experimentally measured kinetic rates of two DNA polymerases, T7 and Polγ, we argue that one of them operates in the kinetic and the other in the energetic regime. Finally, we show how the two mechanisms can be combined in copying schemes implementing error correction through a proofreading pathway. © 2013 American Physical Society.


Leon-Montiel R.D.J.,ICFO - Institute of Photonic Sciences | Torres J.P.,ICFO - Institute of Photonic Sciences | Torres J.P.,Polytechnic University of Catalonia
Physical Review Letters | Year: 2013

Photosynthesis is a biological process that involves the highly efficient transport of energy captured from the Sun to a reaction center, where conversion into useful biochemical energy takes place. Using a quantum description, Rebentrost et al. and Plenio and Huelga have explained this high efficiency as the result of the interplay between the quantum coherent evolution of the photosynthetic system and noise introduced by its surrounding environment. Even though one can always use a quantum perspective to describe any physical process, since everything follows the laws of quantum mechanics, is the use of quantum theory imperative to explain this high efficiency? Recently, it has been shown by Eisfeld and Briggs that a purely classical model can be used to explain main aspects of the energy transfer in photosynthetic systems. Using this approach, we demonstrate explicitly here that highly efficient noise-assisted energy transport can be found as well in purely classical systems. The wider scope of applicability of the enhancement of energy transfer assisted by noise might open new ways for developing new technologies aimed at enhancing the efficiency of a myriad of energy transfer systems, from information channels in microelectronic circuits to long-distance high-voltage electrical lines. © 2013 American Physical Society.


De Barros F.P.J.,Polytechnic University of Catalonia | Ezzedine S.,Lawrence Livermore National Laboratory | Rubin Y.,University of California at Berkeley
Advances in Water Resources | Year: 2012

The significance of conditioning predictions of environmental performance metrics (EPMs) on hydrogeological data in heterogeneous porous media is addressed. Conditioning EPMs on available data reduces uncertainty and increases the reliability of model predictions. We present a rational and concise approach to investigate the impact of conditioning EPMs on data as a function of the location of the environmentally sensitive target receptor, data types and spacing between measurements. We illustrate how the concept of comparative information yield curves introduced in de Barros et al. [de Barros FPJ, Rubin Y, Maxwell R. The concept of comparative information yield curves and its application to risk-based site characterization. Water Resour Res 2009;45:W06401. doi:10.1029/2008WR007324] could be used to assess site characterization needs as a function of flow and transport dimensionality and EPMs. For a given EPM, we show how alternative uncertainty reduction metrics yield distinct gains of information from a variety of sampling schemes. Our results show that uncertainty reduction is EPM dependent (e.g., travel times) and does not necessarily indicate uncertainty reduction in an alternative EPM (e.g., human health risk). The results show how the position of the environmental target, flow dimensionality and the choice of the uncertainty reduction metric can be used to assist in field sampling campaigns. © 2011 Elsevier Ltd.


Versini P.-A.,Polytechnic University of Catalonia
Journal of Hydrology | Year: 2012

Important damages occur in small headwater catchments when they are hit by severe storms with complex spatio-temporal structure, sometimes resulting in flash floods. As these catchments are mostly not covered by sensor networks, it is difficult to forecast these floods. This is particularly true for road submersions, representing major concerns for flood event managers. The use of Quantitative Precipitation Estimates and Forecasts (QPE/QPF) especially based on radar measurements could particularly be adequate to evaluate rainfall-induced risks. Although their characteristic time and space scales would make them suitable for flash flood modelling, the impact of their uncertainties remain uncertain and have to be evaluated. The Gard region (France) has been chosen as case study. This area is frequently affected by severe flash floods, and an application devoted to the road network has also been recently developed for the North part of this region. This warning system combines distributed hydro-meteorological modelling and susceptibility analysis to provide warnings of road inundations. The warning system has been tested on the specific storm of the 29-30 September 2007. During this event, around 200. mm dropped on the South part of the Gard and many roads were submerged. Radar-based QPE and QPF have been used to forecast the exact location of road submersions and the results have been compared to the effective road submersions actually occurred during the event as listed by the emergency services. Used on an area it has not been calibrated, the results confirm that the road submersion warning system represents a promising tool for anticipating and quantifying the consequences of storm events at ground. It rates the submersion risk with an acceptable level of accuracy and demonstrates also the quality of high spatial and temporal resolution radar rainfall data in real time, and the possibility to use them despite their uncertainties. However because of the quality of rainfall forecasts falls drastically with time, it is not often sufficient to provide valuable information for lead times exceeding 1. h. © 2011 Elsevier B.V.


Ostapovets A.,Academy of Sciences of the Czech Republic | Ostapovets A.,ASCR Institute of Physics Prague | Serra A.,Polytechnic University of Catalonia
Philosophical Magazine | Year: 2014

A model of twin growth in magnesium is presented together with the analysis of defects responsible for this growth. The twin interface is represented by and basal-prismatic facets. Disclinations are situated in the facet junctions creating dipoles superimposed on basal-prismatic and conjugate twin facets. The migration of facets is mediated by the conservative motion of interfacial disconnections. The interfaces contain twinning disconnections. The facet junctions serve as sources and sinks for these defects. Two types of disconnections (and) were observed in basal-prismatic boundary. The dipoles of disconnections were nucleated in the vicinity of existing defects of this type. Interaction of this dipole with existing leads to the creation of a disconnection, which is later absorbed in the facet junction. The nucleation of dipoles was not observed. In twin embryo growth, the basal-prismatic segments remain coherent with a fixed length, while the twin segments grow indefinitely. © 2014 © 2014 Taylor & Francis.


Serrat C.,Polytechnic University of Catalonia
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2013

I make evident a method for broadband spectral-phase control of light pulses produced in high-order harmonic generation (HHG). Using a feedback loop scheme, I demonstrate that a broad spectral region of the HHG output field can be coherently added to the intense infrared driving pulse with optimal attenuation and time delay parameters to control the phase of the HHG field generated in a second interaction region. I provide computational evidence of the control scheme by considering an optimal flattening of the spectral phase for the production of isolated attosecond pulses. This is proven by considering different spectral widths and for several central photon energies, such as 36 eV, 70 eV, and 120 eV. An iterative procedure of the method allows one to obtain single pulses with less than 1.2 cycles at a central photon energy of 36 eV. This control scheme is a fundamental tool that can be implemented for amplitude and phase shaping of any suitable spectral region in HHG. © 2013 American Physical Society.


Josa R.,Polytechnic University of Catalonia | Jorba M.,University of Barcelona | Vallejo V.R.,University of Barcelona
Ecological Engineering | Year: 2012

Restoration works in opencast mining are mostly done with fast-growing herbaceous species to control the erosion of embankments during rainfall, especially immediately after their construction. Such species often exhibit a high demand for water and their survival is therefore associated to an adequate hydric supply. If this fails, these species can rapidly disappear when water is scarce. This paper shows the results of a restoration experiment where two stony mineral substrates were used in a quarry area (NE Spain). The experiment was carried out at lysimeter scale and the aim was to evaluate the short time vegetation response to common restoration practices in semiarid areas with two types of substrates and two types of irrigation practices. Lysimeter results were used to indirectly draw implications for the usual irrigation practices in the area. During periods of the first year after sowing, irrigated and non-irrigated lysimeters were monitored for substrate humidity, water leachates and plant development. The population cover of sown plants fluctuated according to the water availability in the substrate before drought. Density of total basal shoots decreased dramatically during the period of water stress, reaching negligible cover values on both types of substrates. Water content in the top substrate (0-20. cm) was dramatically reduced in June, just after the maximum plant cover and water demand were reached (May).According with our results, it is not advisable to aim at a continuous and dense herbaceous cover composed of species that are not drought tolerant in restoration works, especially if embankments are constructed with spoil mine substrates. The restoration of opencast areas should include the use of water stress-resistant plants and careful irrigation plans. © 2012 Elsevier B.V.


Foucaud F.,Polytechnic University of Catalonia
Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics) | Year: 2013

An identifying code is a subset of vertices of a graph such that each vertex is uniquely determined by its nonempty neighbourhood within the identifying code. We study the associated computational problem Minimum Identifying Code, which is known to be NP-hard, even when the input graph belongs to a number of specific graph classes such as planar bipartite graphs. Though the problem is approximable within a logarithmic factor, it is known to be hard to approximate within any sub-logarithmic factor. We extend the latter result to the case where the input graph is bipartite, split or co-bipartite. Among other results, we also show that for bipartite graphs of bounded maximum degree (at least 3), it is hard to approximate within some constant factor. We summarize known results in the area, and we compare them to the ones for the related problem Minimum Dominating Set. In particular, our work exhibits important graph classes for which Minimum Dominating Set is efficiently solvable, but Minimum Identifying Code is hard (whereas in all previously studied classes, their complexity is the same). We also introduce a graph class for which the converse holds. © 2013 Springer-Verlag.


Cubarsi R.,Polytechnic University of Catalonia
Astronomy and Astrophysics | Year: 2014

Solving the Boltzmann collisionless equation under the axisymmetric hypothesis introduces serious limitations on describing the kinematics of a single stellar system according to the local Galactic observables. Instead of relaxing the hypothesis of axisymmetry, one alternative is to assume a mixture model. For a finite mixture of ellipsoidal velocity distributions, the coexistence of several stellar populations sharing a common potential introduces a set of conditions of consistency that may also constrain the population kinematics. For only a few potentials, the populations may have independent mean velocities and unconstrained velocity ellipsoids. In this paper, we determine which axisymmetric potentials are connected with a more flexible superposition of the stellar populations. The conditions of consistency are checked against recent results derived from kinematic surveys of the solar neighbourhood that include RAdial Velocity Experiment (RAVE) data. Several key observables are used to determine whether the axisymmetric mixture model is able to account for the main features of the local velocity distribution, such as the vertex deviation associated with the second central moment μÏ-θ, the population radial mean velocities, the radial gradient of the moment μÏ-z, the tilt of the velocity ellipsoids, and the existence of stars with no net rotation. In addition, the mixture moments for an arbitrary number of populations are derived in terms of the one-to-one mean velocity differences in order to study whether a more populated mixture could add any new features to the velocity distribution that remain unnoticed in a two-component mixture. According to this analysis, the quasi-stationary potential is the only potential allowing arbitrary directions of the population mean velocities. Then, the apparent vertex deviation of the total velocity distribution is due to the difference of the mean velocities of the populations whose velocity ellipsoids have no vertex deviation. For a non-separable potential, the population velocity ellipsoids have the same orientation and point towards the Galactic centre. For a potential separable in addition in cylindrical coordinates, the population velocity ellipsoids may have arbitrary tilt. © 2014 ESO.


Serrat C.,Polytechnic University of Catalonia
Physical Review Letters | Year: 2013

We theoretically study the response of He atoms exposed simultaneously to an intense IR pulse and a weak extreme ultraviolet (XUV) pulse with photon energies far from the principal atomic He resonances. We find that XUV forward scattering from the nonstationary electronic wave packet promoted by the intense IR driving field is strongly enhanced as compared with the normal weak scattering from bound or free electrons. Based on this effect, we predict that large amplification of XUV radiation can be achieved in the cutoff spectral region of high-harmonic generation in He gas. © 2013 American Physical Society.


Lorente-Espin O.,Polytechnic University of Catalonia
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2013

It is well known that little string theory (LST) black holes radiate a purely thermal spectrum of scalar particles. This theory lives in a Hagedorn phase with a fixed Hagedorn temperature that does not depend on its mass. Therefore, the theory keeps a thermal profile even taking into account self-gravitating effects and the backreaction of the metric. This has implications concerning the information loss paradox; one would not be able to recover any information from the LST black hole since the emission of scalar particles is totally uncorrelated. Several studies of the emission spectrum in LST concern scalar fields; it is our aim in this work to extend the study to the emission of fermions in order to verify that the most relevant conclusion for the scalar field remains valid for the fermion fields. Thus, we have calculated the emission probability, the flux, and also the greybody factor corresponding to a fermion field in LST background. © 2013 American Physical Society.


Boguna M.,University of Barcelona | Castellano C.,CNR Institute for Complex Systems | Castellano C.,University of Rome La Sapienza | Pastor-Satorras R.,Polytechnic University of Catalonia
Physical Review Letters | Year: 2013

We develop an analytical approach to the susceptible-infected-susceptible epidemic model that allows us to unravel the true origin of the absence of an epidemic threshold in heterogeneous networks. We find that a delicate balance between the number of high degree nodes in the network and the topological distance between them dictates the existence or absence of such a threshold. In particular, small-world random networks with a degree distribution decaying slower than an exponential have a vanishing epidemic threshold in the thermodynamic limit. © 2013 American Physical Society.


Khoury M.,Departament dEstructura I Constituents de la Materia | Lacasta A.M.,Polytechnic University of Catalonia | Sancho J.M.,Departament dEstructura I Constituents de la Materia | Lindenberg K.,University of California at San Diego
Physical Review Letters | Year: 2011

We carry out a detailed study of the motion of particles driven by a constant external force over a landscape consisting of a periodic potential corrugated by a small amount of spatial disorder. We observe anomalous behavior in the form of subdiffusion and superdiffusion and even subtransport over very long time scales. Recent studies of transport over slightly random landscapes have focused only on parameters leading to normal behavior, and while enhanced diffusion has been identified when the external force approaches the critical value associated with the transition from locked to running solutions, the regime of anomalous behavior had not been recognized. We provide a qualitative explanation for the origin of these anomalies, and make connections with a continuous time random walk approach. © 2011 American Physical Society.


