The project aims at

Studying models that describe the evolution of a fluid and/or of a large number of particles;

Discussing the relevance and the range of validity of these models;

Analyzing connections between different levels of modelling;

Developing efficient numerical methods to compute the solutions of such models.

The scientific activity of the project is concerned with PDEs arising from the physical description of particles and fluids. It covers various viewpoints:

At first, the words “particles and fluids” could simply mean that we are interested independently in models for particles, which can either be considered as individuals
(which leads to “

However, many particle systems can also be viewed as a fluid, via a passage from microscopic to macroscopic viewpoint, that is, a hydrodynamic limit.

Conversely, a fruitful idea to build numerical solvers for hyperbolic conservation laws consists in coming back to a kinetic formulation. This approach has recently motivated the introduction of the so-called kinetic schemes.

Eventually, one of the main topics of the project is to deal with models of particles interacting with a fluid.

By nature these problems describe multiscale phenomena and one of the major difficulties when studying them lies in the interactions between the various scales: number of particles, size, different time and length scales, coupling...

The originality of the project is to consider a wide spectrum of potential applications. In particular, the word “particles” covers various and very different physical situations, like for instance:

charged particles: description of semi-conductor devices or plasmas;

photons, as arising in radiative transfer theory and astrophysics;

neutrons, as arising in nuclear engineering;

bacteria, individuals or genes as in models motivated by biology or population dynamics;

planets or stars as in astrophysics;

vehicles in traffic flow modelling;

droplets and bubbles, as in Fluid/Particles Interaction models which arise in the description of sprays and aerosols, smoke and dust, combustion phenomena (aeronautics or engine design), industrial process in metallurgy...

We aim at focusing on all the aspects of the problem:

Modelling mathematically complex physics requires a deep discussion of the leading phenomena and the role of the physical parameters. With this respect, the asymptotic analysis is a crucial issue, the goal being to derive reduced models which can be solved with a reduced numerical cost but still provide accurate results in the physical situations that are considered.

The mathematical analysis of the equations provides important qualitative properties of the solutions: well-posedness, stability, smoothness of the solutions, large time behavior... which in turn can motivate the design of numerical methods.

Eventually, we aim at developing specific numerical methods and performing numerical simulations for these models, in order to validate the theoretical results and shed some light on the physics.

The team has been composed in order to study these various aspects simultaneously. In particular, we wish to keep a balance between modelling, analysis, development of original codes and simulations.

In the study of kinetic equations, it is a very usual strategy to compute a hydrodynamic limit, and then to get rid of the velocity variable and replace the kinetic equation by a
convection-diffusion model. This kind of derivation is well established, under various forms, and in several fields of applications: neutron transport, semiconductor theory, SHE

The computation of the convection-diffusion coefficients of the limit equation, a question which leads to additional difficulties when the small mean free path asymptotics are combined with a homogeneization limit. This problem is motivated by applications in nuclear engineering. In this case, the effective coefficients are defined through auxiliary equations and suitable averages of the oscillatory coefficients.

Some recent works have revealed the formation of singularities in the solutions of some limit convection-diffusion equations, while the original kinetic equation has globally defined solutions. This is due to a coupling in the definition of the convective term with the macroscopic density. This singularity formation is typical of aggregation dynamics. It occurs in models with gravitational forces in astrophysics, and chemotaxis models in biology. Therefore, the natural problems are either to provide a sharp analysis (theoretical and/or based on numerics) of the singularity formation, or to complete the model to avoid such trouble.

A crucial question for applications is to write models for intermediate regimes, for small but non zero values of the mean free path. Such models are required to remain solvable with a moderate computational cost, and to preserve more features from the kinetic level (as for instance finite speeds of propagation, which is lost with a diffusion equation). An example of such an intermediate model is the moment system obtained by using a closure by Entropy Minimization. We have proved that this model is indeed consistent with the diffusion approximation, and we propose an original scheme to treat these equations numerically. We introduce a relaxation strategy which in turn is naturally amenable to the use of asymptotic preserving splitting methods and anti-diffusive schemes for transport equations that are developed in the team. Therefore, we can compare various limited flux models and discuss on numerics their properties and advantages.

