284 results found
Degond PAA, Deluzet F, Doyen D, 2016, Asymptotic-Preserving Particle-In-Cell methods for the Vlasov-Maxwell system in the quasi-neutral limit, Journal of Computational Physics, Vol: 330, Pages: 467-492, ISSN: 0021-9991
In this article, we design Asymptotic-Preserving Particle-In-Cell methods for the Vlasov-Maxwell system inthe quasi-neutral limit, this limit being characterized by a Debye length negligible compared to the space scaleof the problem. These methods are consistent discretizations of the Vlasov-Maxwell system which, in thequasi-neutral limit, remain stable and are consistent with a quasi-neutral model (in this quasi-neutral model,the electric eld is computed by means of a generalized Ohm law). The derivation of Asymptotic-Preservingmethods is not straightforward since the quasi-neutral model is a singular limit of the Vlasov-Maxwell model.The key step is a reformulation of the Vlasov-Maxwell system which uni es the two models in a single setof equations with a smooth transition from one to another. As demonstrated in various and demandingnumerical simulations, the Asymptotic-Preserving methods are able to treat efficiently both quasi-neutralplasmas and non-neutral plasmas, making them particularly well suited for complex problems involving denseplasmas with localized non-neutral regions.
Degond PAA, Henkes S, Yu H, 2016, Self-organized Hydrodynamics with density-dependent velocity, Kinetic and Related Models, Vol: 10, Pages: 193-213, ISSN: 1937-5093
Motivated by recent experimental and computational results thatshow a motility-induced clustering transition in self-propelled particle systems,we study an individual model and its corresponding Self-Organized Hydrodynamicmodel for collective behaviour that incorporates a density-dependentvelocity, as well as inter-particle alignment. The modal analysis of the hydrodynamicmodel elucidates the relationship between the stability of the equilibriaand the changing velocity, and the formation of clusters. We find, in agreementwith earlier results for non-aligning particles, that the key criterion for stabilityis (ρv(ρ))0 ≥ 0, i.e. a nondecreasing mass flux ρv(ρ) with respect to the density.Numerical simulation for both the individual and hydrodynamic modelswith a velocity function inspired by experiment demonstrates the validity ofthe theoretical results.
Degond PAA, Filbet F, 2016, On the asymptotic limit of the three dimensional Vlasov-Poisson system for large magnetic field : formal derivation., Journal of Statistical Physics, Vol: 165, Pages: 765-784, ISSN: 0022-4715
In this paper we establish the asymptotic limit of the three dimensional Vlasov–Poissonequation with strong external magnetic field. The guiding center approximation is investigated inthe three dimensional case with a non-constant magnetic field. In the long time asymptotic limit,the motion can be split in two parts : one stationary flow along the lines of the magnetic fieldand the guiding center motion in the orthogonal plane of the magnetic field where classical driftvelocities and invariants (magnetic moment) are recovered.
Creppy A, Plouraboué F, Praud O, et al., 2016, Symmetry-breaking phase-transitions in highly concentrated semen, Journal of the Royal Society Interface, Vol: 13, ISSN: 1742-5689
New experimental evidence of self-motion of aconfined active suspension is presented. Depositingfresh semen sample in an annular shaped micro-fluidic chip leads to a spontaneous vortex state ofthe fluid at sufficiently large sperm concentration.The rotation occurs unpredictably clockwise orcounterclockwise and is robust and stable. Furthermore,for highly active and concentrated semen, richerdynamics can occur such as self-sustained or dampedrotation oscillations. Experimental results obtainedwith systematic dilution provide a clear evidence of aphase transition toward collective motion associatedwith local alignment of spermatozoa akin to theVicsek model. A macroscopic theory based onpreviously derived Self-Organized Hydrodynamics(SOH) models is adapted to this context and providespredictions consistent with the observed stationarymotion.
Degond P, Liu J-G, Pego RL, 2016, Coagulation-fragmentation model for animal group-size statistics, Journal of Nonlinear Science, Vol: 27, Pages: 379-424, ISSN: 1432-1467
We study coagulation-fragmentation equations inspired by a simple modelproposed in fisheries science to explain data for the size distribution ofschools of pelagic fish. Although the equations lack detailed balance and admitno $H$-theorem, we are able to develop a rather complete description ofequilibrium profiles and large-time behavior, based on recent developments incomplex function theory for Bernstein and Pick functions. In thelarge-population continuum limit, a scaling-invariant regime is reached inwhich all equilibria are determined by a single scaling profile. This universalprofile exhibits power-law behavior crossing over from exponent $-\frac23$ forsmall size to $-\frac32$ for large size, with an exponential cut-off.