Haro J.,Polytechnic University of Catalonia
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2016

Background bouncing cosmologies, driven by a single scalar field, having a quasi-matter domination period during the contracting phase, i.e., depicting the so-called Matter Bounce Scenario, are reconstructed for cosmologies with spatial positive curvature. These cosmologies lead to a nearly flat power spectrum of the fluctuation curvature in co-moving coordinates for modes that leave the Hubble radius during this quasi-matter domination period, and whose spectral index and its running, which are related with the effective Equation of State parameter given by the quotient of the pressure over the energy density, are compatible with experimental data. © 2016


Gordillo M.C.,Pablo De Olavide University | Boronat J.,Polytechnic University of Catalonia
Journal of Low Temperature Physics | Year: 2013

By means of diffusion Monte Carlo calculations, we obtained the phase diagrams of a first and second layer of 4He on graphene and on the outside of different isolated armchair carbon nanotubes with radii in the range 3.42 to 10.85 Å. That corresponds to tubes between the (5, 5) and (16, 16) in standard nomenclature. In both cases, the ground state is either a liquid (second layer on graphene and on nanotubes whose radii is greater than ∼7 Å) or an incommensurate solid (for thinner tubes). In the former case, upon a density increase, the system undergoes a first-order phase transition to another incommensurate solid. A study of the influence of the C-He potential (isotropic or anisotropic) on the phase diagrams is also presented. © 2012 Springer Science+Business Media New York.


Walasik W.,Aix - Marseille University | Walasik W.,Polytechnic University of Catalonia | Renversez G.,Aix - Marseille University
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2016

We present two complementary models to study stationary nonlinear solutions in one-dimensional plasmonic slot waveguides made of a finite-thickness nonlinear dielectric core surrounded by metal regions. The considered nonlinearity is of focusing Kerr type. In the first model, it is assumed that the nonlinear term depends only on the transverse component of the electric field and that the nonlinear refractive index change is small compared to the linear part of the refractive index. This first model allows us to describe analytically the field profiles in the whole waveguide using Jacobi elliptic special functions. It also provides a closed analytical formula for the nonlinear dispersion relation. In the second model, the full dependency of the Kerr nonlinearity on the electric-field components is taken into account and no assumption is required on the amplitude of the nonlinear term. The disadvantage of this approach is that the field profiles must be computed numerically. Nevertheless, analytical constraints are obtained to reduce the parameter space where the solutions of the nonlinear dispersion relations are sought. © 2016 American Physical Society.


Sandoval C.,University of Santiago de Chile | Roca P.,Polytechnic University of Catalonia
Construction and Building Materials | Year: 2012

A study of the influence and interaction of the main parameters involved on the buckling behaviour of masonry walls subjected to vertical loading is presented. For this purpose, the effects of the variation of the slenderness ratio, the eccentricity of applied load, the stiffness of the wall and the tensile strength of the unit-mortar interface on the load bearing capacity of masonry walls are analyzed by means of a numerical model based on the micro-modelling approach. The study has confirmed the possibility of describing the combined effect of both slenderness and stiffness by means of a single non-dimensional parameter. In addition, the research shows that, in the case of slender walls, the tensile strength of the unit-mortar interface may influence significantly on the maximum load eccentricity that the walls can resist. The paper also includes a comparison of the results obtained in the parametric study carried out with the analytical method provided by EN 1996-1-1:2005. © 2012 Elsevier Ltd. All rights reserved.


Sempau J.,Polytechnic University of Catalonia | Badal A.,U.S. Food and Drug Administration | Brualla L.,Universitatsklinikum Essen
Medical Physics | Year: 2011

Purpose: Two new codes, PENEASY and PENEASYLINAC, which automate the Monte Carlo simulation of Varian Clinacs of the 600, 1800, 2100, and 2300 series, together with their electron applicators and multileaf collimators, are introduced. The challenging case of a relatively small and far-from-axis field has been studied with these tools. Methods: PENEASY is a modular, general-purpose main program for the PENELOPE Monte Carlo system that includes various source models, tallies and variance-reduction techniques (VRT). The code includes a new geometry model that allows the superposition of voxels and objects limited by quadric surfaces. A variant of the VRT known as particle splitting, called fan splitting, is also introduced. PENEASYLINAC, in turn, automatically generates detailed geometry and configuration files to simulate linacs with PENEASY. These tools are applied to the generation of phase-space files, and of the corresponding absorbed dose distributions in water, for two 6 MV photon beams from a Varian Clinac 2100 CD: a 40 40 cm 2 centered field; and a 3 5 cm 2 field centered at (4.5, -11.5) cm from the beam central axis. This latter configuration implies the largest possible over-traveling values of two of the jaws. Simulation results for the depth dose and lateral profiles at various depths are compared, by using the gamma index, with experimental values obtained with a PTW 31002 ionization chamber. The contribution of several VRTs to the computing speed of the more demanding off-axis case is analyzed. Results: For the 40 × 40 cm 2 field, the percentages 1 and 1.2 of voxels with gamma indices (using 0.2 cm and 2 criteria) larger than unity and larger than 1.2 are 0.2 and 0, respectively. For the 3 5 cm 2 field, 1 0. These figures indicate an excellent agreement between simulation and experiment. The dose distribution for the off-axis case with voxels of 2.5 × 2.5 × 2.5 mm 3 and an average standard statistical uncertainty of 2 (1σ) is computed in 3.1 h on a single core of a 2.8 GHz Intel Core 2 Duo processor. This result is obtained with the optimal combination of the tested VRTs. In particular, fan splitting for the off-axis case accelerates execution by a factor of 240 with respect to standard particle splitting. Conclusions: PENEASY and PENEASYLINAC can simulate the considered Varian Clinacs both in an accurate and efficient manner. Fan splitting is crucial to achieve simulation results for the off-axis field in an affordable amount of CPU time. Work to include Elekta linacs and to develop a graphical interface that will facilitate user input is underway. © 2011 American Association of Physicists in Medicine.


Martinez-Ramirez S.,CSIC - Institute for the Structure of Matter | Fernandez-Carrasco L.,Polytechnic University of Catalonia
Construction and Building Materials | Year: 2012

The main hydration reaction product in the ternary system fly ash, calcium sulphate and calcium aluminate cement (40/20/40) at 20 °C is a hydrated calcium sulfoaluminate compound, an AFt phase slightly different from "traditional ettringite". The carbonation of ettringite develops gypsum but in this case rapidcreekite is formed. For the first time it has been observed that carbonation of the mentioned calcium sulfoaluminate compound (AFt), an hydrated calcium sulphate carbonated phase (Ca2(SO 4)(CO3)·4H2O) is formed with the replacement of an SO4 row in a (0 4 0) layer of the gypsum by CO 3 groups. The developed carbonated phase has been studied and analyzed through XRD, FTIR, Raman spectroscopy, and scanning electron microscopy. © 2011 Elsevier Ltd. All rights reserved.


Sanrom G.,Rovira i Virgili University | Alquezar R.,Polytechnic University of Catalonia | Serratosa F.,Rovira i Virgili University
Computer Vision and Image Understanding | Year: 2012

Finding correspondences between two point-sets is a common step in many vision applications (e.g., image matching or shape retrieval). We present a graph matching method to solve the point-set correspondence problem, which is posed as one of mixture modelling. Our mixture model encompasses a model of structural coherence and a model of affine-invariant geometrical errors. Instead of absolute positions, the geometrical positions are represented as relative positions of the points with respect to each other. We derive the Expectation-Maximization algorithm for our mixture model. In this way, the graph matching problem is approximated, in a principled way, as a succession of assignment problems which are solved using Softassign. Unlike other approaches, we use a true continuous underlying correspondence variable. We develop effective mechanisms to detect outliers. This is a useful technique for improving results in the presence of clutter. We evaluate the ability of our method to locate proper matches as well as to recognize object categories in a series of registration and recognition experiments. Our method compares favourably to other graph matching methods as well as to point-set registration methods and outlier rejectors. © 2011 Elsevier Inc. All rights reserved.


Gimenez O.,Polytechnic University of Catalonia | Jonsson A.,University Pompeu Fabra
Artificial Intelligence | Year: 2012

A planning problem is k-dependent if each action has at most k pre-conditions on variables unaffected by the action. This concept is of interest because k is a constant for all but a few of the current benchmark domains in planning, and is known to have implications for tractability. In this paper, we present an algorithm for solving planning problems in P(k), the class of k-dependent planning problems with binary variables and polytree causal graphs. We prove that our algorithm runs in polynomial time when k is a fixed constant. If, in addition, the causal graph has bounded depth, we show that plan generation is linear in the size of the input. Although these contributions are theoretical due to the limited scope of the class P(k), suitable reductions from more complex planning problems to P(k) could potentially give rise to fast domain-independent heuristics. © 2011 Elsevier B.V. All rights reserved.


Aneggi E.,University of Udine | De Leitenburg C.,University of Udine | Llorca J.,Polytechnic University of Catalonia | Trovarelli A.,University of Udine
Catalysis Today | Year: 2012

High temperature treatment under air of samples of polycrystalline ceria and ceria-zirconia is shown to induce a rearrangement of crystals, with a surface structural evolution of the oxides toward the formation of more reactive exposed planes belonging to the {1 0 0} and {1 1 0} families, associated with a decrease of the exposure of the less reactive {1 1 1} facets. This is dependent on the amount of ceria, being more evident with pure CeO 2, and less important with increasing ZrO 2 content. A correlation between the exposure of more reactive planes and the specific rate of carbon soot oxidation is found, in agreement with previous results obtained in monodimensional nanoshaped and polycrystalline ceria samples for CO oxidation. This suggests that Diesel soot oxidation catalysts might be designed by keeping into consideration also these effects. © 2012 Elsevier B.V.


Senturk O.S.,ABB | Helle L.,Vestas Inc. | Munk-Nielsen S.,University of Aalborg | Rodriguez P.,Polytechnic University of Catalonia | Teodorescu R.,University of Aalborg
IEEE Transactions on Power Electronics | Year: 2012

Wind turbine power capability is an essential set of data for both wind turbine manufacturers/operators and transmission system operators since the power capability determines whether a wind turbine is able to fulfill transmission system reactive power requirements and how much it is able to provide reactive power support as an ancillary service. For multimegawatt full-scale wind turbines, power capability depends on converter topology and semiconductor switch technology. As power capability limiting factors, switch current, semiconductor junction temperature, and converter output voltage are addressed in this study for the three-level neutral-point-clamped voltage source converter (3L-NPC-VSC) and 3L Active NPC VSC (3L-ANPC-VSC) with press-pack insulated gate bipolar transistors employed as a grid-side converter. In order to investigate these VSCs power capabilities under various operating conditions with respect to these limiting factors, a power capability generation algorithm based on the converter electrothermal model is developed. Built considering the VSCs operation principles and physical structure, the model is validated by a 2MVA single-phase 3L-ANPC-VSC test setup. The power capability investigations regarding a sample grid codes reactive power requirement show that 3L-ANPC-VSC results in 32 better power capability than 3L-NPC-VSC at the switching frequency of 1050Hz. Furthermore, 3L-ANPC-VSC with 57 higher switching frequency (1650Hz) and 33 smaller switching ripple filter can yield close power capability compared to 3L-NPC-VSC with 1050Hz. © 1986-2012 IEEE.


Gordillo M.C.,Pablo De Olavide University | Marti J.,Polytechnic University of Catalonia
Journal of Physics Condensed Matter | Year: 2010

In this paper, we summarize the main results obtained in our group about the behavior of water confined inside or close to different graphene surfaces by means of molecular dynamics simulations. These include the inside and outside of carbon nanotubes, and the confinement inside a slit pore or a single graphene sheet. We paid special attention to some thermodynamical (binding energies), structural (hydrogen-bond distributions) and dynamic (infrared spectra) properties, and their comparison to their bulk counterparts. © 2010 IOP Publishing Ltd.


Jaen X.,Polytechnic University of Catalonia | Molina A.,University of Barcelona
General Relativity and Gravitation | Year: 2013

In an effort to contribute to a better understanding of General Relativity, here we lay the foundations of generalized Newtonian gravity, which unifies inertial forces and gravitational fields. We also formulate a kind of equivalence principle for this generalized Newtonian theory. Finally, we prove that the theory we propose here can be obtained as the non-relativistic limit of General Relativity. © 2013 Springer Science+Business Media New York.


Reverter F.,Polytechnic University of Catalonia
IEEE Transactions on Instrumentation and Measurement | Year: 2013

This paper analyzes theoretically and experimentally the current consumption in direct interface circuits, i.e., circuits in which the sensor is directly connected to a microcontroller (μC) without using either a signal conditioning circuit or an analog-to-digital converter. The theoretical analysis, which takes into account the current consumed by both the internal electronics of the μC and the external components, proposes formulas to estimate the average current consumption in active mode. The estimated values fairly agree with those obtained in the experimental tests carried out by an AVR ATtiny2313 μC at different operating conditions. For example, the current consumption in active mode at 3 V-4 MHz was about 1.5 mA for the measurement of a 1-kω resistive sensor and 0.6 mA for the measurement of a 177-pF capacitive sensor. The results reported herein are expected to be useful for the design of direct interface circuits intended for battery-powered measurement systems. © 1963-2012 IEEE.