We are interested in the equations of the radiative transfer theory which are motivated by the description of high temperature combustion processes (spacecraft propulsion, reentry problems), space observation, nuclear weapons engineering, or inertial confinement fusion. Such problems can be described by a coupling between kinetic and macroscopic equations that comes from the “collision term”, through energy, or energy-impulsion, exchanges. The hydrodynamic limit yields coupled macroscopic equations, with possibly two distinct temperatures: the temperature of the radiations and the temperature of the material. Taking into account Doppler and relativistic effects adds convective terms, which in turn might give rise to the formation of specific singularities.

The interesting points can be summarized as follows:

The derivation of the reduced models, based on modeling arguments, is an issue, bearing in mind to describe a complete hierarchy of models;

The coupling induces non trivial effects on the structure of the hydrodynamics system, which can modify strongly the qualitative properties of the solutions. In particular, the radiative transfer equations might exhibit non standard shocks profiles, with possible discontinuities. The computation of such discontinuous shock profiles requires a very accurate and nondiffusive numerical scheme for the convective terms. This also leads to the delicate question of the stability of travelling waves solutions.

These topics are the object of a very intense research activity e.g. at the Department of Computational Physics of the Los Alamos National Laboratory as well as at the French Atomic Energy Agency (CEA). We develop alternative numerical methods, based on tricky splitting approaches. When dealing with kinetic models, such methods have to be specifically designed to preserve the asymptotic properties of the model. In this approach, one computes on a time step the evolution of the unknown due the convective terms, which will be handled by antidiffusive schemes (see the paragraph Conservation Laws below), and on the next time step, we treat source and interaction terms, that can be nonlocal and/or stiff. This leads to a fully explicit scheme which provides accurate results for a cheap numerical cost and which does not require a tedious inversion step as the implicit methods usually do. We are able to treat numerically the full coupling of radiation with hydrodynamics (Euler equations) in the non equilibrium diffusion regime.

These models arise in the modelling of disperse suspensions in fluids, say droplet or bubble motion. Their study is motivated by applications to combustion, rocket propulsor engineering,
biology, aerosols engineering, or for certain industrial processes. The main effect to take into account is the Stokes drag force, which is proportional to the relative velocity between the
particle and the surrounding fluid

Complementary effects can be taken into account: the so-called Basset force, or the added mass effect, etc. For instance, when particles flow in a pipe, a phenomenological lift force has been proposed to mimic the tendency of particles to concentrate at the center of the pipe. Even though it is moderate in strength, such a force can have crucial effects on blood flows, or on industrial processes of steel production.

Up to now, there are only a few contributions on the description of size variations, by coagulation or fragmentation and break-up. However, in practical situations, as for combustion or biology applications, these phenomena cannot be neglected.

Of course, the coupling with the evolution of the surrounding fluid is a crucial question that leads naturally to problems of asymptotics. Effects of “turbulence”,
which roughly means high and fast variations of

The coupling with the Navier-Stokes or the Euler equations is a privileged subject for SIMPAF. Some asymptotics lead to two-phase flows models, that we are interested in investigating both from a theoretical and numerical point of view. In particular, the effect of an external force (gravitational or centrifugal) can lead to sedimentation profiles that are suspected to be stable; we would like to confirm these heuristics by a thorough numerical and theoretical study. Of course, such investigations require efficient numerical schemes to solve the fluid equations with source terms, which will be detailed in the next sections. To this end, we adapt to this framework the numerical schemes we develop for radiative transfer problems, based on splitting methods and a suitable use of the asymptotic expansion.

Homogenization methods aim at replacing a PDE with highly oscillatory coefficients by an effective PDE with smoother coefficients, whose solution captures the averaged behavior of the true oscillatory solution. The effective determination of the homogenized PDE is however not trivial (especially in the nonlinear or/and stochastic cases). Numerical approximations of the solution of the homogenized PDE is the heart of numerical homogenization.

Homogenization methods are used in many application fields. The two applications we are specifically interested in are material sciences (in particular the determination of macroscopic constitutive laws for rubber starting from polymer-chain networks) and nuclear waste storage (in particular the evolution of nuclear wastes in complex storage devices).

The team in interested in qualitative as well as quantitative results, and theoretical as well as numerical results. Challenging questions are mainly related to nonlinear problems (nonlinear elasticity for instance) and stochastic problems (especially regarding quantitative results).

Plasmas, the fourth state of the matter, play an important role in many branches of physics, such as thermonuclear fusion research, astrophysics and space physics. A plasma is a
(partially) ionized gas where charged particles interact via electromagnetic fields. Since the announcement of the creation of the experimental fusion reactor ITER, and with the progress on
the ICF

The nuclear fusion mechanisms result from the strong confinement of charged particles, either by inertial confinement (nuclear fusion reactions are initiated by heating and compressing a target - a pellet that most often contains deuterium and tritium - by the use of intense laser or ion beams) or by the - more promising - magnetic fusion confinement. The tokamaks are experimental devices which produce a toroidal magnetic field for confining a plasma.

