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Perinet N.,University of Ontario Institute of Technology | Juric D.,CNRS Computer Science Laboratory for Mechanics and Engineering Sciences | Tuckerman L.S.,University Pierre and Marie Curie
Physical Review Letters | Year: 2012

A direct numerical simulation of Faraday waves is carried out in a minimal hexagonal domain. Over long times, we observe the alternation of patterns we call quasihexagons and beaded stripes. The symmetries and spatial Fourier spectra of these patterns are analyzed. © 2012 American Physical Society.

Duguet Y.,CNRS Computer Science Laboratory for Mechanics and Engineering Sciences | Schlatter P.,KTH Royal Institute of Technology
Physical Review Letters | Year: 2013

Localized structures such as turbulent stripes and turbulent spots are typical features of transitional wall-bounded flows in the subcritical regime. Based on an assumption for scale separation between large and small scales, we show analytically that the corresponding laminar-turbulent interfaces are always oblique with respect to the mean direction of the flow. In the case of plane Couette flow, the mismatch between the streamwise flow rates near the boundaries of the turbulence patch generates a large-scale flow with a nonzero spanwise component. Advection of the small-scale turbulent fluctuations (streaks) by the corresponding large-scale flow distorts the shape of the turbulence patch and is responsible for its oblique growth. This mechanism can be easily extended to other subcritical flows such as plane Poiseuille flow or Taylor-Couette flow. © 2013 American Physical Society.

Duguet Y.,CNRS Computer Science Laboratory for Mechanics and Engineering Sciences | Monokrousos A.,KTH Royal Institute of Technology | Brandt L.,KTH Royal Institute of Technology | Henningson D.S.,KTH Royal Institute of Technology
Physics of Fluids | Year: 2013

Subcritical transition to turbulence requires finite-amplitude perturbations. Using a nonlinear optimisation technique in a periodic computational domain, we identify the perturbations of plane Couette flow transitioning with least initial kinetic energy for Re le; 3000. We suggest a new scaling law Ec = O(Re-2.7) for the energy threshold vs. the Reynolds number, in quantitative agreement with experimental estimates for pipe flow. The route to turbulence associated with such spatially localised perturbations is analysed in detail for Re = 1500. Several known mechanisms are found to occur one after the other: Orr mechanism, oblique wave interaction, lift-up, streak bending, streak breakdown, and spanwise spreading. The phenomenon of streak breakdown is analysed in terms of leading finite-time Lyapunov exponents of the associated edge trajectory. © 2013 AIP Publishing LLC.

Duguet Y.,KTH Royal Institute of Technology | Duguet Y.,CNRS Computer Science Laboratory for Mechanics and Engineering Sciences | Schlatter P.,KTH Royal Institute of Technology | Henningson D.S.,KTH Royal Institute of Technology
Journal of Fluid Mechanics | Year: 2010

The formation of turbulent patterns in plane Couette flow is investigated near the onset of transition, using numerical simulation in a very large domain of size 800 h 2 h 356 h. Based on a maximum observation time of 20 000 inertial units, the threshold for the appearance of sustained turbulent motion is Rec = 324 1. For Rec < Re 380, turbulent-banded patterns form, irrespective of whether the initial perturbation is a noise or localized disturbance. Measurements of the turbulent fraction versus Re show evidence for a discontinuous phase transition scenario where turbulent spots play the role of the nuclei. Using a smaller computational box, the angle selection of the turbulent bands in the early stages of their development is shown to be related to the amplitude of the initial perturbation. Copyright © Cambridge University Press 2010.