Pigolotti S.,Polytechnic University of Catalonia | Cencini M.,CNR Institute for Complex Systems
Journal of Theoretical Biology | Year: 2013

We study a stochastic community model able to interpolate from a neutral regime to a niche partitioned regime upon varying a single parameter tuning the intensity of niche stabilization, namely the difference between intraspecific and interspecific competition. By means of a self-consistent approach, we obtain an analytical expression for the species abundance distribution, in excellent agreement with stochastic simulations of the model. In the neutral limit, the Fisher log-series is recovered, while upon increasing the stabilization strength the species abundance distribution develops a maximum for species at intermediate abundances, corresponding to the emergence of a carrying capacity. Numerical studies of species extinction-time distribution show that niche-stabilization strongly affects also the dynamical properties of the system by increasing the average species lifetimes, while suppressing their fluctuations. The results are discussed in view of the niche-neutral debate and of their potential relevance to field data. © 2013 Elsevier Ltd.


Guasch O.,Ramon Llull University | Codina R.,Polytechnic University of Catalonia
Computer Methods in Applied Mechanics and Engineering | Year: 2013

Numerical simulations have proved that Variational Multiscale Methods (VMM) perform well as pure numerical large eddy simulation (LES) models. In this paper we focus on the orthogonal subgrid scale (OSS) finite element method and make an analysis of the statistical behavior of its stabilization terms in the quasi static approximation. This is done by resorting to results from classical statistical fluid mechanics concerning two point velocity, pressure and combined correlation functions of various orders. Given a fine enough mesh with characteristic element size h in the inertial subrange of a turbulent flow, it is shown that the rate of transfer of subgrid kinetic energy provided by the OSS stabilization terms does not depend on h and that it equals the molecular physical dissipation rate (up to a dimensionless constant that only depends on the finite element shapes) for a proper redesign of the standard parameters of the formulation. This is a noteworthy fact taking into account that the subgrid stabilization terms do not arise from physical considerations, but from the mathematical necessity to allow equal interpolation for the pressure and velocity fields, as well as to control convection. Therefore, the obtained results contribute somehow to the line of reasoning supporting that pure numerical approaches (i.e., without introducing additional physical models) could probably suffice in the LES simulation of turbulent flows. © 2013 Elsevier B.V.


The performance analysis of microwave radiometers is often simplified by assuming that most or all circuits are perfectly matched and that all are at the same physical temperature. However, the increasingly high performance demanded of future instruments requires assessment of their performance, including the following effects: actual complex S parameters of the various subsystems (nonzero insertion losses, imperfect matching, finite isolation etc.), physical temperature gradients etc. in order to correct them in software, when it is not possible to eliminate these nonidealities by design. A full noise-wave analysis is presented in this paper. Linearity and bias errors are evaluated, as well as their sensitivity to thermal gradients and characterization errors. Finally, the required intervals between calibrations can be estimated based on this analysis. Copyright 2010 by the American Geophysical Union.


Mirzadeh H.,Isfahan University of Technology | Cabrera J.M.,Polytechnic University of Catalonia | Cabrera J.M.,Fundacio Center Tecnologic | Najafizadeh A.,Isfahan University of Technology
Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science | Year: 2012

The modeling of hot flow stress and prediction of flow curves for unseen deformation conditions are important in metal-forming processes because any feasible mathematical simulation needs accurate flow description. In the current work, in an attempt to summarize, generalize, and introduce efficient methods, the dynamic recrystallization (DRX) flow curves of a 17-4 PH martensitic precipitation hardening stainless steel, a medium carbon microalloyed steel, and a 304 H austenitic stainless steel were modeled and predicted using (1) a hyperbolic sine equation with strain dependent constants, (2) a developed constitutive equation in a simple normalized stress-normalized strain form and its modified version, and (3) a feed-forward artificial neural network (ANN). These methods were critically discussed, and the ANN technique was found to be the best for the modeling available flow curves; however, the developed constitutive equation showed slightly better performance than that of ANN and significantly better predicted values than those of the hyperbolic sine equation in prediction of flow curves for unseen deformation conditions. © 2011 The Minerals, Metals & Materials Society and ASM International.


Gordillo M.C.,Pablo De Olavide University | Boronat J.,Polytechnic University of Catalonia
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

The phase diagrams of 4He adsorbed on the external surfaces of single armchair carbon nanotubes with radii in the range 3.42-10.85 are calculated using the diffusion Monte Carlo method. For nanotubes narrower than a (10,10) one, the ground state is an incommensurate solid similar to the one found for H 2 on the same substrates. For wider nanotubes, the phase with the minimum energy per particle is a liquid layer. Curved √3×√3 registered solids similar to the ones found on graphene and graphite were unstable for all the tubes considered. © 2012 American Physical Society.


Oladyshkin S.,University of Stuttgart | de Barros F.P.J.,Polytechnic University of Catalonia | Nowak W.,University of Stuttgart
Advances in Water Resources | Year: 2012

When investigating, modeling or operating uncertain systems, sensitivity analysis with respect to uncertain model parameters yields valuable information. It helps to quantify the relevance of parameters, to estimate their individual contributions to prediction uncertainty, and to better direct further data acquisition. In this work, we propose a response surface method for global sensitivity analysis (based on the arbitrary polynomial chaos expansion, aPC). The key advantages of our proposed technique are: (1) aPC alleviates the computational burden associated with conventional global sensitivity analysis methods that require many evaluations of a simulation model; (2) the proposed method incorporates arbitrary independent probability densities or weighting functions for the investigated model parameters, thus generalizing several existing methods to reflect the expected relevance of parameters values within their allowable ranges; and (3) our framework allows to incorporate this information while requiring only a finite number of statistical moments for the investigated parameters. We generalize the polynomial-based computation of Sobol indices to arbitrary distributions, and suggest an associated complementary new sensitivity measure based on polynomial representation, which allows both univariate and multivariate global analysis. Compared to Sobol indices, our new weighted measure is absolute rather than relative, and converges faster with increasing order of expansion. We use analytical and hybrid analytical-numerical formulations that further improve computational efficiency. Altogether, we can conduct global sensitivity analysis at computational costs that are almost as low as those of local sensitivity analyses. We illustrate our approach for a 3D groundwater quality and human health risk problem in heterogeneous porous media. © 2011 Elsevier Ltd.


Van Der Pijl F.,Technical University of Delft | Bauer P.,Technical University of Delft | Castilla M.,Polytechnic University of Catalonia
IEEE Transactions on Industrial Electronics | Year: 2013

Recent improvements in semiconductor technology make efficient switching possible at higher frequencies, which benefits the application of wireless inductive energy transfer. However, a higher frequency does not alter the magnetic coupling between energy transmitter and receiver. Due to the still weak magnetic coupling between transmitting and receiving sides that are separated by a substantial air gap, energy circulates in the primary transmitting side without being transferred to the secondary receiving side. This paper introduces an energy control method that reduces energy circulation in the primary to zero. The method makes use of the fact that energy can be stored in a magnetic field by the primary side and absorbed by the secondary side. Furthermore, the secondary side converter topology is modified in order to boost the damping as seen by the primary converter at required times. Essentially, the control method realizes an energetic coupling factor of one between the air coils of the wireless transformer. The working principle of the control method has been verified with an experimental setup. © 2012 IEEE.


De Palencia A.J.F.,ONGAWA | Perez-Foguet A.,Polytechnic University of Catalonia
Water Policy | Year: 2012

This paper reports the findings of three water point mapping (WPM) studies carried out in three rural districts of Tanzania covering 3,363 water points. The methodology was designed to estimate the influence that consideration of the quality of the water supplied and the year-round functionality of each water point would have on each district's coverage figures. To this end, the study included measurements of basic quality parameters (692 analyses) and characterisation of the year-round continuity of service, in addition to the data collected in standard WPM campaigns. Both the quality and year-round continuity results were analysed in a disaggregated form by water point technology. The results show that 22% of the improved water points analysed had more than 10 CFU (colony-forming units)/100 ml of water and 19% were seasonal. Moreover, water service coverage in the districts studied fell 40% on average when quality and year-round continuity were considered. There is a strong need to include these basic factors in the minimum standards to be delivered and monitored in rural areas. Simple and efficient methodologies for including quality and year-round continuity measurements in the information routines, such as the one presented here, are a necessary step to this end. © IWA Publishing 2012.


Martinez-Salamero L.,Rovira i Virgili University | Garcia G.,CNRS Laboratory for Analysis and Architecture of Systems | Orellana M.,Polytechnic University of Catalonia | Lahore C.,CNRS Laboratory for Analysis and Architecture of Systems | Estibals B.,CNRS Laboratory for Analysis and Architecture of Systems
IEEE Transactions on Industrial Electronics | Year: 2013

This paper presents the analysis and design of a sliding-mode control of a boost converter operating in continuous conduction mode that minimizes the inrush current and provides output voltage regulation in front of input voltage perturbations and load changes. The analytical expression of the inrush current is obtained, and the conditions to minimize the current peak are derived. The resulting controller is implemented analogically and employs two switching surfaces. Namely, one surface is combined with the conventional auxiliary diode branch for the start-up, while another one is used for the voltage regulation in steady state. Experimental results are in perfect agreement with the theoretical predictions. © 1982-2012 IEEE.


Pereira S.S.,Polytechnic University of Catalonia | Lopez-Valcarce R.,University of Vigo | Pages-Zamora A.,University of Vigo
IEEE Signal Processing Letters | Year: 2013

We address the problem of distributed estimation of a parameter from a set of noisy observations collected by a sensor network, assuming that some sensors may be subject to data failures and report only noise. In such scenario, simple schemes such as the Best Linear Unbiased Estimator result in an error floor in moderate and high signal-to-noise ratio (SNR), whereas previously proposed methods based on hard decisions on data failure events degrade as the SNR decreases. Aiming at optimal performance within the whole range of SNRs, we adopt a Maximum Likelihood framework based on the Expectation-Maximization (EM) algorithm. The statistical model and the iterative nature of the EM method allow for a diffusion-based distributed implementation, whereby the information propagation is embedded in the iterative update of the parameters. Numerical examples show that the proposed algorithm practically attains the Cramer-Rao Lower Bound at all SNR values and compares favorably with other approaches. © 1994-2012 IEEE.


Tort-Martorell X.,Polytechnic University of Catalonia
Quality Engineering | Year: 2015

Xavier Tort-Martorell, Department of Statistics and Operational Research, Universitat PoliteÁcnica de Catalunya, Spain, congratulates and thank Jose RamiÁrez for his excellent overview of George Box's thinking, personality, and contributions and for doing it in such an interesting and entertaining way. As is well known, George considered statistics to be the catalyst of learning by the scientific method. In the early 1980s, Box was already involved in disseminating statistics among engineers and technical managers. Together with Bill Hunter and Brian Joiner, The Three Tenors, they delivered seminars on design of experiments (DOE) and statistical process control (SPC). Together with Bill Hunter and Soren Bisgaard, he was the driving force behind the creation of the CQPI. The Center hosted many activities of collaboration with industry and of research based on these collaborations.


Valderrama-Blavi H.,Rovira i Virgili University | Bosque J.M.,Rovira i Virgili University | Guinjoan F.,Polytechnic University of Catalonia | Marroyo L.,Public University of Navarra | Martinez-Salamero L.,Rovira i Virgili University
IEEE Transactions on Industrial Electronics | Year: 2013

This paper presents a power adaptor device, referred to as smart panel device, allowing the connection of additional energy sources and storage elements to a domestic photovoltaic (PV) grid-connected system. The adaptor output port is designed to behave as a power source/sink, thus enabling its hot-swap parallel connection to renewable power sources without modifying their maximum power point (MPP). Moreover, the adaptor device features a power characteristic with a single controllable MPP and allows the control of the injected power within the operating range of the dc-ac grid-connected inverter. The work presents the design principles of such device by describing the operation of a sliding-mode controlled quadratic-boost converter. The proper operation of the device is experimentally verified for several scenarios in a small PV-based microgrid system including a fuel-cell stack, a 1-kW three-phase wind turbine, a battery charger-discharger, and commercial grid-connected PV inverters. © 2012 IEEE.


Beltran H.,Jaume I University | Bilbao E.,IKERLAN - IK4 | Bilbao E.,Ecole Polytechnique Federale de Lausanne | Belenguer E.,Jaume I University | And 2 more authors.
IEEE Transactions on Industrial Electronics | Year: 2013

This paper analyzes the minimum energy capacity ratings that an energy-storage (ES) system should accomplish in order to achieve a defined constant power production in a photovoltaic (PV) power plant. ES is a key issue for the further integration of intermittent and stochastic renewable energy sources, which are not currently dispatchable due to their dependence on real-time weather conditions, as is the case of PV technology. This paper proposes and describes an energy management strategy (EMS) for operating PV power plants with ES in the future. The goal of this EMS is to endow these power plants with a constant production that can be controlled and hence traded on electricity markets. This paper presents simulated results on the percentage of time throughout the year during which a large-scale PV$+$ES plant operates properly with different ES ratings and according to different configurations of this EMS. Finally, a test bench was developed, and experimental results validating this EMS were obtained. © 2012 IEEE.