The description of these phenomena is extremely complex and leads to delicate problems in mathematical analysis and numerical simulation. Actually, plasmas may be described with various levels of detail. The simplest possibility is to treat the plasma as a single fluid governed by the Navier Stokes Equations. A more general description is the two-fluid picture, where the ions and electrons are considered to be distinct. If electric or magnetic fields are present, then the Maxwell equations will be needed to describe them. The coupling of the description of a conductive fluid to electromagnetic fields is known generally as magnetohydrodynamics, or simply MHD.

For some cases the fluid description is not sufficient. Then, the kinetic models may become useful. Kinetic models include information on distortions of the velocity distribution functions with respect to a Maxwell-Boltzmann distribution. This may be important when currents flow, when waves are involved, or when gradients are very steep.

The main mathematical difficulties are therefore linked to the conjunction of the following elements

These two types of models are strongly nonlinear.

The unknowns depend on the time and space variables and, in the case of kinetic models, also on the velocity variables. Therefore, we can be led to work with variables of 1 + 3 + 3 dimensions.

There exist many very different scales (time scale, characteristic length, etc).

The numerical resolution of a complete system of equations, with meshes adapted to the lower scales, leads to prohibitive computational costs both in terms of memory and time. The derivation of new reduced models, corresponding to relevant asymptotic regimes (high magnetic field for example), is therefore a crucial issue. Moreover, very serious efforts must be done on the numerical methods that are used in order to reproduce the typical phenomena. This work depends on the one hand on seriously thinking over the models, the physical parameters, their typical respective scales, and on the other hand over some arguments of asymptotic analysis, which can particularly call on deterministic or random homogeneization.

Satellites in geostationary and low Earth orbits naturally evolve in a plasma. This ionized environment induces some perturbations which may lead to many kind of faults and to the partial or complete loss of a mission. The satellites are covered by dielectric coatings in order to protect them against thermal radiations. Electrons and ions species of the space plasma interact with the external surfaces of the satellite and modify their electrostatic charges. This effect produces potential differences between the satellite surfaces and its electric mass. When the electric field exceeds a certain level, an electrostatic discharge appears. This electric current pulse is able to disrupt the equipments, to damage the external surfaces and even to destroy some electronic components. The plasma may also be created by an other source : the electric thrusters. This new propulsion device uses the electric energy supplied by solar arrays to speed up charged species. It is more and more used in satellite industry and has preference over the classical chemical propulsion. Indeed, the latter needs a very large amount of propellant inducing an expensive rocket launch. On the one hand, the electric thrusters allow to significantly reduce the satellite weight. On the other hand, it is necessary to understand their potential impacts on the other systems of the satellite.

This line of research, which continues former works of the team CAIMAN at Sophia Antipolis, was the object of a strong collaboration with the Department Research and Technology of the
company Thales Alenia Space. N. Vauchelet proposed several evolutions for the SPARCS code, including parallel procedures. A comparizon of different numerical schemes (Finite Volume,
Semi-Lagrangian, Back Trajectory) to treat the Vlasov equations of spacecraft charging has been discussed in details. Moreover, the PhD of S. Borghol shows that at LEO or PEO altitudes

In models of charge transport, say transport of electrons, a phenomenological friction force is generally introduced, which is proportional to the velocity

A major issue in the numerical approximation of systems of conservation laws is the preservation of singularities (shocks, contact discontinuities...). Indeed, the derivatives of the solutions usually blow-up in finite time. The numerical scheme should be able to reproduce this phenomenon with accuracy, i.e. with a minimum number of points, by capturing the profile of the singularity (discontinuity), and by propagating it with the correct velocity. The scheme should also be able to give some insight on the interactions between the possible singularities. Quite recently, new anti-diffusive strategies have been introduced, and successfully used on fluid mechanics problems. We focus on multidimensional situations, as well as on boundary value problems. Since a complete theory is not yet available, the numerical analysis of some prototype systems of conservation laws is a good starting point to understand multi-dimensional problems. In particular, a good understanding of the linear case is necessary. This is not achieved yet on the numerical point of view on general meshes. This question is particularly relevant in industrial codes, where one has to solve coupled systems of PDEs involving a complex coupling of different numerical methods, which implies we will have to deal with unstructured meshes. Thus, deriving non-dissipative numerical schemes for transport equations on general meshes is an important issue. Furthermore, transport phenomena are the major reason why a numerical diffusion appears in the simulation of nonlinear hyperbolic conservation laws and contact discontinuities are more subject to this than shocks because of the compressivity of shock waves (this is another reason why we focus at first on linear models).