Barbaro ABT, Canizo JA, Carrillo de la plata J, et al., 2016, Phase Transitions in a Kinetic Flocking Model of Cucker-Smale Type, Multiscale Modeling & Simulation, Vol: 14, Pages: 1063-1088, ISSN: 1540-3467
We consider a collective behavior model in which individuals try to imitate each others' velocity and have a preferred speed. We show that a phase change phenomenon takes place as diffusion decreases, bringing the system from a “disordered” to an “ordered” state. This effect is related to recently noticed phenomena for the diffusive Vicsek model. We also carry out numerical simulations of the system and give further details on the phase transition.
Blanchet A, Degond PAA, 2016, Topological interactions in a Boltzmann-type framework, Journal of Statistical Physics, Vol: 163, Pages: 41-60, ISSN: 1572-9613
We consider a finite number of particles characterised by their positionsand velocities. At random times a randomly chosen particle, the follower,adopts the velocity of another particle, the leader. The follower choosesits leader according to the proximity rank of the latter with respect to theformer. We study the limit of a system size going to infinity and, under theassumption of propagation of chaos, show that the limit equation is akinto the Boltzmann equation. However, it exhibits a spatial non-localityinstead of the classical non-locality in velocity space. This result relies onthe approximation properties of Bernstein polynomials. We illustrate thedynamics with numerical simulations.
Degond P, Delebecque F, Peurichard D, 2015, Continuum model for linked fibers with alignment interactions, Mathematical Models and Methods in Applied Sciences, Vol: 26, ISSN: 0218-2025
We introduce an individual-based model for fiber elements having the abilityto cross-link or unlink each other and to align with each other at the cross links.We first formally derive a kinetic model for the fiber and cross-links distributionfunctions. We then consider the fast linking/unlinking regime in which the modelcan be reduced to the fiber distribution function only and investigate its diffusionlimit. The resulting macroscopic model consists of a system of nonlinear diffusionequations for the fiber density and mean orientation. In the case of a homogeneousfiber density, we show that the model is elliptic.
Muljadi B, Narski J, Lozinski A, et al., 2015, Non-conforming multiscale finite element method for stokes flows in heterogeneous media. Part I: Methodologies and numerical experiments, Multiscale Modeling & Simulation, Vol: 13, Pages: 1146-1172, ISSN: 1540-3467
The Multiscale Finite Element Method (MsFEM) is developed in the vein of Crouzeix-Raviart element for solving viscous incompressible ows in genuine heterogeneous media. Such owsare relevant in many branches of engineering, often at multiple scales and at regions where analyticalrepresentations of the microscopic features of the ows are often unavailable. Full accounts to theseproblems heavily depend on the geometry of the system under consideration and are computationallyexpensive. Therefore, a method capable of solving multiscale features of the ow without con ningitself to ne scale calculations is sought after.The approximation of boundary condition on coarse element edges when computing the multiscalebasis functions critically inuences the eventual accuracy of any MsFEM approaches. The weaklyenforced continuity of Crouzeix - Raviart function space across element edges leads to a naturalboundary condition for the multiscale basis functions which relaxes the sensitivity of our method tocomplex patterns of obstacles exempt from the needs of implementing any oversampling techniques.Additionally, the application of penalization method makes it possible to avoid complex unstructureddomain and allows extensive use of simpler Cartesian meshes.Key words. Crouzeix-Raviart Element, Multiscale Finite Element Method, Stokes Equations,Penalization Method
Degond P, Fibich G, Piccoli B, et al., 2015, SPECIAL ISSUE ON MODELING AND CONTROL IN SOCIAL DYNAMICS, NETWORKS AND HETEROGENEOUS MEDIA, Vol: 10, Pages: I-II, ISSN: 1556-1801
Degond P, Navoret L, 2015, A multi-layer model for self-propelled disks interacting through alignment and volume exclusion, Mathematical Models and Methods in Applied Science, Vol: 25, Pages: 2439-2475, ISSN: 0218-2025
We present an individual-based model describing disk-like self-propelled particles movinginside parallel planes. The disk directions of motion follow alignment rules insideeach layer. Additionally, the disks are subject to interactions with those of the neighboringlayers arising from volume exclusion constraints. These interactions affect the diskinclinations with respect to the plane of motion. We formally derive a macroscopic modelcomposed of planar self-organized hydrodynamic (SOH) models describing the transportof mass and evolution of mean direction of motion of the disks in each plane, supplementedwith transport equations for the mean disk inclination. These planar modelsare coupled due to the interactions with the neighboring planes. Numerical comparisonsbetween the individual-based and macroscopic models are carried out. These modelscould be applicable, for instance, to describe sperm-cell collective dynamics.