Cazorla C.,CSIC - Institute of Materials Science | Lutsyshyn Y.,University of Rostock | Boronat J.,Polytechnic University of Catalonia
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

We study the elasticity of perfect solid 4He at zero temperature using the diffusion Monte Carlo method and a realistic semiempirical pairwise potential to describe t(C ij) as a funche He-He interactions. We calculate the value of the elastic constants of hcp helium tion of pressure from zero up to ∼110 bar. It is found that the pressure dependence of all nonzero elastic constants is linearly increasing and we provide accurate parametrization of each of them. Our C ij results are compared to previous variational calculations and low-temperature measurements and, in general, we find notably good agreement among them. Furthermore, we report results for the Grüneisen parameters, sound velocities, and Debye temperature over a wide range of pressures. This work represents a comprehensive quantum atomistic calculation of the elastic properties of solid helium under compression. © 2012 American Physical Society.


Huguet G.,Polytechnic University of Catalonia
Journal of vision | Year: 2014

We study the dynamics of perceptual switching in ambiguous visual scenes that admit more than two interpretations/percepts to gain insight into the dynamics of perceptual multistability and its underlying neural mechanisms. We focus on visual plaids that are tristable and we present both experimental and computational results. We develop a firing-rate model based on mutual inhibition and adaptation that involves stochastic dynamics of multiple-attractor systems. The model can account for the dynamic properties (transition probabilities, distributions of percept durations, etc.) observed in the experiments. Noise and adaptation have both been shown to play roles in the dynamics of bistable perception. Here, tristable perception allows us to specify the roles of noise and adaptation in our model. Noise is critical in considering the time of a switch. On the other hand, adaptation mechanisms are critical in considering perceptual choice (in tristable perception, each time a percept ends, there is a possible choice between two new percepts).


Morales-Narvaez E.,Catalan Institute of Nanoscience and Nanotechnology | Hassan A.-R.,Catalan Institute of Nanoscience and Nanotechnology | Hassan A.-R.,Polytechnic University of Catalonia | Hassan A.-R.,Catalan Institution for Research and Advanced Studies | Merkoci A.,Catalan Institute of Nanoscience and Nanotechnology
Angewandte Chemie - International Edition | Year: 2013

Turned ON by a pathogen: A highly sensitive pathogen-detection system has been designed and evaluated for the sensing of E. coli bacteria in diverse matrices. It employs antibody-quantum dot (Ab-QD) probes and exploits the extraordinary two-dimensional structure and fluorescence-quenching capabilities of graphene oxide. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


Gordillo M.C.,Pablo De Olavide University | Boronat J.,Polytechnic University of Catalonia
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

The phase diagram at zero temperature of 4He adsorbed on a helium incommensurate triangular solid on top of a single graphene sheet has been obtained using the diffusion Monte Carlo method. We have found that, in accordance with previous experimental and simulation results for graphite, the ground state of 4He on this setup is a liquid that, upon compression, transforms into a triangular solid. To define the stability limits of both liquid and solid phases, we considered not only the adsorption energies of the atoms located on the second layer but the average energy of the atoms in both layers. Our results show that the lower density limit for a stable liquid in the second layer is 0.163±0.005 Å -2 and that the lower limit for the existence of an incommensurate solid on the second layer is 0.186±0.003 Å -2. Both values are in overall agreement with the results of torsional oscillator experiments and heat capacity measurements on graphite. The 4/7 and 7/12 registered solids are found to be metastable with respect to triangular incommensurate arrangements of the same density. © 2012 American Physical Society.


Casas J.R.,Polytechnic University of Catalonia
Construction and Building Materials | Year: 2011

The paper presents a methodology for the probabilistic assessment of masonry arches at the serviceability and Ultimate Limit States. First, it explains the definition of the different failure modes and corresponding limit state functions that may occur depending on the type of masonry construction (single-ring and multi-ring). The most reported modes of failure are the four-hinge mechanism, the ring separation in multi-ring arches and the slippage at the foundations. Because of the lack of reliable material data (in the statistic sense) or available response models, only those more prone to be analyzed using reliability-based methods are shown in this paper: four-hinge mechanism and ring separation. The possibility of fatigue failure of masonry arch bridges under service loads and the proposal of reliability-based assessment methods at the ultimate level of the four-hinge mechanism are also analyzed. Finally, the proposed methodology is applied to an existing bridge. © 2010 Elsevier Ltd. All rights reserved.


Quintanilla R.,Polytechnic University of Catalonia | Sivaloganathan J.,University of Bath
European Journal of Mechanics, A/Solids | Year: 2012

In this paper we study the equations of the nonlinear theory of type II Thermoelasticity proposed by Green and Naghdi. We propose a constitutive framework for the free energy function arising in this theory and study various consequences including the existence and uniqueness of quasistatic solutions and the possibility of thermally activated instability. © 2011 Elsevier Masson SAS. All rights reserved.


Mellibovsky F.,Polytechnic University of Catalonia | Eckhardt B.,University of Marburg | Eckhardt B.,Technical University of Delft
Journal of Fluid Mechanics | Year: 2012

We study numerically a succession of transitions in pipe Poiseuille flow that lead from simple travelling waves to waves with chaotic time-dependence. The waves at the origin of the bifurcation cascade are twofold azimuthally periodic, shift-reflect symmetric, and have a non-dimensional axial wavelength of 1. 927 diameters. As the Reynolds number is increased, successive transitions result in a wide range of time-dependent solutions that include spiralling, modulated travelling, modulated spiralling, doubly modulated spiralling and mildly chaotic waves. Numerical evidence suggests that the latter spring from heteroclinic tangles of the stable and unstable invariant manifolds of two shift-reflect symmetric, modulated travelling waves. The chaotic set thus produced is confined to a limited range of Reynolds numbers, bounded by the occurrence of manifold tangencies. The subspace of discrete symmetry to which the states studied here belong makes many of the bifurcation and path-following investigations presented readily accessible. However, we expect that most of the phenomenology carries over to the full state space, thus suggesting a mechanism for the formation and break-up of invariant states that can give rise to chaotic dynamics. © 2012 Cambridge University Press.


Bravo E.,Polytechnic University of Catalonia
Astronomy and Astrophysics | Year: 2013

Type Ia supernova (SNIa) explosions synthesize a few tenths to several tenths of a solar mass, whose composition is the result of incomplete silicon burning that reaches peak temperatures of 4 GK to 5 GK. The elemental abundances are sensitive to the physical conditions in the explosion, making their measurement a promising clue to uncovering the properties of the progenitor star and of the explosion itself. Using a parameterized description of the thermodynamic history of matter undergoing incomplete silicon burning, we computed the final composition for a range of parameters wide enough to encompass current models of SNIa. Then, we searched for combinations of elemental abundances that trace the parameters values and are potentially measurable. For this purpose, we divide the present study into two epochs of SNIa, namely the optical epoch, from a few weeks to several months after the explosion, and the X-ray epoch, which refers to the time period in which the supernova remnant is young, starting one or two hundred years age and ending a thousand years after the event. During the optical epoch, the only SNIa property that can be extracted from the detection of incomplete silicon burning elements is the neutron excess of the progenitor white dwarf at thermal runaway, which can be determined through measuring the ratio of the abundance of manganese to that of titanium, chromium, or vanadium. Conversely, in the X-ray epoch, any abundance ratio built using a couple of elements from titanium, vanadium, chromium, or manganese may constrain the initial neutron excess. Furthermore, measuring the ratio of the abundances of vanadium to manganese in the X-ray might shed light on the timescale of the thermonuclear explosion. © 2013 ESO.


Domingo M.C.,Polytechnic University of Catalonia
Information Sciences | Year: 2013

Congestion control in Underwater Wireless Sensor Networks (UWSNs) is an important but challenging issue. TCP performs very poorly in UWSNs due to its inability to distinguish between packet losses due to congestion and those due to channel error. The existing congestion control protocols developed for terrestrial sensor networks cannot be applied to UWSNs because the characteristics of the underwater channel such as high bit error rates, high attenuation and propagation delays, multipath and Doppler distortion have not been considered. In this paper, a biologically-inspired congestion control protocol has been proposed for UWSNs based on the ability of marine communities to terminate with phytoplankton blooms and move the system back to equilibrium between species. The proposed distributed algorithm distinguishes between packet losses due to congestion and those due to high link error rates. It eliminates flow starvation and provides flow fairness. The channel effects of underwater propagation on packet losses are captured, the shadow zones are detected and the throughput of the flows from different nodes at the receiver is restored even with channel fading. Consequently, the system goes back to a stable state, where the different event flows coexist. The theoretical analysis and numerical evaluations show the performance of the proposed congestion control protocol in UWSNs. © 2012 Elsevier Inc. All rights reserved.


Argelaguet F.,French Institute for Research in Computer Science and Automation | Andujar C.,Polytechnic University of Catalonia
Computers and Graphics (Pergamon) | Year: 2013

Computer graphics applications controlled through natural gestures are gaining increasing popularity these days due to recent developments in low-cost tracking systems and gesture recognition technologies. Although interaction techniques through natural gestures have already demonstrated their benefits in manipulation, navigation and avatar-control tasks, effective selection with pointing gestures remains an open problem. In this paper we survey the state-of-the-art in 3D object selection techniques. We review important findings in human control models, analyze major factors influencing selection performance, and classify existing techniques according to a number of criteria. Unlike other components of the application's user interface, pointing techniques need a close coupling with the rendering pipeline, introducing new elements to be drawn, and potentially modifying the object layout and the way the scene is rendered. Conversely, selection performance is affected by rendering issues such as visual feedback, depth perception, and occlusion management. We thus review existing literature paying special attention to those aspects in the boundary between computer graphics and human-computer interaction. © 2012 Elsevier Ltd. All rights reserved.


Aneggi E.,University of Udine | Wiater D.,University of Udine | De Leitenburg C.,University of Udine | Llorca J.,Polytechnic University of Catalonia | Trovarelli A.,University of Udine
ACS Catalysis | Year: 2014

In this study, a series of conventional polycrystalline ceria and single-crystalline ceria nanorods and nanocubes were prepared by hydrothermal methods, and their structural, redox, and morphological properties were investigated using XRD, SEM, HRTEM, BET, temperature-programmed reduction, and oxygen storage capacity measurements. According to HRTEM, they are characterized by exposure of different surfaces: {100} surface for nanocubes; {100}, {110}, and in part {111} for nanorods; and mainly {111} for conventional polycrystalline ceria, with a morphology dominated by {111}-enclosed octahedral particles. The presence of more-reactive exposed surfaces affects the reaction of soot oxidation positively, with an increase in activity in nanoshaped materials compared with conventional ceria. Thermal aging, although detrimental for surface area, is shown to affect morphology by promoting irregular truncation of edges and corners and development of more reactive surface combinations in all crystal shapes. It is likely that thermal treatment, starting from either cubes or octahedral particles, induces the formation of a similar particle geometry whose activity is dependent on the type of plane exposed and by the number an extension of edge and corners, thus linking reactivity of octahedral particles in conventional ceria powders with that of cubes in nanoshaped materials. The results indicate that soot oxidation is also a surface-dependent reaction, and catalyst design for this purpose should allow for surface structure morphology and its evolution against temperature. © 2013 American Chemical Society.


Igualada L.,Economy Energy | Corchero C.,Economy Energy | Cruz-Zambrano M.,Economy Energy | Heredia F.-J.,Polytechnic University of Catalonia
IEEE Transactions on Smart Grid | Year: 2014

An optimization model is proposed to manage a residential microgrid including a charging spot with a vehicle-to-grid system and renewable energy sources. In order to achieve a realistic and convenient management, we take into account: (1) the household load split into three different profiles depending on the characteristics of the elements considered; (2) a realistic approach to owner behavior by introducing the novel concept of range anxiety; (3) the vehicle battery management considering the mobility profile of the owner and (4) different domestic renewable energy sources. We consider the microgrid operated in grid-connected mode. The model is executed one-day-ahead and generates a schedule for all components of the microgrid. The results obtained show daily costs in the range of 2.82¢= to 3.33¢; the proximity of these values to the actual energy costs for Spanish households validate the modeling. The experimental results of applying the designed managing strategies show daily costs savings of nearly 10%. © 2014 IEEE.


Foucher S.,Computer Research Institute of Montreal | Lopez-Martinez C.,Polytechnic University of Catalonia
IEEE Transactions on Image Processing | Year: 2014

Speckle noise filtering on polarimetric SAR (PolSAR) images remains a challenging task due to the difficulty to reduce a scatterer-dependent noise while preserving the polarimetric information and the spatial information. This challenge is particularly acute on single look complex images, where little information about the scattering process can be derived from a rank-1 covariance matrix. This paper proposes to analyze and to evaluate the performances of a set of PolSAR speckle filters. The filter performances are measured by a set of ten different indicators, including relative errors on incoherent target decomposition parameters, coherences, polarimetric signatures, point target, and edge preservation. The result is a performance profile for each individual filter. The methodology consists of simulating a set of artificial PolSAR images on which the various filters will be evaluated. The image morphology is stochastic and determined by a Markov random field and the number of scattering classes is allowed to vary so that we can explore a large range of image configurations. Evaluation on real PolSAR images is also considered. Results show that filters performances need to be assessed using a complete set of indicators, including distributed scatterer parameters, radiometric parameters, and spatial information preservation. © 1992-2012 IEEE.


