Emergence and selforganization in complex systemsComplex systems are characterized by the spontaneous formation of spatiotemporal structures as a result of simple local interactions between agents without leaders. Complex systems mostly appear in the biological and social contexts but can also be encountered in physics, chemistry, etc. The project aims at understanding the emergence of selforganization through the investigation of global, macroscopic models. ...Micromacro passage in models of complex systemsComplex systems are commonly investigated through microscopic, Agent (or Individual)Based Models (ABM or IBM), which predict the evolution of each agent in time. When macroscopic models are considered in the literature, they are usually based on phenomenological considerations.This project aims at rigorously establishing macroscopic models from their microscopic (AgentBased Model) counterpart. By doing so, macroscopic models gain in predictive character and may become an invaluable aid for experimental data analysis. ...
SelfOrganized HydrodynamicsThe 'SelfOrganizedHydrodynamics' (SOH) model describes selforganization phenomena at the macroscopic level. It shows some prominent differences from usual compressible hydrodynamic models. For instance, it is nonconservative and involves a constraint on the flow speed. The SOH model is one of the very first examples of nonconservative hyperbolic systems that can be derived from kinetic theory. Its derivation highlights one of the specific difficulties in complex systems, namely that elementary particle interactions may lack conservation properties. ... Symmetrybreaking phase transitionsIn kinetic models of selforganized dynamics inspired from the Vicsek model, the distribution of particle orientations exhibits symmetrybreaking phase transitions due to multiple 'Local Thermodynamical' equilibria (LTE)....
Propagation of chaosThe derivation of macroscopic models relies on the 'propagation of chaos', i.e. the fact that the particles become statistically independent when their number becomes large. In complex systems, selforganization may prevent propagation of chaos to arise. ... EmergenceEmergence refers to the ability of complex systems to spontaneously form largescale structures which are not directly encoded in the agents' interaction rules. In models, emergence results from various mechanisms. In this page are found examples of
Jamming transitionThe typical phenomenology is that of a traffic jam: in the jammed phase, the vehicle density is maximal, corresponding to the minimal allowed distance between two vehicles, while in the free flow phase, the vehicle density is below this threshold. Pedestrian crowds or mammal herds are twodimensional analogs of traffic jams. A fish school or a tumor may also, to some extent, be viewed as three dimensional analogs. These systems can be modeled on the macroscopic scale by fluid models with density constraints. ...
ApplicationsEconomics and social sciencesAnalogy between thermodynamic equilibria and Nash equilibria: We propose a framework for noncooperative nonatomic anonymous games with a continuum of players (also known as a MeanField Game) which exhibit strong scale separation.Fish schoolsExperimental measurements of fish trajectories (J. Gautrais et al, J. Math. Biol. (2009) 58: 429445) suggest that individual fish behaviour follows the 'Persistent Turning Walker' (PTW) process. The PTW model assumes that individual trajectories consist of pieces of circles with randomly varying radii, which are patched together (see picture below). ...
Models of network formationSelforganized dynamics often leads to the organization of agents into networks. An IndividualBased Model of network generation by selfpropelled particles has been developed. Each agent deposits directed pheromones and interacts with them through alignment interaction. ...
HerdsHerds of gregarious mammals offers a natural laboratory for the observation of emergence phenomena. Like in vehicular traffic or crowds, the spatial extension of the agents gives rise to congestion phenomena. Congestion phenomena and their morphogenetic capability within herds have been studied and simulated ...
Pedestrians and crowdsInteractions between pedestrians in crowds produce emerging selforganized structures. In this work, we have studied this phenomenon under controlled laboratory conditions, in the framework of the 'PEDIGREE' ANR project. We have developed both microscopic (IndividualBased) and macroscopic (fluidlike) models. ...
Collective cell dynamicsCollective cell dynamics is of key importance in such processes as embryogenesis, tissue regeneration or tumor growth. Tumors grown in vitro in 3D (or tumor spheroids, see IP3D web page and pictures) offer a fascinating laboratory system. A microscopic, individual model of cell growth, division and motion is under development... Numerical methods for asymptotic problemsAsymptoticPreserving (AP) methodsThe concept of an AsymptoticPreserving (AP) method makes a breakthrough in the numerical resolution of asymptotic perturbations of PartialDifferential Equations. It has first been introduced by Shi Jin for transport equations in diffusive regimes (see S. Jin, SIAM J. Sci. Comp. 21, 441454, 1999). ... Below are presented several new applications of this concept to fluid and plasma problems, with a large potential of other kinds of applications. Quasineutral limit in plasmasAt large time and space scales, plasmas tend to be quasineutral, i.e. the local charge vanishes. However, at small time and space scales, quasineutrality breaks down. The typical breakdown scales are the electron plasma period and the Debye length. At large plasma densities, both are very small, compared to the usual time and space scales of interest. APschemes applied to plasma models in the quasineutral limit are described below. ... Strongly anisotropic diffusion problems ; application to large magnetic fields in plas masStrongly anisotropic diffusion problems occur in various areas of physics such as structural mechanics of plates and shells, geophysical and fast rotating flows, or strongly magnetized plasmas for instance. APmethods for strongly anisotropic diffusion problems have been developed and applied to plasmas under large magnetic fields. ... Compressible fluids under small Machnumber (i.e. when the fluid motion is slow compared to the thermal speed) are subject to very fast acoustic waves which contribute to instantaneously equilibrate the pressure over the whole domain. AP methods (aka 'allspeed' schemes) are necessary when the Machnumber is small in some regions and order unity in other regions. ... Multiscale methodsMultiscale methods are needed when detailed information about the microscopic structure of the solution (i.e. the structure of the solution at scale 'eps') are needed. This may occur for instance when nonlinear interactions give rise to macroscopic structures which persist in the limit eps > 0 . Localized ModelUpscaling (LMU) and Macroguided micro (MGM) methodsThe localized modelupscaling (LMU) method consists in coupling a perturbation model and its asymptotic limit model (when the perturbation parameter is sent to zero) through a transition zone. ... The MacroGuided Micro method (MGM) relies on the joint solution of the microscopic and the macroscopic scales ... MultiScale FiniteElement Method (MSFEM)This projects investigates the applicability of the MultiScale FiniteElement Method (MSFEM) of T. Hou to diffusion equations in perforated domains. One targetted application is pollutant dispersion in cities. Pollutant dispersion is extremely dependent of the geometry of the city but its full account leads to very time consuming simulations. The MSFEM method is able to provide realtime responses to critical events, which is extremely useful in crisis management. ... Other topicsOther works concerning the following topics can be found on the 'publications' page

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Souspages (19) :
Allspeed schemes for compressible fluids, multiphase flows and fluids with density constraints
AsymptoticPreserving (AP) methods
Collective cell dynamics
Economics and social sciences
Emergence and selforganization in complex systems: general view
Fish schools
General principle of multiscale methods
Herds
Jamming transition and nonoverlapping constraints
Localized ModelUpscaling (LMU) and Macroguided micro (MGM) methods
Micromacro passage in models of complex systems: motivation
MultiScale FiniteElement Method (MSFEM)
Network formation models and traffic on networks
Pedestrians and crowds
Propagation of chaos
Quasineutral limit in plasmas
SelfOrganized Hydrodynamics
Strongly anisotropic diffusion problems ; application to large magnetic fields in plasmas
Symmetrybreaking phase transitions
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