Northwestern Institute on Complex Systems

Northwestern, United States

Northwestern Institute on Complex Systems

Northwestern, United States
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Hay J.B.,University of Canterbury | Pierrehumbert J.B.,University of Canterbury | Pierrehumbert J.B.,Northwestern University | Pierrehumbert J.B.,Northwestern Institute on Complex Systems | And 2 more authors.
Cognition | Year: 2015

Contemporary New Zealand English has distinctive pronunciations of three characteristic vowels. Did the evolution of these distinctive pronunciations occur in all words at the same time or were different words affected differently? We analyze the changing pronunciation of New Zealand English in a large set of recordings of speakers born over a 130. year period. We show that low frequency words were at the forefront of these changes and higher frequency words lagged behind. A long-standing debate exists between authors claiming that high frequency words lead regular sound change and others claiming that there are no frequency effects. The leading role of low frequency words is surprising in this context. It can be elucidated in models of lexical processing that include detailed word-specific memories. © 2015 The Authors.

Panaggio M.J.,Northwestern University | Panaggio M.J.,Rose - Hulman Institute of Technology | Abrams D.M.,Northwestern University | Abrams D.M.,Northwestern Institute on Complex Systems
Nonlinearity | Year: 2015

A chimera state is a spatio-temporal pattern in a network of identical coupled oscillators in which synchronous and asynchronous oscillation coexist. This state of broken symmetry, which usually coexists with a stable spatially symmetric state, has intrigued the nonlinear dynamics community since its discovery in the early 2000s. Recent experiments have led to increasing interest in the origin and dynamics of these states. Here we review the history of research on chimera states and highlight major advances in understanding their behaviour. © 2015 IOP Publishing Ltd & London Mathematical Society.

Lemey P.,Catholic University of Leuven | Rambaut A.,University of Edinburgh | Rambaut A.,U.S. National Institutes of Health | Bedford T.,University of Edinburgh | And 13 more authors.
PLoS Pathogens | Year: 2014

Information on global human movement patterns is central to spatial epidemiological models used to predict the behavior of influenza and other infectious diseases. Yet it remains difficult to test which modes of dispersal drive pathogen spread at various geographic scales using standard epidemiological data alone. Evolutionary analyses of pathogen genome sequences increasingly provide insights into the spatial dynamics of influenza viruses, but to date they have largely neglected the wealth of information on human mobility, mainly because no statistical framework exists within which viral gene sequences and empirical data on host movement can be combined. Here, we address this problem by applying a phylogeographic approach to elucidate the global spread of human influenza subtype H3N2 and assess its ability to predict the spatial spread of human influenza A viruses worldwide. Using a framework that estimates the migration history of human influenza while simultaneously testing and quantifying a range of potential predictive variables of spatial spread, we show that the global dynamics of influenza H3N2 are driven by air passenger flows, whereas at more local scales spread is also determined by processes that correlate with geographic distance. Our analyses further confirm a central role for mainland China and Southeast Asia in maintaining a source population for global influenza diversity. By comparing model output with the known pandemic expansion of H1N1 during 2009, we demonstrate that predictions of influenza spatial spread are most accurate when data on human mobility and viral evolution are integrated. In conclusion, the global dynamics of influenza viruses are best explained by combining human mobility data with the spatial information inherent in sampled viral genomes. The integrated approach introduced here offers great potential for epidemiological surveillance through phylogeographic reconstructions and for improving predictive models of disease control. © 2014 Lemey et al.

Umbanhowar P.,Northwestern University | Vose T.H.,Northwestern University | Mitani A.,Sapporo City University | Hirai S.,Ritsumeikan University | And 2 more authors.
Proceedings - IEEE International Conference on Robotics and Automation | Year: 2012

This paper explores the role of anisotropic friction properties in vibratory parts manipulation. We show that direction-dependent surface friction properties can be used in conjunction with a vibrating plate to help design friction-induced velocity fields on the surface of the plate. Theoretical, simulation, and experimental results are presented quantifying the anisotropic friction effects of textured surfaces such as micromachined silicon and fabrics. © 2012 IEEE.

Ryu J.-C.,Northwestern University | Ruggiero F.,University of Naples Federico II | Lynch K.M.,Northwestern University | Lynch K.M.,Northwestern Institute on Complex Systems
Proceedings - IEEE International Conference on Robotics and Automation | Year: 2012

This paper presents stabilization control of a rolling manipulation system called the disk-on-disk. The system consists of two disks in which the upper disk (object) is free to roll on the lower disk (hand) under the influence of gravity. The goal is to stabilize the object at the unstable upright position directly above the hand. We use backstepping to derive a control law yielding global asymptotic stability. We present simulation as well as experimental results demonstrating the controller. © 2012 IEEE.