Torrades F.,Polytechnic University of Catalonia | Garcia-Montano J.,Technological Center
Dyes and Pigments | Year: 2014

This work focuses on the use of Fenton reagent and UV-irradiation, in a lab-scale experiment, for the treatment of real dye wastewater coming from a Spanish textile manufacturer. Response surface methodology and a 23 factorial design were used to evaluate the effects of the three independent variables considered for the optimization of the oxidative process: temperature, Fe (II) and H2O2 concentrations, for a textile wastewater generated during a dyeing process with chemical oxygen demand of 1705 mg L -1 O2 at pH = 3. Wastewater degradation was followed in terms of chemical oxygen demand reduction. In the optimization, the correlation coefficients for the model (R2) were 0.985 and 0.990 for Fenton and photo-Fenton treatments respectively. Optimum reaction conditions at pH = 3 and temperature = 298 K were [H2O2] = 73.5 mM and [Fe(II)] = 1.79 mM. The combination of Fenton, Fenton-like and photon-Fenton reactions has been proved to be highly effective for the treatment of such a type of wastewaters, and several advantages for this technique arise from the study. Under optimum conditions, 120 min of treatment resulted in a 62.9% and 76.3% reduction in chemical oxygen demand after Fenton and photo-Fenton treatments respectively. © 2013 Elsevier Ltd. All rights reserved.


Huguet G.,Polytechnic University of Catalonia | Rinzel J.,Courant Institute of Mathematical Sciences | Hupe J.-M.,Toulouse 1 University Capitole
Journal of Vision | Year: 2014

We study the dynamics of perceptual switching in ambiguous visual scenes that admit more than two interpretations/percepts to gain insight into the dynamics of perceptual multistability and its underlying neural mechanisms. We focus on visual plaids that are tristable and we present both experimental and computational results. We develop a firing-rate model based on mutual inhibition and adaptation that involves stochastic dynamics of multiple-attractor systems. The model can account for the dynamic properties (transition probabilities, distributions of percept durations, etc.) observed in the experiments. Noise and adaptation have both been shown to play roles in the dynamics of bistable perception. Here, tristable perception allows us to specify the roles of noise and adaptation in our model. Noise is critical in considering the time of a switch. On the other hand, adaptation mechanisms are critical in considering perceptual choice (in tristable perception, each time a percept ends, there is a possible choice between two new percepts).


Ferreira L.,University of Lisbon | De Brito J.,University of Lisbon | Barra M.,Polytechnic University of Catalonia
Magazine of Concrete Research | Year: 2011

The properties of structural concrete which incorporates recycled coarse concrete aggregates (RCCA) may be affected by the RCCA's properties, especially in terms of water absorption, if no technological control methods are adopted in its production. Based on the knowledge of RCCA's water absorption behaviour, this research is aimed at determining the influence of the pre-saturation of the RCCA method on concrete's fresh and hardened properties, compared with a conventional concrete and a recycled aggregates concrete produced using the mixing water compensation method. Both methods can be easily adopted at concrete plants or construction sites. For the establishment of the pre-saturation method procedure, RCCA absorption was thoroughly characterised. A reference concrete and three mixes with replacement ratios of 20%, 50% and 100% of natural coarse aggregates by RCCA were produced, for each method. Tests were performed to determine slump, density, compressive strength, elasticity modulus, shrinkage, water absorption by immersion and capillary water absorption; the surfaces of the mixes were also observed with a magnifying glass. Results showed that mixes produced with the pre-saturation method exhibited slightly worse fresh and hardened properties, and therefore it is concluded that both the pre-saturation method and the mixing water compensation method can be adopted. Thomas Telford Ltd © 2011.


Lipinska M.,Technical University of Lodz | Hutchinson J.M.,Polytechnic University of Catalonia
Composites Science and Technology | Year: 2012

In polymer layered silicate nanocomposites, significant differences have been reported between the effects of the nano-reinforcement on rigid and elastomeric nanocomposites. In this paper, we have studied elastomeric nanocomposites based upon DGEBA epoxy resin filled with montmorillonite (MMT) and cured with a long-chain polyoxypropylene diamine, for comparison with analogous rigid nanocomposites. Ultrasonic mixing was used to disperse the MMT in the matrix to improve homogeneity and decrease the agglomerate size. Two different methods of nanocomposite preparation were used in which the MMT was first swollen with either the curing agent or the epoxy before the addition of, respectively, DGEBA or diamine. A better dispersion of the nanoclay in the matrix and a greater amount of intercalation occurred when the MMT was first swollen with the diamine. The effect of MMT concentrations up to 8. wt.% on the mechanical behaviour of the epoxy/MMT nanocomposites was investigated. It was found that the addition of MMT increased the tensile strength and modulus, although SAXS and TEM indicated that a significant fraction of the clay layers were not exfoliated. Nevertheless, the addition of the clay resulted in changes in the fracture surfaces, as indicated by SEM, consistent with the tensile results and indicative of toughening. © 2012 Elsevier Ltd.


Martinez J.A.,Polytechnic University of Catalonia
IEEE Power and Energy Society General Meeting | Year: 2011

The increasing production of energy from intermittent renewable sources (solar, wind) is also increasing the necessity of energy storage systems, which can provide a wide array of solutions to issues that affect electric power systems. This paper is aimed at summarizing the main characteristics of energy storage technologies, the factors that have to be considered for their selection, their main applications, and the type of models proposed for representing these technologies in economic and power system studies. © 2011 IEEE.


Cazorla C.,CSIC - Institute of Materials Science | Boronat J.,Polytechnic University of Catalonia
Physical Review B - Condensed Matter and Materials Physics | Year: 2013

We theoretically investigate the ground-state properties of a molecular para-hydrogen (p-H2) film in which crystallization is energetically frustrated by embedding sodium (Na) atoms periodically distributed in a triangular lattice. In order to fully deal with the quantum nature of p-H 2 molecules, we employ the diffusion Monte Carlo method and realistic semiempirical pairwise potentials describing the interactions between H 2-H2 and Na-H2 species. In particular, we calculate the energetic, structural, and superfluid properties of two-dimensional Na-H2 systems within a narrow density interval around equilibrium at zero temperature. In contrast to previous computational studies considering other alkali metal species such as rubidium and potassium, we find that the p-H2 ground state is a liquid with a significantly large superfluid fraction of ρs/ρ=0.29(2). The appearance of p-H2 superfluid response is due to the fact that the interactions between Na atoms and H2 molecules are less attractive than between H2 molecules. This induces a considerable reduction of the hydrogen density which favors the stabilization of the liquid phase. © 2013 American Physical Society.


Betancur R.,ICFO - Institute of Photonic Sciences | Romero-Gomez P.,ICFO - Institute of Photonic Sciences | Martinez-Otero A.,ICFO - Institute of Photonic Sciences | Elias X.,ICFO - Institute of Photonic Sciences | And 3 more authors.
Nature Photonics | Year: 2013

Organic solar cells have unique properties that make them very attractive as a renewable energy source. Of particular interest are semi-transparent cells, which have the potential to be integrated into building façades yet not completely block light. However, making organic cells transparent limits the metal electrode thickness to a few nanometres, drastically reducing its reflectivity and the device photon-harvesting capacity. Here, we propose and implement an ad hoc path for light-harvesting recovery to bring the photon-to-charge conversion up to almost 80% that of its opaque counterpart. We report semi-transparent PTB7:PC 71 BM cells that exhibit 30% visible light transmission and 5.6% power conversion efficiency. Non-periodic photonic crystals are used to trap near-infrared and near-ultraviolet photons. By modifying the layer structure it is possible to tune the device colour without significantly altering cell performance. © 2013 Macmillan Publishers Limited. All rights reserved.


Ikhouane F.,Polytechnic University of Catalonia
Mathematics of Control, Signals, and Systems | Year: 2013

Hysteresis is a nonlinear phenomenon exhibited by systems stemming from various science and engineering areas. To detect experimentally the presence of hysteresis in a system, its graph output versus input is plotted for different frequencies of the input. For hysteresis systems, these graphs converge to a limit set when frequency goes to zero. Moreover, this limit approaches asymptotically a periodic orbit. The relevance of hysteresis in applications makes it important to characterize it mathematically, which is the purpose of this paper. The systems that are considered are operators that map an input signal and initial condition to an output signal, all belonging to specified sets. The main result of this paper is a criterion for the mathematical characterization of hysteresis. The tools introduced in this paper are illustrated by means of a case study. © 2012 Springer-Verlag London.


Perra N.,Northeastern University | Baronchelli A.,Northeastern University | Mocanu D.,Northeastern University | Goncalves B.,Northeastern University | And 3 more authors.
Physical Review Letters | Year: 2012

The random walk process underlies the description of a large number of real-world phenomena. Here we provide the study of random walk processes in time-varying networks in the regime of time-scale mixing, i.e., when the network connectivity pattern and the random walk process dynamics are unfolding on the same time scale. We consider a model for time-varying networks created from the activity potential of the nodes and derive solutions of the asymptotic behavior of random walks and the mean first passage time in undirected and directed networks. Our findings show striking differences with respect to the well-known results obtained in quenched and annealed networks, emphasizing the effects of dynamical connectivity patterns in the definition of proper strategies for search, retrieval, and diffusion processes in time-varying networks. © 2012 American Physical Society.


Pigolotti S.,Niels Bohr Institute | Pigolotti S.,Polytechnic University of Catalonia | Benzi R.,University of Rome Tor Vergata | Jensen M.H.,Niels Bohr Institute | Nelson D.R.,Harvard University
Physical Review Letters | Year: 2012

We study competition between two biological species advected by a compressible velocity field. Individuals are treated as discrete Lagrangian particles that reproduce or die in a density-dependent fashion. In the absence of a velocity field and fitness advantage, number fluctuations lead to a coarsening dynamics typical of the stochastic Fisher equation. We investigate three examples of compressible advecting fields: a shell model of turbulence, a sinusoidal velocity field and a linear velocity sink. In all cases, advection leads to a striking drop in the fixation time, as well as a large reduction in the global carrying capacity. We find localization on convergence zones, and very rapid extinction compared to well-mixed populations. For a linear velocity sink, one finds a bimodal distribution of fixation times. The long-lived states in this case are demixed configurations with a single interface, whose location depends on the fitness advantage. © 2012 American Physical Society.


Masoller C.,Polytechnic University of Catalonia | Atay F.M.,Max Planck Institute for Mathematics in the Sciences
European Physical Journal D | Year: 2011

A network of delay-coupled logistic maps exhibits two different synchronization regimes, depending on the distribution of the coupling delay times. When the delays are homogeneous throughout the network, the network synchronizes to a time-dependent state [F.M. Atay, J. Jost, A. Wende, Phys. Rev. Lett. 92, 144101 (2004)], which may be periodic or chaotic depending on the delay; when the delays are sufficiently heterogeneous, the synchronization proceeds to a steady-state, which is unstable for the uncoupled map [C. Masoller, A.C. Marti, Phys. Rev. Lett. 94, 134102 (2005)]. Here we characterize the transition from time-dependent to steady-state synchronization as the width of the delay distribution increases. We also compare the two transitions to synchronization as the coupling strength increases. We use transition probabilities calculated via symbolic analysis and ordinal patterns. We find that, as the coupling strength increases, before the onset of steady-state synchronization the network splits into two clusters which are in anti-phase relation with each other. On the other hand, with increasing delay heterogeneity, no cluster formation is seen at the onset of steady-state synchronization; however, a rather complex unsynchronized state is detected, revealed by a diversity of transition probabilities in the network nodes. © 2011 EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.


Torres R.,Polytechnic University of Catalonia
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2014

A general class of quantum improved stellar models with interiors composed of non-interacting ( dust) particles is obtained and analyzed in a framework compatible with asymptotic safety. First, the effective exterior, based on the Quantum Einstein Gravity approach to asymptotic safety is presented and, second, its effective compatible dust interiors are deduced. The resulting stellar models appear to be devoid of shell-focusing singularities. © 2014 The Author.


De Haro J.,Polytechnic University of Catalonia
EPL | Year: 2014

We study the introduction of holonomy corrections in f(R) gravity. We will show that there are infinitely many ways, as many as canonical transformations, to introduce this kind of corrections, depending on the canonical variables (two coordinates and its conjugate momenta) used to obtain the Hamiltonian. In each case, these corrections lead, at effective level, to different modified holonomy-corrected Friedmann equations in f(R) gravity, which are in practice analytically unworkable, i.e. only numerical analysis can be used to understand its dynamics. In fact, there are infinitely many quantum theories, as many as canonical transformations, leading to different quantum cosmologies. Then, the problem is to know which of these quantum theories apply correctly to our Universe. We do not have an answer to this problem, and we could only give the following argument in favour of one preferred set of variables: the one that conformally maps f(R) to Einstein gravity, because for these variables the Hamiltonian of the system has the same form as in standard Loop Quantum Cosmology, which provides a map between the dynamics obtained in holonomy-corrected f(R) gravity and the one of standard Loop Quantum Cosmology. © CopyrightEPLA, 2014.