Duguet Y.,CNRS Computer Science Laboratory for Mechanics and Engineering Sciences | Schlatter P.,KTH Royal Institute of Technology | Henningson D.S.,KTH Royal Institute of Technology | Eckhardt B.,University of Marburg | Eckhardt B.,Technical University of Delft
Physical Review Letters | Year: 2012

When a boundary layer starts to develop spatially over a flat plate, only disturbances of sufficiently large amplitude survive and trigger turbulence subcritically. Direct numerical simulation of the Blasius boundary-layer flow is carried out to track the dynamics in the region of phase space separating transitional from relaminarizing trajectories. In this intermediate regime, the corresponding disturbance is fully localized and spreads slowly in space. This structure is dominated by a robust pair of low-speed streaks, whose convective instabilities spawn hairpin vortices evolving downstream into transient disturbances. A quasicyclic mechanism for the generation of offspring is unfolded using dynamical rescaling with the local boundary-layer thickness. © 2012 American Physical Society.

Raji A.,Sultan Moulay Slimane University | Hasnaoui M.,Cadi Ayyad University | Firdaouss M.,CNRS Computer Science Laboratory for Mechanics and Engineering Sciences | Firdaouss M.,University Pierre and Marie Curie | Ouardi C.,Sultan Moulay Slimane University
Numerical Heat Transfer; Part A: Applications | Year: 2013

The present article reports numerical results of natural convection within an air filled square cavity with its horizontal walls submitted to different heating models. The temperature of the bottom horizontal surface (hot temperature) is maintained constant, while that of the opposite surface (cold temperature) is varied sinusoidally with time. The remaining vertical walls are considered adiabatic. The parameters governing the problem are the amplitude (0 ≤ a ≤ 0.8) and the period (τ ≥ 0.001) of the variable temperature, the Rayleigh number (103 ≤ Ra ≤ 7 × 106), and the Prandtl number (Pr = 0.71). In constant cooling conditions (a = 0), up to three different solutions (monocellular flow MF, bicellular vertical flow BVF, and bicellular horizontal flow BHF) are obtained. Their existence ranges are delineated and, in the limits of the existence range of each solution, the transitions observed are identified and described. In the variable cooling conditions, the effect of the amplitude and the period of the exciting temperature on fluid flow and heat transfer is examined in the case of the MF, and BHF for specific values of Ra. Results are presented in terms of Ψ max (t), Ψ min (t), Nu(t) and streamlines, heatlines, and isotherms during the evolutions of selected flow cycles. In comparison with the constant heating conditions, it is found that the variable cooling temperature could lead to a drastic change in the flow structure and the corresponding heat transfer, especially at specific low periods of the cold variable temperature. This leads to a resonance phenomenon characterized by an important increase in heat transfer by about 46.1% compared to the case of a constant cold temperature boundary condition. © 2013 Taylor & Francis Group, LLC.

Yahiaoui S.,ESPCI ParisTech | Feuillebois F.,CNRS Computer Science Laboratory for Mechanics and Engineering Sciences
Journal of Fluid Mechanics | Year: 2010

The lift on a solid sphere moving along a wall in a parabolic shear flow is obtained as a regular perturbation problem for low Reynolds number when the sphere is in the inner region of expansion. Comprehensive results are given for the 10 terms of the lift, which involve the sphere translation and rotation, the linear and quadratic parts of the shear flow and all binary couplings. Based on very accurate earlier results of a creeping flow in bispherical coordinates, precise results for these lift terms are obtained for a large range of sphere-to-wall distances, including the lubrication region for sphere-to-wall gaps down to 0.01 of a sphere radius. Fitting formulae are also provided in view of applications. The migration velocity of an inertialess spherical particle is given explicitly, for a non-rotating sphere with a prescribed translation velocity and for a freely moving sphere in a parabolic shear flow. Values of the lift and migration velocity are in good agreement with earlier results whenever available. © 2010 Cambridge University Press.