Sales-Pardo M.,Rovira i Virgili University | Sales-Pardo M.,Northwestern University | Diermeier D.,Northwestern University | Diermeier D.,Northwestern Institute on Complex Systems | And 4 more authors.
PLoS ONE | Year: 2013

Social groups of interacting agents display an ability to coordinate in the absence of a central authority, a phenomenon that has been recently amplified by the widespread availability of social networking technologies. Models of opinion formation in a population of agents have proven a very useful tool to investigate these phenomena that arise independently of the heterogeneities across individuals and can be used to identify the factors that determine whether widespread consensus on an initial small majority is reached. Recently, we introduced a model in which individual agents can have conservative and partisan biases. Numerical simulations for finite populations showed that while the inclusion of conservative agents in a population enhances the population's efficiency in reaching consensus on the initial majority opinion, even a small fraction of partisans leads the population to converge on the opinion initially held by a minority. To further understand the mechanisms leading to our previous numerical results, we investigate analytically the noise driven transition from a regime in which the population reaches a majority consensus (efficient), to a regime in which the population settles in deadlock (non-efficient). We show that the mean-field solution captures what we observe in model simulations. Populations of agents with no opinion bias show a continuous transition to a deadlock regime, while populations with an opinion bias, show a discontinuous transition between efficient and partisan regimes. Furthermore, the analytical solution reveals that populations with an increasing fraction of conservative agents are more robust against noise than a population of naive agents because in the efficient regime there are relatively more conservative than naive agents holding the majority opinion. In contrast, populations with partisan agents are less robust to noise with an increasing fraction of partisans, because in the efficient regime there are relatively more naive agents than partisan agents holding the majority opinion. © 2013 Sales-Pardo et al.

Thiemann C.,Northwestern University | Thiemann C.,Max Planck Institute for Dynamics and Self-Organization | Theis F.,Helmholtz Center Munich | Theis F.,TU Munich | And 5 more authors.
PLoS ONE | Year: 2010

Territorial subdivisions and geographic borders are essential for understanding phenomena in sociology, political science, history, and economics. They influence the interregional flow of information and cross-border trade and affect the diffusion of innovation and technology. However, it is unclear if existing administrative subdivisions that typically evolved decades ago still reflect the most plausible organizational structure of today. The complexity of modern human communication, the ease of long-distance movement, and increased interaction across political borders complicate the operational definition and assessment of geographic borders that optimally reflect the multi-scale nature of today's human connectivity patterns. What border structures emerge directly from the interplay of scales in human interactions is an open question. Based on a massive proxy dataset, we analyze a multi-scale human mobility network and compute effective geographic borders inherent to human mobility patterns in the United States. We propose two computational techniques for extracting these borders and for quantifying their strength. We find that effective borders only partially overlap with existing administrative borders, and show that some of the strongest mobility borders exist in unexpected regions. We show that the observed structures cannot be generated by gravity models for human traffic. Finally, we introduce the concept of link significance that clarifies the observed structure of effective borders. Our approach represents a novel type of quantitative, comparative analysis framework for spatially embedded multi-scale interaction networks in general and may yield important insight into a multitude of spatiotemporal phenomena generated by human activity. © 2010 Thiemann et al.

Vose T.H.,Northwestern University | Umbanhowar P.,Northwestern University | Lynch K.M.,Northwestern University | Lynch K.M.,Northwestern Institute on Complex Systems
IEEE International Conference on Automation Science and Engineering | Year: 2012

We examine the dynamics of point parts in frictional contact with a periodically vibrating, flat, rigid plate that is nominally tilted with respect to horizontal. If the friction law satisfies the maximum power inequality, then part dynamics on the tilted plate are equivalent to part dynamics on a horizontal plate with a different friction law that also satisfies the maximum power inequality. For small angles of tilt, this equivalence means that every periodic plate motion induces a position-dependent velocity field through which parts slide. Assuming Coulomb friction, some of the fields obtainable with a tilted plate are impossible to generate with a horizontal plate; other fields that require complicated motions on a horizontal plate can be generated with simpler motions on a tilted plate. © 2012 IEEE.

Vose T.H.,Northwestern University | Umbanhowar P.,Northwestern University | Lynch K.M.,Northwestern University | Lynch K.M.,Northwestern Institute on Complex Systems
Robotics: Science and Systems | Year: 2012

We model the full dynamics of a rigid part in three-point frictional sliding contact with a flat rigid 6-degree-offreedom plate. Given a periodic plate motion and the geometric, inertial, and frictional properties of the part, we define an asymptotic twist field mapping each part configuration to a unique part twist (linear and angular velocity). Asymptotic twist vectors in the field approximate the part's cycle-averaged twist at each configuration and are independent of time or the system's initial state. Simulations and experiments show that the trajectory of the part's configuration as it slides on the plate is well described by the field. With the ability to program arbitrary plate motions, part manipulation reduces to finding plate motions that generate asymptotic twist fields to accomplish desired tasks. Several simple fields useful for manipulation tasks (e.g., sensorless part alignment) are verified in simulation and experiment. For the special case of a rigid part with infinitesimal thickness, we show that the part's cycle-averaged twist for any configuration asymptotically converges to a unique asymptotic twist vector.

Christov I.C.,Northwestern University | Lueptow R.M.,Northwestern University | Ottino J.M.,Northwestern Institute on Complex Systems
American Journal of Physics | Year: 2011

We compare and contrast two types of deformations inspired by mixing applications-one from the mixing of fluids (stretching and folding) and the other from the mixing of granular matter (cutting and shuffling). The connection between mechanics and dynamical systems is discussed in the context of the kinematics of deformation, emphasizing the equivalence between stretches and Lyapunov exponents. The stretching and folding motion exemplified by the baker's map is shown to give rise to a dynamical system with a positive Lyapunov exponent, the hallmark of chaotic mixing. In contrast, cutting and shuffling does not stretch. When an interval exchange transformation is used as the basis for cutting and shuffling, we establish that all of the map's Lyapunov exponents are zero. Mixing, as quantified by the interfacial area per unit volume, is shown to be exponential when there is stretching and folding but linear when there is only cutting and shuffling. We also discuss how a simple computational approach can discern stretching in discrete data. © 2011 American Association of Physics Teachers.

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