The next step is to combine non-dissipation with nonlinear stability. An example of such a combination of preservation of sharp shocks and entropy inequalities has been recently proposed for scalar equations and is still at study. It has also been partially done in dimension one for Euler equations.

Of course, there are plenty of applications for the development of such explicit methods for conservation laws. We are particularly interested in simulation of macroscopic models of radiative hydrodynamics, as mentioned above. Another field of application is concerned with polyphasic flows and it is worth specifying that certain numerical methods designed by F. Lagoutière are already used in codes at the CEA for that purpose. We also wish to apply these methods for coagulation-fragmentation problems and for PDEs modelling the growth of tumoral cells; concerning these applications, the capture of the large time state is a particularly important question.

Flow control techniques are widely used to improve the performances of planes or vehicles, or to drive some internal flows arising for example in combustion chambers. Indeed, they can sensibly reduce energy consumption, noise disturbances, or prevent the flow from undesirable behaviors.

Recently, open and closed active flow control were carried out in order to study the flow behavior over a backward-facing step in a transitional regime. It was done either by a global frequency destabilization at the entry of the domain, or by a local blowing or suction through the lower and upper parts of the step by the use of small jets ( , E. Creusé, A. Giovannini (IMFT Toulouse) and I. Mortazavi (MC2 INRIA EPI, Bordeaux)). The numerical computations were based on a vortex-in-cell method. Such controls were shown to be efficient in reducing the average recirculation length value, the global flow energy, as well as the global flow enstrophy. We have now in mind to apply such a strategy on cavity-stent flows, in order to study the effect of passive and/or active control on the average emptying time of the cavity, corresponding to a lot of possible industrial or health applications (combustion, blood circulation in arteries,...).

Passive as well as active control were also performed on the "Ahmed body geometry", which can be considered as a first approximation of a vehicle profile. This work was
carried out in collaboration with the EPI INRIA MC2 team in Bordeaux (C.H. Bruneau, I. Mortazavi and D. Depeyras), as well as with Renault car industry (P. Gillieron). We recently combined
active and passive control strategies in order to reach efficient results, especially concerning the drag coefficient, for two and three dimensional simulations
. We are now interested in the same kind of study, but for a

In another field of applications, a work was performed with the TEMPO Laboratory of Valenciennes. The objective of this collaboration was to study the pressure wave generated by high-speed trains entering tunnels in order to improve the shape of the tunnel sections.

Numerical investigations are very useful to check the behavior of systems of equations which modeling very complicate dynamics. In order to simulate the motion of mixtures of immiscible fluids having different densities, a recent contribution of the team was to develop an hybrid Finite Element / Finite Volume scheme for the resolution of the variable density 2D incompressible Navier-Stokes equations. The main points of this work were to ensure the consistency of the new method as well as its stability for high density ratios . In order to answer to these questions, we have developed respectively a MATLAB code and a C++ code. In the following of this work, we now have in mind the following objectives :

To allow the corresponding MATLAB code distribution, in order to promote some further collaborations with other researchers in the domain, and to make the comparison of our results with alternative numerical methods possible. For this objective, a graphics interface was developed as well as some post-processing tools and an accurate documentation. This was the object of the Manuel Bernard internship in the SIMPAF team (Mars 2011 – Sept. 2011). For exemple, the code was already used to study the influence of new strategies for updating LU factors of existing preconditioners. In C. Calgaro et al. address the problem of computing preconditioners for solving linear systems of equations with a sequence of slowly varying matrices. This problem arises in many important applications, for example in computational fluid dynamics, when the equations change only slightly possibly in some parts of the domain.