David I, Kohnke P, Lagriffoul G, et al., 2015, Mass sperm motility is associated with fertility in sheep, Animal Reproduction Science, Vol: 161, Pages: 75-81, ISSN: 0378-4320
The study was to focus on the relationship between wave motion (mass sperm motility, measured by a mass sperm motility score, manually assessed by artificial insemination (AI) center operators) and fertility in male sheep. A dataset of 711,562 artificial inseminations performed in seven breeds by five French AI centers during the 2001 to 2005 time period was used for the analysis. Factors influencing the outcome of the insemination, which is a binary response observed at lambing of either success (1) or failure (0), were studied using a joint model within each breed and AI center (eight separate analyses). The joint model is a multivariate model where all information related to the female, the male and the insemination process were included to improve the estimation of the factor effects. Results were consistent for all analyses. The male factors affecting AI results were the age of the ram and the mass motility. After correction for the other factors of variation, the lambing rate increased quasi linearly from three to more than ten points with the mass sperm motility score depending on the breed and the AI center. The consistency of the relationship for all breeds indicated that mass sperm motility is predictive of the fertility resulting when sperm are used from a specific ejaculate. Nonetheless, predictability could be improved if an objective measurement of mass sperm motility were available as a substitute for the subjective scoring currently in use in AI centers.
Fehrenbach J, Narski J, Hua J, et al., 2015, Time-delayed follow-the-leader model for pedestrians walking in line, Networks and Heterogeneous Media, Vol: 10, Pages: 579-608, ISSN: 1556-181X
We use the results of a pedestrian tracking experiment to identify a follow-the-leader model for pedestrians walking-in-line. We demonstrate the existence of a time-delay between a subject's response and the predecessor's corresponding behavior. This time-delay induces an instability which can be damped out by a suitable relaxation. By comparisons with the experimental data, we show that the model reproduces well the emergence of large-scale structures such as congestions waves. The resulting model can be used either for modeling pedestrian queuing behavior or can be incorporated into bi-dimensional models of pedestrian traffic.
Carlen E, Carvalho MC, Degond P, et al., 2015, A Boltzmann model for rod alignment and schooling fish, Nonlinearity, Vol: 28, Pages: 1783-1803, ISSN: 1361-6544
We consider a Boltzmann model introduced by Bertin, Droz and Grégoire as a binary interaction model of the Vicsek alignment interaction. This model considers particles lying on the circle. Pairs of particles interact by trying to reach their mid-point (on the circle) up to some noise. We study the equilibria of this Boltzmann model and we rigorously show the existence of a pitchfork bifurcation when a parameter measuring the inverse of the noise intensity crosses a critical threshold. The analysis is carried over rigorously when there are only finitely many non-zero Fourier modes of the noise distribution. In this case, we can show that the critical exponent of the bifurcation is exactly 1/2. In the case of an infinite number of non-zero Fourier modes, a similar behavior can be formally obtained thanks to a method relying on integer partitions first proposed by Ben-Naïm and Krapivsky.
Degond P, Lozinski A, Muljadi BP, et al., 2015, Crouzeix-Raviart MsFEM with Bubble Functions for Diffusion and Advection-Diffusion in Perforated Media, Communications in Computational Physics, Vol: 17, Pages: 887-907, ISSN: 1991-7120
The adaptation of Crouzeix-Raviart finite element in the context of multi-scale finite element method (MsFEM) is studied and implemented on diffusion and advection-diffusion problems in perforated media. It is known that the approximation of boundary condition on coarse element edges when computing the multiscale basis functions critically influences the eventual accuracy of any MsFEM approaches. The weakly enforced continuity of Crouzeix-Raviart function space across element edges leads to a natural boundary condition for the multiscale basis functions which relaxes the sensitivity of our method to complex patterns of perforations. Another ingredient to our method is the application of bubble functions which is shown to be instrumental in maintaining high accuracy amid dense perforations. Additionally, the application of penalization method makes it possible to avoid complex unstructured domain and allows extensive use of simpler Cartesian meshes.