Julio G.,Polytechnic University of Catalonia
Cornea | Year: 2016

PURPOSE:: To establish determining factors for fast corneal sensitivity (CS) recovery after pterygium excision. METHODS:: Thirty-two eyes of 14 males and 18 females with primary nasal pterygium were recruited. Differences in CS (in the 4 quadrants and the center using a Cochet–Bonnet esthesiometer), pterygium corneal area (PCA), tear osmolarity, tear break-up time, Schirmer test, and ocular symptoms were analyzed before and 1 month after lesion excision. The relationship between CS recovery (difference between the 2 time points; CS1 – CS0) and the other features was assessed. RESULTS:: All the studied locations exhibited normal (6 cm) or near-normal mean CS at the 2 time points, except a tendency for moderate hypoesthesia in nasal CS0 (median 4.5; range: 1.5–6.0 cm). Point by point comparison revealed significant postoperative improvements in nasal location (P = 0.008; Wilcoxon signed-rank test) with normal values in 17 eyes (53%) and a median CS1 = 5.0 cm (2.5–5.5 cm) in 15 eyes with no complete recovery. No significant correlation was found between CS0 and the studied variables, and CS1 was only significantly correlated with PCA (rho: −0.441; P < 0.05). CS recovery also showed significant correlation with PCA (rho = −0.516; P < 0.01). CONCLUSIONS:: CS recovery after pterygium excision showed important variability, and the only studied factor that seems to be determinant could be PCA. It would be advisable to operate when the lesion is relatively small, with lower surgical injury and faster and complete recovery, thus protecting ocular surface homeostasis. Copyright © 2016 Wolters Kluwer Health, Inc. All rights reserved.


Shen Y.,Polytechnic University of Catalonia
Computer Methods in Applied Mechanics and Engineering | Year: 2014

We have developed an efficient method to model the fluid lag in fluid-driven fracture propagation via a variational inequality formulation. The distinct feature of this method is that the configurations with and without a lag can be handled in a unified framework and no change of formulation is needed during the simulation at the time the fracturing liquid reaches the fracture tip. This is achieved by formulating the problem as solving for the nonnegative pressure field in the fracture via a time-dependent (parabolic) variational inequality. Without introducing extra assumptions but merely based on mass conservation, this method is able to predict whether a fluid lag is going to remain or completely disappear as the fracturing progresses. © 2014 Elsevier B.V.


Garcia-Rudolph A.,Institute Guttmann | Gibert K.,Polytechnic University of Catalonia
Expert Systems with Applications | Year: 2014

Cognitive rehabilitation (CR) treatment consists of hierarchically organized tasks that require repetitive use of impaired cognitive functions in a progressively more demanding sequence. Active monitoring of the progress of the subjects is therefore required, and the difficulty of the tasks must be progressively increased, always pushing the subjects to reach a goal just beyond what they can attain. There is an important lack of well-established criteria by which to identify the right tasks to propose to the patient. In this paper, the NeuroRehabilitation Range (NRR) is introduced as a means of identifying formal operational models. These are to provide the therapist with dynamic decision support information for assigning the most appropriate CR plan to each patient. Data mining techniques are used to build data-driven models for NRR. The Sectorized and Annotated Plane (SAP) is proposed as a visual tool by which to identify NRR, and two data-driven methods to build the SAP are introduced and compared. Application to a specific representative cognitive task is presented. The results obtained suggest that the current clinical hypothesis about NRR might be reconsidered. Prior knowledge in the area is taken into account to introduce the number of task executions and task performance into NRR models and a new model is proposed which outperforms the current clinical hypothesis. The NRR is introduced as a key concept to provide an operational model identifying when a patient is experiencing activities in his or her Zone of Proximal Development and, consequently, experiencing maximum improvement. For the first time, data collected through a CR platform has been used to find a model for the NRR. © 2014 Elsevier Ltd. All rights reserved.


Shen Y.,Polytechnic University of Catalonia | Lew A.J.,Stanford University
Computer Methods in Applied Mechanics and Engineering | Year: 2014

The extended finite element method (XFEM) is an efficient way to include discontinuities, such as a crack, into a finite element mesh. The singularity at the crack tip restricts standard finite element methods to converge with a rate of at most 1/2 for the stresses, and 1 for the displacements, with respect to the mesh size. This is true for cracks in incompressible materials as well, when any of the standard techniques to sidestep locking is adopted. To attain an optimal convergence rate of 1 for stresses and of 2 for displacements with piecewise affine elements, it is necessary to enrich the finite element space with singular basis functions. The support of these singular functions is the entire plane, but to avoid decreasing the sparsity of the stiffness matrix too much, each of them is then generally localized to a neighborhood of the crack tip by multiplying by a cutoff function or a subset of a partition-of-unity basis. For nearly incompressible materials, however, the resulting basis functions no longer contain incompressible displacement fields, and hence they either lead to locking or suboptimal convergence rates. To overcome this problem, we introduce here an XFEM with optimal convergence rate and without the problem of locking for nearly incompressible materials, i.e., it possesses an error bound that does not diverge as Poisson's ratio approaches 0.5. The method is based on a primal, or one-field formulation of a discontinuous Galerkin method that we introduced earlier. This one-field formulation is obtained through the introduction of a lifting operator, but unlike most lifting operators which map inter-element discontinuities into elementwise polynomials, ours maps such discontinuities into spaces enriched with the singular behavior of the solution. This is the key idea for the method to be simultaneously locking-free and optimally convergent. © 2014 Elsevier B.V.


Walasik W.,Aix - Marseille University | Walasik W.,Polytechnic University of Catalonia | Renversez G.,Aix - Marseille University | Kartashov Y.V.,Russian Academy of Sciences
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2014

We present three complementary methods to study stationary nonlinear solutions in one-dimensional nonlinear metal-dielectric structures. Two of them use an approximate treatment of the Kerr-type nonlinear term taking into account only the leading electric-field component, while the third one allows for an exact treatment of the nonlinearity. A direct comparison of the results obtained with all three models is presented and the excellent agreement between them justifies the assumptions that have been used to construct the models. A systematic study of the configurations made of two, three, or four layers that contain a semi-infinite Kerr-type nonlinear dielectric, a metal film, and linear dielectrics is presented. A detailed analysis of properties, type, and number of solutions in these three types of structures is performed. The parameter ranges where plasmon-soliton waves exist are found. Structures with realistic optogeometric parameters where plasmon solitons exist at power levels already used in spatial soliton studies are proposed and studied. © 2014 American Physical Society.


Romero E.,Polytechnic University of Catalonia
Engineering Geology | Year: 2013

A review of the literature is initially presented to bring into light the important microstructural effects on the hydraulic properties of different compacted clayey soils. Experimental data coming from microstructural and macroscopic studies on different compacted clayey soils with dominant multi-modal pore size distribution are analysed to provide a comprehensive picture of different phenomenological features of hydraulic soil behaviour. The data come from clayey soils compacted on the dry side (i.e., with an intrinsic and permanent aggregated structure) or alternatively with dominant coarse fraction (i.e., sand-bentonite mixture with shielding skeleton and well-developed inter-grain porosity), which undergo important microstructural changes on wetting and drying. A microstructural model, already developed to take into account microstructural aspects on water retention curves, is used to plot in the Proctor plane the microstructure set up by compaction and its evolution along hydraulic paths. The model is also used to explain the evolution of hydraulic properties (water and air permeability, water retention) along wetting and drying paths on an artificially prepared mixture of sand and bentonite with dominant granular fraction. The bentonite strongly reacts to changes in water content and allows studying the transition from a granular soil with large pores between sand grains to a low-permeability material on wetting. The key point of this simple model is the introduction of the dependence of the microvoid volume (admitting saturated aggregates) on water content, following an equivalent behavioural response to the macroscopic shrinkage curve. © 2013 Elsevier B.V.


Knops R.J.,Heriot - Watt University | Quintanilla R.,Polytechnic University of Catalonia
International Journal of Engineering Science | Year: 2015

Uniqueness and spatial stability are investigated for smooth solutions to boundary value problems in non-classical linearised and linear thermoelasticity subject to certain conditions on material coefficients. Uniqueness is derived for standard boundary conditions on bounded regions using a generalisation of Kirchhoff's method. Spatial stability is discussed for the semi-infinite prismatic cylinder in the absence of specified axial asymptotic behaviour. Alternative growth and decay estimates are established principally for the cross-sectional energy flux that is shown to satisfy a first order differential inequality. Uniqueness in the class of solutions with bounded energy follows as a corollary. Separate discussion is required for the linearised and linear theories. Although the general approach is similar for both theories, the argument must be considerably modified for the treatment of the linear theory. © 2014 Elsevier Ltd. All rights reserved.


Orlandi J.G.,University of Barcelona | Soriano J.,University of Barcelona | Alvarez-Lacalle E.,Polytechnic University of Catalonia | Teller S.,University of Barcelona | Casademunt J.,University of Barcelona
Nature Physics | Year: 2013

At early stages of development, neuronal cultures in vitro spontaneously reach a coherent state of collective firing in a pattern of nearly periodic global bursts. Although understanding the spontaneous activity of neuronal networks is of chief importance in neuroscience, the origin and nature of that pulsation has remained elusive. By combining high-resolution calcium imaging with modelling in silico, we show that this behaviour is controlled by the propagation of waves that nucleate randomly in a set of points that is specific to each culture and is selected by a non-trivial interplay between dynamics and topology. The phenomenon is explained by the noise focusing effect - a strong spatio-temporal localization of the noise dynamics that originates in the complex structure of avalanches of spontaneous activity. Results are relevant to neuronal tissues and to complex networks with integrate-and-fire dynamics and metric correlations, for instance, in rumour spreading on social networks. © 2013 Macmillan Publishers Limited.


Santos S.,Polytechnic University of Catalonia
Applied Physics Letters | Year: 2014

In amplitude modulation atomic force microscopy the attractive and the repulsive force regimes induce phase shifts above and below 90°, respectively. In the more recent multifrequency approach, however, multiple operation regimes have been reported and the theory should be revisited. Here, a theory of phase contrast in multifrequency atomic force microscopy is developed and discussed in terms of energy transfer between modes, energy dissipation and the kinetic energy and energy transfer associated with externally driven harmonics. The single frequency virial that controls the phase shift might undergo transitions in sign while the average force (modal virial) remains positive (negative). © 2014 AIP Publishing LLC.


Dobre C.,Polytechnic University of Bucharest | Xhafa F.,Polytechnic University of Catalonia
Future Generation Computer Systems | Year: 2014

Cities are areas where Big Data is having a real impact. Town planners and administration bodies just need the right tools at their fingertips to consume all the data points that a town or city generates and then be able to turn that into actions that improve peoples' lives. In this case, Big Data is definitely a phenomenon that has a direct impact on the quality of life for those of us that choose to live in a town or city. Smart Cities of tomorrow will rely not only on sensors within the city infrastructure, but also on a large number of devices that will willingly sense and integrate their data into technological platforms used for introspection into the habits and situations of individuals and city-large communities. Predictions say that cities will generate over 4.1 terabytes per day per square kilometer of urbanized land area by 2016. Handling efficiently such amounts of data is already a challenge. In this paper we present our solutions designed to support next-generation Big Data applications. We first present CAPIM, a platform designed to automate the process of collecting and aggregating context information on a large scale. It integrates services designed to collect context data (location, user's profile and characteristics, as well as the environment). Later on, we present a concrete implementation of an Intelligent Transportation System designed on top of CAPIM. The application is designed to assist users and city officials better understand traffic problems in large cities. Finally, we present a solution to handle efficient storage of context data on a large scale. The combination of these services provides support for intelligent Smart City applications, for actively and autonomously adaptation and smart provision of services and content, using the advantages of contextual information. © 2013 Elsevier B.V. All rights reserved.


Gordillo M.C.,Pablo De Olavide University | Boronat J.,Polytechnic University of Catalonia
Physical Review B - Condensed Matter and Materials Physics | Year: 2010

The phase diagram of the first layer of H2 adsorbed on top of a single graphene sheet has been calculated by means of a series of diffusion Monte Carlo simulations. We have found that, as in the case of H4 e, the ground state of molecular hydrogen is a √3×√3 commensurate structure, followed, upon a pressure increase, by an incommensurate triangular solid. A striped phase of intermediate density was also considered and found lying on top of the equilibrium curve separating both commensurate and incommensurate solids. © 2010 The American Physical Society.