Shin S.,Hongik University | Yoon I.,Hongik University | Juric D.,CNRS Computer Science Laboratory for Mechanics and Engineering Sciences
Journal of Computational Physics | Year: 2011

We present a new interface reconstruction technique, the Local Front Reconstruction Method (LFRM), for incompressible multiphase flows. This new method falls in the category of Front Tracking methods but it shares automatic topology handling characteristics of the previously proposed Level Contour Reconstruction Method (LCRM). The LFRM tracks the phase interface explicitly as in Front Tracking but there is no logical connectivity between interface elements thus greatly easing the algorithmic complexity. Topological changes such as interfacial merging or pinch off are dealt with automatically and naturally as in the Level Contour Reconstruction Method. Here the method is described for both two- and three-dimensional flow geometries. The interfacial reconstruction technique in the LFRM differs from that in the LCRM formulation by foregoing using an Eulerian distance field function. Instead, the LFRM uses information from the original interface elements directly to generate the new interface in a mass conservative way thus showing significantly improved local mass conservation. Because the reconstruction procedure is independently carried out in each individual reconstruction cell after an initial localization process, an adaptive reconstruction procedure can be easily implemented to increase the accuracy while at the same time significantly decreasing the computational time required to perform the reconstruction. Several benchmarking tests are performed to validate the improved accuracy and computational efficiency as compared to the LCRM. The results demonstrate superior performance of the LFRM in maintaining detailed interfacial shapes and good local mass conservation especially when using low-resolution Eulerian grids. © 2011 Elsevier Inc.

Grouin C.,CNRS Computer Science Laboratory for Mechanics and Engineering Sciences | Neveol A.,CNRS Computer Science Laboratory for Mechanics and Engineering Sciences
Journal of Biomedical Informatics | Year: 2014

Background: To facilitate research applying Natural Language Processing to clinical documents, tools and resources are needed for the automatic de-identification of Electronic Health Records. Objective: This study investigates methods for developing a high-quality reference corpus for the de-identification of clinical documents in French. Methods: A corpus comprising a variety of clinical document types covering several medical specialties was pre-processed with two automatic de-identification systems from the MEDINA suite of tools: a rule-based system and a system using Conditional Random Fields (CRF). The pre-annotated documents were revised by two human annotators trained to mark ten categories of Protected Health Information (PHI). The human annotators worked independently and were blind to the system that produced the pre-annotations they were revising. The best pre-annotation system was applied to another random selection of 100 documents. After revision by one annotator, this set was used to train a statistical de-identification system. Results: Two gold standard sets of 100 documents were created based on the consensus of two human revisions of the automatic pre-annotations. The annotation experiment showed that (i) automatic pre-annotation obtained with the rule-based system performed better (F= 0.813) than the CRF system (F= 0.519), (ii) the human annotators spent more time revising the pre-annotations obtained with the rule-based system (from 102 to 160. minutes for 50 documents), compared to the CRF system (from 93 to 142. minutes for 50 documents), (iii) the quality of human annotation is higher when pre-annotations are obtained with the rule-based system (F-measure ranging from 0.970 to 0.987), compared to the CRF system (F-measure ranging from 0.914 to 0.981).Finally, only 20 documents from the training set were needed for the statistical system to outperform the pre-annotation systems that were trained on corpora from a medical speciality and hospital different from those in the reference corpus developed herein. Conclusion: We find that better pre-annotations increase the quality of the reference corpus but require more revision time. A statistical de-identification method outperforms our rule-based system when as little as 20 custom training documents are available. © 2013 Elsevier Inc.

Feuillebois F.,CNRS Computer Science Laboratory for Mechanics and Engineering Sciences | Bazant M.Z.,Massachusetts Institute of Technology | Vinogradova O.I.,RWTH Aachen
Physical Review E - Statistical, Nonlinear, and Soft Matter Physics | Year: 2010

We provide some general theoretical results to guide the optimization of transverse hydrodynamic phenomena in superhydrophobic channels. Our focus is on the canonical micro- and nanofluidic geometry of a parallel-plate channel with an arbitrary two-component (low-slip and high-slip) coarse texture, varying on scales larger than the channel thickness. By analyzing rigorous bounds on the permeability, over all possible patterns, we optimize the area fractions, slip lengths, geometry, and orientation of the surface texture to maximize transverse flow. In the case of two aligned striped surfaces, very strong transverse flows are possible. Optimized superhydrophobic surfaces may find applications in passive microfluidic mixing and amplification of transverse electrokinetic phenomena. © 2010 The American Physical Society.

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