Degond P, Yu H, 2015, Self-organized hydrodynamics in an annular domain: Modal analysis and nonlinear effects, MATHEMATICAL MODELS & METHODS IN APPLIED SCIENCES, Vol: 25, Pages: 495-519, ISSN: 0218-2025
Degond P, Dimarco G, Thi BNM, et al., 2015, Macroscopic models of collective motion with repulsion, Communications in Mathematical Sciences, Vol: 13, Pages: 1615-1638, ISSN: 1945-0796
Degond P, Liu J-G, Ringhofer C, 2014, Evolution of wealth in a non-conservative economy driven by local Nash equilibria, PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES, Vol: 372, ISSN: 1364-503X
Degond P, Frouvelle A, Liu J-G, 2014, Phase Transitions, Hysteresis, and Hyperbolicity for Self-Organized Alignment Dynamics, Archive for Rational Mechanics and Analysis, Vol: 216, Pages: 63-115, ISSN: 1432-0673
We provide a complete and rigorous description of phase transitions for kineticmodels of self-propelled particles interacting through alignment. These modelsexhibit a competition between alignment and noise. Both the alignment frequencyand noise intensity depend on a measure of the local alignment. We show that, inthe spatially homogeneous case, the phase transition features (number and nature ofequilibria, stability, convergence rate, phase diagram, hysteresis) are totally encodedin how the ratio between the alignment and noise intensities depend on the localalignment. In the spatially inhomogeneous case, we derive the macroscopic modelsassociated to the stable equilibria and classify their hyperbolicity according to thesame function.
Degond P, Frouvelle A, Raoul G, 2014, Local Stability of Perfect Alignment for a Spatially Homogeneous Kinetic Model, JOURNAL OF STATISTICAL PHYSICS, Vol: 157, Pages: 84-112, ISSN: 0022-4715
Barbaro ABT, Degond P, 2014, PHASE TRANSITION AND DIFFUSION AMONG SOCIALLY INTERACTING SELF-PROPELLED AGENTS, DISCRETE AND CONTINUOUS DYNAMICAL SYSTEMS-SERIES B, Vol: 19, Pages: 1249-1278, ISSN: 1531-3492
Chae D, Degond P, Liu J-G, 2014, Well-posedness for Hall-magnetohydrodynamics, ANNALES DE L INSTITUT HENRI POINCARE-ANALYSE NON LINEAIRE, Vol: 31, Pages: 555-565, ISSN: 0294-1449
Degond P, Liu J-G, Ringhofer C, 2014, Large-Scale Dynamics of Mean-Field Games Driven by Local Nash Equilibria, JOURNAL OF NONLINEAR SCIENCE, Vol: 24, Pages: 93-115, ISSN: 0938-8974
Degond P, Dimarco G, Thi BNM, 2014, HYDRODYNAMICS OF THE KURAMOTO-VICSEK MODEL OF ROTATING SELF-PROPELLED PARTICLES, MATHEMATICAL MODELS & METHODS IN APPLIED SCIENCES, Vol: 24, ISSN: 0218-2025
Degond P, Liu J-G, Ringhofer C, 2014, Evolution of the Distribution of Wealth in an Economic Environment Driven by Local Nash Equilibria, JOURNAL OF STATISTICAL PHYSICS, Vol: 154, Pages: 751-780, ISSN: 0022-4715
Degond P, Frouvelle A, Liu J-G, 2014, A NOTE ON PHASE TRANSITIONS FOR THE SMOLUCHOWSKI EQUATION WITH DIPOLAR POTENTIAL, HYPERBOLIC PROBLEMS: THEORY, NUMERICS, APPLICATIONS, Vol: 8, Pages: 179-192
Appert-Rolland C, Cividini J, Hilhorst HJ, et al., 2014, Pedestrian flows: from individuals to crowds, Conference on Pedestrian and Evacuation Dynamics (PED), Publisher: ELSEVIER SCIENCE BV, Pages: 468-476, ISSN: 2352-1465
Cordier F, Degond P, Kumbaro A, 2014, Phase Appearance or Disappearance in Two-Phase Flows, JOURNAL OF SCIENTIFIC COMPUTING, Vol: 58, Pages: 115-148, ISSN: 0885-7474
Degond P, Herty M, Liu J-G, 2014, FLOW ON SWEEPING NETWORKS, MULTISCALE MODELING & SIMULATION, Vol: 12, Pages: 538-565, ISSN: 1540-3459
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