Caner F.C.,Polytechnic University of Catalonia | Caner F.C.,Northwestern University | Bazant Z.P.,Northwestern University
Journal of Engineering Mechanics | Year: 2013

Mathematical modeling of the nonlinear triaxial behavior and damage of such a complexmaterial as concrete has been a long-standing challenge in which progress has been made only in gradual increments. The goal of this study is a realistic and robust material model for explicit finite-element programs for concrete structures that computes the stress tensor from the given strain tensor and some history variables. Themicroplanemodels, which use a constitutive equation in a vectorial rather than tensorial form and are semimultiscale by virtue of capturing interactions among phenomena of different orientation, can serve this goal effectively. This paper presents a new concrete microplane model, M7, which achieves this goal much better than the previous versions M1-M6 developed at Northwestern University since 1985. The basic mathematical structure of M7 is logically correlated to thermodynamic potentials for the elastic regime, the tensile and compressive damage regimes, and the frictional slip regime. Given that the volumetric-deviatoric (V-D) split of strains is inevitable for distinguishing between compression failures at low and high confinement, the key idea is to apply the V-Dsplit only to the microplane compressive stress-strain boundaries (or strain-dependent yield limits), the sumof which is compared with the total normal stress from the microplane constitutive relation. This avoids the use of the V-D split of the elastic strains and of the tensile stress-strain boundary, which caused various troubles in M3-M6 such as excessive lateral strains and stress locking in far postpeak uniaxial extension, poor representation of unloading and loading cycles, and inability to represent high dilatancy under postpeak compression in lower-strength concretes. Moreover, the differences between high hydrostatic compression and compressive uniaxial strain are accurately captured by considering the compressive volumetric boundary as dependent on the principal strain difference. The model is verified extensively in the companion paper. © 2013 American Society of Civil Engineers.


Ghias A.M.Y.M.,University of New South Wales | Pou J.,Polytechnic University of Catalonia | Agelidis V.G.,University of New South Wales | Ciobotaru M.,University of New South Wales
IEEE Transactions on Power Electronics | Year: 2014

This letter reports a method for the initial charging of capacitors in grid-connected flying capacitor (FC) multilevel converters. A resistor is inserted between each phase of the FC converter and the grid. A voltage balancing algorithm is activated from the beginning of the process and the FC converter generates proper output voltages to achieve balanced charging of both the dc-bus capacitor and the FCs. The proposed initial charging method achieves low voltage and current stress on the power devices and the passive components. The method is simple to implement and can be applied to an FC with any number of levels. Experimental results demonstrate the effectiveness of the proposed method. © 2013 IEEE.


Sebestyen E.,University Pompeu Fabra | Zawisza M.,Polytechnic University of Catalonia | Eyras E.,University Pompeu Fabra | Eyras E.,Catalan Institution for Research and Advanced Studies
Nucleic Acids Research | Year: 2015

The determination of the alternative splicing isoforms expressed in cancer is fundamental for the development of tumor-specific molecular targets for prognosis and therapy, but it is hindered by the heterogeneity of tumors and the variability across patients. We developed a new computational method, robust to biological and technical variability, which identifies significant transcript isoform changes across multiple samples. We applied this method to more than 4000 samples from the The Cancer Genome Atlas project to obtain novel splicing signatures that are predictive for nine different cancer types, and find a specific signature for basal-like breast tumors involving the tumor-driver CTNND1. Additionally, our method identifies 244 isoform switches, for which the change occurs in the most abundant transcript. Some of these switches occur in known tumor drivers, including PPARG, CCND3, RALGDS, MITF, PRDM1, ABI1 and MYH11, for which the switch implies a change in the protein product. Moreover, some of the switches cannot be described with simple splicing events. Surprisingly, isoform switches are independent of somatic mutations, except for the tumor-suppressor FBLN2 and the oncogene MYH11. Our method reveals novel signatures of cancer in terms of transcript isoforms specifically expressed in tumors, providing novel potential molecular targets for prognosis and therapy. Data and software are available at: http: //dx.doi.org/10.6084/m9.figshare.1061917 and https: //bitbucket.org/regulatorygenomicsupf/iso-ktsp. © The Author(s) 2015. Published by Oxford University Press on behalf of Nucleic Acids Research.


Kumar S.,MINES ParisTech | Bergada J.M.,Polytechnic University of Catalonia
International Journal of Mechanical Sciences | Year: 2013

In the design of high pressure piston pumps/motors, some manufacturers use pistons which have grooves being cut along the piston length, while others do not use grooves at all. The present paper clarifies the effect of grooves cut along the pistons surface via studding the effect of the number of grooves and their location over the piston surface. Piston force, torque, leakage, areas where cavitation might appear and piston stability, will be, in the present paper and for a set of piston/grooves configurations, carefully evaluated. A finite volume based Reynolds equation model has been formulated for the piston-cylinder clearance which considers the piston eccentricity and the relative tangential movement between piston and barrel. Different grooves configurations have been evaluated in search of finding minimum leakage, minimum appearance of cavitation and maximum restoring torque. Design instructions to optimize the piston behavior are also given, therefore the information and methodology presented in the present paper, is expected to be used as a designing tool for future machines. The piston/cylinder leakage is considered as laminar under all working conditions. The Fluid used is hydraulic oil ISO 32. © 2012 Elsevier Ltd. All rights reserved.


Musso G.,Polytechnic University of Turin | Romero E.,Polytechnic University of Catalonia | della Vecchia G.,Polytechnic of Milan
Geotechnique | Year: 2013

This work presents an insight into double-structure effects on the coupled chemo-hydro-mechanical behaviour of a compacted active clay. In the first part, selected pore size distribution curves are introduced, to highlight the influence of solute concentration on the evolution of the microstructure of compacted samples. An aggregated structure with dual-pore network is induced by compaction even at relatively high water contents. This structural arrangement is enhanced by salinisation, and has a notable influence on transient volume change behaviour - that is, the occurrence of different stages of swelling upon pore water dilution and higher volume change rates upon salinisation. A coupled chemo-hydro-mechanical model, taking into consideration double-structural features from a chemo-mechanical viewpoint, is described and then used to interpret these behavioural responses and present complementary information on local transient processes. The model is designed to identify an intra-aggregate and an inter-aggregate domain, and assigns different values of hydraulic pressure and osmotic suction to each domain. Distinct constitutive laws for both domains are formulated, and the flow of salt and water between the two domains is accounted for by a physically based mass exchange term. The model is used to simulate salt diffusion tests run in an oedometer at constant vertical stress. Parameters used in the formulation are calibrated based on separate experimental evidence, both through direct test results and through back-analyses of laboratory experiments. © 2013 Thomas Telford Ltd.


Pabby A.K.,Bhabha Atomic Research Center | Sastre A.M.,Polytechnic University of Catalonia
Journal of Membrane Science | Year: 2013

This review will examine the most recent research and developments in hollow fibre contactor technology and membrane-based extraction processes, including the latest improvements with regard to stability and flux. The described classification attempts to cover all studies performed by means of non-dispersive contact using hydrophilic/hydrophobic microporous polymeric supports, either by impregnating the membrane or filling its pores with the bulk of the aqueous/organic solution. All membrane processes covered under these categories will be compared with improved versions in terms of performance, mass transfer modelling, stability issues, applications and the state of the art in membrane-based separation techniques. In general, an attempt will be made to review the literature published between 2005 and 2012 (August 2012) in order to focus on the real status of hollow fibre technology and membrane-based extraction processes. In a modern approach, the prospects for the use of ionic liquid (IL) as a membrane carrier for different applications with different membrane morphologies are also presented. In addition, new highly stabilised techniques developed by different researchers, such as hollow fibre renewal liquid membranes (HFRLMs) and pseudo-emulsion-based hollow fibre strip dispersion (PEHFSD), are also discussed. © 2012 Elsevier B.V.


Jimenez P.,Polytechnic University of Catalonia
Journal of Intelligent Manufacturing | Year: 2013

A systematic overview on the subject of assembly sequencing is presented. Sequencing lies at the core of assembly planning, and variants include finding a feasible sequence - respecting the precedence constraints between the assembly operations -, or determining an optimal one according to one or several operational criteria. The different ways of representing the space of feasible assembly sequences are described, as well as the search and optimization algorithms that can be used. Geometry plays a fundamental role in devising the precedence constraints between assembly operations, and this is the subject of the second part of the survey, which treats also motion in contact in the context of the actual performance of assembly operations. © 2011 Springer Science+Business Media, LLC.


Tallada M.G.,Polytechnic University of Catalonia
Proceedings of the ACM SIGPLAN Symposium on Principles and Practice of Parallel Programming, PPOPP | Year: 2016

Deep neural networks (DNN) have recently achieved extraordinary results in domains like computer vision and speech recognition. An essential element for this success has been the introduction of high performance computing (HPC) techniques in the critical step of training the neural network. This paper describes the implementation and analysis of a network-agnostic and convergence-invariant coarse-grain parallelization of the DNN training algorithm. The coarse-grain parallelization is achieved through the exploitation of the batch-level parallelism. This strategy is independent from the support of specialized and optimized libraries. Therefore, the optimization is immediately available for accelerating the DNN training. The proposal is compatible with multi-GPU execution without altering the algorithm convergence rate. The parallelization has been implemented in Caffe, a state-of-the-art DNN framework. The paper describes the code transformations for the parallelization and we also identify the limiting performance factors of the approach. We show competitive performance results for two state-of-the-art computer vision datasets, MNIST and CIFAR-10. In particular, on a 16-core Xeon E5-2667v2 at 3.30GHz we observe speedups of 8× over the sequential execution, at similar performance levels of those obtained by the GPU optimized Caffe version in a NVIDIA K40 GPU. © 2016 ACM.


Pierobon M.,Georgia Institute of Technology | Akyildiz I.F.,Georgia Institute of Technology | Akyildiz I.F.,Polytechnic University of Catalonia
IEEE Transactions on Signal Processing | Year: 2011

Molecular communication (MC) is a promising bio-inspired paradigm, in which molecules are used to encode, transmit and receive information at the nanoscale. Very limited research has addressed the problem of modeling and analyzing the MC in nanonetworks. One of the main challenges in MC is the proper study and characterization of the noise sources. The objective of this paper is the analysis of the noise sources in diffusion-based MC using tools from signal processing, statistics and communication engineering. The reference diffusion-based MC system for this analysis is the physical end-to-end model introduced in a previous work by the same authors. The particle sampling noise and the particle counting noise are analyzed as the most relevant diffusion-based noise sources. The analysis of each noise source results in two types of models, namely, the physical model and the stochastic model. The physical model mathematically expresses the processes underlying the physics of the noise source. The stochastic model captures the noise source behavior through statistical parameters. The physical model results in block schemes, while the stochastic model results in the characterization of the noises using random processes. Simulations are conducted to evaluate the capability of the stochastic model to express the diffusion-based noise sources represented by the physical model. © 2011 IEEE.


Haro A.,University of Barcelona | Puig J.,Polytechnic University of Catalonia
Nonlinearity | Year: 2013

In this paper, we study the dynamical properties of a class of ergodic linear skew-products which includes the linear skew-products defined by quasi-periodic Schrödinger operators and their duals, in Aubry sense, when the potential is a trigonometric polynomial. Notably, these linear skew-products preserve an adapted complex-symplectic structure. We prove a Thouless formula relating the sum of the positive Lyapunov exponents and the logarithmic potential associated with the density of states of the corresponding operator. In particular, for quasi-periodic Schrödinger operators and their duals, we prove an identity for the upper Lyapunov exponent of the skew-product and the sum of the positive Lyapunov exponents of their dual, which generalizes the well-known formula for the Almost Mathieu. We illustrate these identities with some numerical illustrations. © 2013 IOP Publishing Ltd & London Mathematical Society.


Perez E.,Jaume I University | Beltran H.,Jaume I University | Aparicio N.,Jaume I University | Rodriguez P.,Polytechnic University of Catalonia
IEEE Transactions on Sustainable Energy | Year: 2013

This work presents a model predictive control (MPC) approach to manage in real-time the energy generated by a grid-tied photovoltaic (PV) power plant with energy storage (ES), optimizing its economic revenue. This MPC approach stands out because, when a long enough prediction horizon is used, the saturation of the ES system (ESS) can be advanced by means of a prediction model of the PV panels production. Therefore, the PV${+}$ES power plant can modify its production so as to manage the power deviations with regard to that committed in the daily and intraday electricity markets, with the objective of reducing economic penalties. The initial power commitment is supposed in this work to be given by a higher level energy management operator. By a proper definition of its objective function, the predictive control allows us to economically optimize the PV${+}$ES power plant performance. This control strategy is tested in simulations with actual data measured for different days with varying meteorological conditions. Results provide a good reference on the economic benefits which can be obtained thanks to the MPC introduction. © 2010-2012 IEEE.


Nabona N.,Polytechnic University of Catalonia
IEEE Transactions on Power Systems | Year: 2013

The medium-term planning for a company participating in a pure pool market can be considered under different behavioral principles: cartel, endogenous cartel with respect to hydro generation, and equilibrium. One of the important aspects of the model is the medium-term load matching, where load is represented by the load duration curves of each period into which the medium-term horizon is subdivided. The probabilistically exact Bloom and Gallant formulation of load matching could be employed, but its exponential number of load-matching inequality constraints limits its use to small cases. A heuristic has been proposed where only a small subset of the load-matching constraints, hopefully containing the active ones at the optimizer, is employed. This heuristic does not perform totally well with the more complex models, as those of endogenous cartel and equilibrium behavior, and a more comprehensive heuristic procedure is presented here. A well-known alternative method of medium-term load matching is the multi-block approximation to the load duration curve. The three load-matching techniques mentioned above are here described and are compared, and computational results for realistic systems are analyzed for the different behavioral models, showing the advantages of each procedure. © 2012 IEEE.


Beltran H.,Jaume I University | Perez E.,Jaume I University | Aparicio N.,Jaume I University | Rodriguez P.,Polytechnic University of Catalonia
IEEE Transactions on Sustainable Energy | Year: 2013

This paper proposes an optimized energy management strategy (EMS) for photovoltaic (PV) power plants with energy storage (ES) based on the estimation of the daily solar energy production. This EMS produces a constant-by-hours power reference which mitigates the stochastic nature of PV production typically associated to the solar resource, and enables PV power plants to take part in the day and intraday electricity markets. The possibility of using the intraday market sessions to refine the plant's power reference paves the way to minimizing the energy capacity ratings of the ES system required to operate the PV power plant without incurring excessive production deviations. This proposal is analyzed on an annual basis using actual irradiance data and theoretical irradiance models extracted from official databases. © 2010-2012 IEEE.


Serra T.,Polytechnic University of Catalonia
Food Policy | Year: 2011

Recent incidents of contaminated food products coupled with the widespread diffusion of news by mass media and the growing social concerns about food safety, have resulted in significant food market crises. One of the most highly publicized recent food scares involved Bovine Spongiform Encephalopathy (BSE). In our analysis, we evaluate the impacts from a BSE outbreak on the price volatility transmission along the Spanish food marketing chain by using a smooth transition conditional correlation (STCC) GARCH model. Our work is the first to assess price volatility responses to food scares. Results suggest that two distinct regimes involving different price volatility behavior can be distinguished, one characterized by turbulent markets and another where markets are calming down. © 2010 Elsevier Ltd.


Serra T.,Polytechnic University of Catalonia | Zilberman D.,University of California at Berkeley
Energy Economics | Year: 2013

In this article, an extensive review of the rapidly growing biofuel-related time-series literature is carried out. The data used, the modeling techniques and the main findings of this literature are discussed. Providing a review of this flourishing research area is relevant as a guidepost for future research. This literature concludes that energy prices drive long-run agricultural price levels and that instability in energy markets is transferred to food markets. © 2013 Elsevier B.V.


Rius J.,Polytechnic University of Catalonia
IEEE Transactions on Very Large Scale Integration (VLSI) Systems | Year: 2013

A compact IR-drop model for on-chip power distribution networks in array and wire-bonded ICs is analyzed. Chip dimensions, size, and location of the supply pads, metal coverage, piecewise distribution of IC consumption, and the resistance between the pads and the power supply are considered to obtain closed-form expressions for the IR-drop. The IR-drop model is validated by comparing its results with electrical simulations. The obtained error is in the range of 1%. © 1993-2012 IEEE.


Cerda-Alabern L.,Polytechnic University of Catalonia
International Conference on Wireless and Mobile Computing, Networking and Communications | Year: 2012

In this paper it is analyzed the topology of Guifi.net Wireless Community Network (WCN). WCN have emerged in recent years organized and deployed by the cooperation of its own customers. WCN keep parallelisms with the construction of the Internet, and seems logical to look for common characteristics among them. More specifically, it is investigated whether there are topology patterns of Guifi.net, and to what extend they share the power-law properties that have been found to obey many topology parameters of the Internet. The paper also proposes a topology generator algorithm that it is able to reproduce specific graph properties that have been obtained for Guifi.net. © 2012 IEEE.


Cheng C.,Michigan State University | Yaroshchuk A.,Polytechnic University of Catalonia | Bruening M.L.,Michigan State University
Langmuir | Year: 2013

Membranes composed of multilayer poly(4-styrenesulfonate) (PSS)/protonated poly(allylamine) (PAH) films on porous alumina supports exhibit high monovalent/divalent cation selectivities. Remarkably, the diffusion dialysis K+/Mg2+ selectivity is >350. However, in nanofiltration this selectivity is only 16, suggesting some convective ion transport through film imperfections. Under MgCl2 concentration gradients across either (PSS/PAH)4- or (PSS/PAH)4PSS-coated alumina, transmembrane potentials indicate Mg2+ transference numbers approaching 0. The low Mg2+ transference numbers with both polycation- and polyanion-terminated films likely stem from exclusion of Mg 2+ due to its large size or hydration energy. However, these high anion/cation selectivities decrease as the solution ionic strength increases. In nanofiltration, the high asymmetry of membrane permeabilities to Mg 2+ and Cl- creates transmembrane diffusion potentials that lead to negative rejections (the ion concentration in the permeate is larger than in the feed) as low as -200% for trace monovalent cations such as K + and Cs+. Moreover, rejection becomes more negative as the mobility of the trace cation increases. Knowledge of single-ion permeabilities is vital for predicting the performance of polyelectrolyte films in the separation and purification of mixed salts. © 2013 American Chemical Society.


Quintanilla R.,Polytechnic University of Catalonia
Journal of Thermal Stresses | Year: 2010

Green and Naghdi proposed three theories of thermoelasticity labelled as type I, II and III, respectively. Here we investigate the spatial behavior of solutions of a problem determined by the type III thermoelasticity. We study the growth/decay alternative in the spatial variable for the linear theory in the case that the thermoelastic deformations at the instant T are constrained to be proportional to the ones at the instant zero. In the case that the solutions decay, an upper bound for the amplitude term by means of the data of the problem can be obtained. We sketch how to obtain it. We finish by noting (through examples) that for several families of cases the solutions can be periodic in the spatial variable and the Phragmen-Lindelof alternative is not satisfied. © 2010 Taylor & Francis.


Bancal J.-D.,Ecole Polytechnique Federale de Lausanne | Pastor-Satorras R.,Polytechnic University of Catalonia
European Physical Journal B | Year: 2010

Many sociological networks, as well as biological and technological ones, can be represented in terms of complex networks with a heterogeneous connectivity pattern. Dynamical processes taking place on top of them can be very much influenced by this topological fact. In this paper we consider a paradigmatic model of non-equilibrium dynamics, namely the forest fire model, whose relevance lies in its capacity to represent several epidemic processes in a general parametrization. We study the behavior of this model in complex networks by developing the corresponding heterogeneous mean-field theory and solving it in its steady state. We provide exact and approximate expressions for homogeneous networks and several instances of heterogeneous networks. A comparison of our analytical results with extensive numerical simulations allows to draw the region of the parameter space in which heterogeneous mean-field theory provides an accurate description of the dynamics, and enlights the limits of validity of the mean-field theory in situations where dynamical correlations become important. © 2010 EDP Sciences, SIF, Springer-Verlag Berlin Heidelberg.


Perez-Foguet A.,Polytechnic University of Catalonia
International Journal of Climatology | Year: 2014

In this work, the wind patterns in high mountain areas with complex orography are characterized using hourly data provided by a network of weather stations. The key novelty of the study is the methodology. Data are grouped separately by wind speed and wind direction using two cluster analyses. The groups are analysed and described according to measurements at key stations in the network and their hourly presence. Both classifications are subsequently compared using contingency tables, and the main wind patterns are identified. The uncertainties associated with the average values of each wind pattern are quantified by principal component analysis of the wind vectors at each station. One year of data from nine stations located in the area of La Oroya, Peru was used to validate the proposed method. The local wind behaviour was characterized, the wind patterns were compared with respect to the seasons, and the winter morning transitions were analysed in detail. The methodology allows quantitative description of the local wind patterns and their temporal dynamics in complex mountain valleys. Both wind speed and direction were found to be relevant in wind pattern characterization. In particular, both parameters have proven helpful in identifying and quantifying the prevailing winds during cold dawns and thermal inversion periods. © 2013 Royal Meteorological Society.


Astrakharchik G.E.,Polytechnic University of Catalonia | Krutitsky K.V.,University of Duisburg - Essen
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2011

Exact calculation of the condensate fraction in multidimensional inhomogeneous interacting Bose systems in a confining potential of arbitrary shape is a difficult computational problem. We have developed an iterative procedure which allows us to calculate the condensate fraction as well as the corresponding eigenfunction of the one-body density matrix. We successfully validate this procedure in diffusion Monte Carlo simulations of a Bose gas in an optical lattice at zero temperature. We also discuss the relation between different criteria used for testing coherence in cold Bose systems, such as the fraction of particles that are superfluid, condensed, or in the zero-momentum state. © 2011 American Physical Society.


Yaroshchuk A.,Polytechnic University of Catalonia
Advances in Colloid and Interface Science | Year: 2012

The problem is considered theoretically of dynamics of current-induced concentration polarization of interfaces between ideally perm-selective and non-ideally perm-selective (leaky) ion-exchange media in binary electrolyte solutions under galvanostatic conditions and at negligible volume flow. In contrast to the previous studies, the analysis is systematically carried out in terms of local thermodynamic equilibrium in the approximation of local electric neutrality in virtual solution. For macroscopically homogeneous media, this enables one to obtain model-independent results in quadratures for the stationary state as well as an approximate scaling-form solution for the transient response to the step-wise increase in electric-current density. These results are formulated in terms of such phenomenological properties of the leaky medium as ion transport numbers, diffusion permeability to salt and specific chemical capacity. An easy-to-solve numerically 1D PDE is also formulated in the same terms. A systematic parametric study is carried out within the scope of fine-pore model of leaky medium in terms of such properties as volumetric concentration of fixed electric charges and diffusivities of ions of symmetrical electrolyte. While previous studies paid principal attention to the shape and propagation rate of the so-called deionization shocks, we also consider in detail the time evolution of voltage drop and interface salt concentration. Our analysis confirms the previously predicted pattern of propagating deionization shocks within the leaky medium but also reveals several novel features. In particular, we demonstrate that the deionization-shock pattern is really pronounced only at intermediate ratios of fixed-charge concentration to the initial salt concentration and at quite high steady-state voltages where the model used in this and previous studies is applicable only at relatively early stages of concentration-polarization process. © 2012 Elsevier B.V.


Yaroshchuk A.,Polytechnic University of Catalonia
Journal of Membrane Science | Year: 2010

It is demonstrated theoretically that the diffusion flow of solute from concentrated solutions through composite/asymmetric (reverse osmosis) membranes can be considerably reduced due to the concomitant osmosis. Therefore, the diffusion permeability of such membranes is strongly dependent on the hydraulic conditions (zero trans-membrane hydrostatic pressure difference vs. zero trans-membrane volume flow). Besides that, under osmosis conditions, the trans-membrane diffusion flow of solute is demonstrated to be a strongly non-linear function of concentration in the source solution despite the fact that the diffusion permeabilities of constituent layers are assumed independent of solute concentration. The diffusion flow is also shown to noticeably depend on the membrane orientation with respect to the concentration gradient. © 2010 Elsevier B.V. All rights reserved.


Ball S.,Polytechnic University of Catalonia | De Beule J.,Ghent University
Designs, Codes, and Cryptography | Year: 2012

This article contains a proof of the MDS conjecture for k ≤ 2p - 2. That is, that if S is a set of vectors of F q ^k in which every subset of S of size k is a basis, where q = p h, p is prime and q is not and k ≤ 2p - 2, then |S| ≤ q + 1. It also contains a short proof of the same fact for k ≤ p, for all q. © 2012 Springer Science+Business Media, LLC.


Morales-Narvaez E.,Biosensors | Morales-Narvaez E.,Polytechnic University of Catalonia | Merkoci A.,Biosensors | Merkoci A.,Catalan Institution for Research and Advanced Studies
Advanced Materials | Year: 2012

Since graphene exhibits innovative mechanical, electrical, thermal, and optical properties, this 2D material is increasingly attracting attention and is under active research. Among the various graphene forms with lattice-like nanostructure, graphene oxide (GO) displays advantageous characteristics as a biosensing platform due to its excellent capabilities for direct wiring with biomolecules, a heterogeneous chemical and electronic structure, the possibility to be processed in solution and the ability to be tuned as insulator, semiconductor or semi-metal. Moreover, GO photoluminescences with energy transfer donor/acceptor molecules exposed in a planar surface and is even proposed as a universal highly efficient long-range quencher, which is opening the way to several unprecedented biosensing strategies. Here, the rationale behind the use of GO in optical biosensing applications is discussed by describing different potentially exploitable properties of GO, and an overview of the current approaches are presented along with future perspectives and challenges. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.


Guerrero J.M.,Polytechnic University of Catalonia
IEEE International Symposium on Industrial Electronics | Year: 2010

• Droop-controlled microgrids can be used in islanded mode. • New concept of flexible microgrids can operate in both grid-connected and islanded mode. • Improvements to the conventional droop method are required for integrate inverter-based energy resources: • Improvement of the transient response • Virtual impedance: harmonic power sharing and hot-swapping • Adaptive droop control laws • The hierarchical control is required for a AC microgrid, analogous as those proposed in the DC-coupled microgrid: • Primary control is based on the droop method allowing the connection of different AC sources without any intercommunication. • Secondary control avoids the voltage and frequency deviation produced by the primary control. Only low bandwidth communications are needed to perform this control level. A synchronization loop can be add in this level to transfer from islanding to grid connected modes. • Tertiary control allows to import/export active and reactive power to the grid. • Additional features are also required to the flexible microgrids: • Voltage ride-through • Black-start operation • Grid impedance estimation • Storage energy management and control. © 2010 IEEE.


Odor G.,Hungarian Academy of Sciences | Pastor-Satorras R.,Polytechnic University of Catalonia