To modify the already existing C++ code to treat certain more general hydrodynamic models arising in combustion theory, as well as models describing mixing of compressible fluids arising for instance when describing the transport of pollutants. The interesting thing is that this kind of model can be derived by a completely different approach through a kinetic model. Besides, this model presents interesting features, since it is not clear at all whether solutions can be globally defined without smallness assumptions on the data. Then, a numerical investigation is very useful to check what the actual behavior of the system is. Accordingly, our program is two-fold. On the one hand, we develop the density dependent Navier-Stokes code, where the incompressibility condition is replaced by a non standard condition on the velocity field. In particular, if the closure model is the Fick's law, one obtain the so called Kazhikhov-Smagulov model. The first phenomena we try to reproduce are the powder-snow avalanches. The influence of the characteristic Froude number on the front progression is also investigate. On the other hand, we wish to extend our kinetic asymptotic-based schemes to such problems. This work is being developed in collaboration between C. Calgaro, E. Creusé and T. Goudon (INRIA Sophia Antipolis, team COFFEE).

In the case of the PhD of M. Ezzoug, co-advised by C. Calgaro and E. Zahrouni (Monastir University, Tunisie), to study numerically and theoretically the influence of a specific stress tensor, introduced for the first time by Korteweg.

Finally, to prove the convergence of the numerical scheme in order to ensure the theoretically performance of the method. This work started very recently by a collaboration between C. Calgaro, E. Creusé and E. Zahrouni (Monastir University, Tunisie).

E. Creusé works on a posteriori error estimators for finite element methods, applied to the resolution of several partial differential equations. The objective is to derive useful tools in order to control the global error between the exact solution and the approximated one (reliability of the estimator), and to control the local error leading to adaptive mesh refinement strategies (efficiency of the estimator).

A recent work, in collaboration with S. Nicaise (LAMAV, Valenciennes), was devoted to the derivation of some so-called "reconstruction estimators" based on gradient averaging, in order to provide lower and upper bounds of the error arising from a discontinuous Galerkin approximation of a diffusion problem .

At the same time, some equilibrated-type estimators were developed for the Reissner-Mindlin system arising in solid mechanics applications, for conforming and locking-free approximations, in the context of the PhD. of E. Verhille.

At last, a collaboration with the "Laboratoire d'électrotechnique et d'électronique de puissance de Lille (L2EP)" began two years ago, to derive a residual-based a posteriori error estimator
for the Maxwell system in its vectorial and scalar potential formulation

In collaboration with M. Taki (PhLAM laboratory, Lille), Christophe Besse and Guillaume Dujardin are considering dispersive equations modelling the propagation of a laser beam in an optic fiber. They are trying to explain the possible ways of creating rogue waves in the propagation of laser beams. More generally, they are trying to explain which terms in the dispersive Schrödinger-like equations obtained by the physicists allow which physical behaviour of the solutions (e.g. the creation of rogue waves).

In collaboration with G. Reinish (Nice Observatory), Christophe Besse and Guillaume Dujardin are working on the numerical computation of the ground state and the first bound states of the non linear Schrödinger-Poisson system with confining quadratic potential in 2 space dimension modelling quatum dot helium. The goal is to perform after that numerical time stepping methods to simulate the dynamics of the NLSP system and compute accurately some quantities of physical interest as functions of time.

The DDNS2 code is a parallel solver for unsteady incompressible Navier-Stokes flows in 2D geometries and primitive variables written in Fortran 95 with MPI as a message-passage library. Mixed finite element methods, with hierarchical basis, are used to discretize the equations and a non overlapping domain decomposition approach leads to an interface problem which involves a Lagrange multiplier corresponding to the velocity (the FETI approach). A dynamical multilevel method is developed locally on each subdomain. Several numerical estimates on the evolution of linear and nonlinear terms allow to construct the multilevel strategy which produces auto-adaptive cycles in time during which different mesh sizes, one for each subdomain, can be considered.

Here is the self-assessment of the team effort following the grid provided by Inria (see :
http://

The NS3ED code is a solver for steady incompressible Navier-Stokes flows in three-dimensional exterior domains, written in C++. The truncated problem is discretized using an exponential mesh and an equal-order velocity-pressure finite element method, with additional stabilization terms. A bloc-triangular preconditioner is performed for the generalized saddle-point problem.

Here is the self-assessment of the team effort following the grid provided by Inria (see :
http://

The NS2DDV-M code is based on a hybrid method coupling FV and FE approaches for solving the variable density Navier-Stokes equation in dimension 2. This original approach for variable density flows is described in . The NS2DDV-M code will be avalaible on the SIMPAF team web page before the end of 2011.

Here is the self-assessment of the team effort following the grid provided by Inria (see :
http://

Software web site :
http://

The NS2DVD-C++ code is based on a hybrid method coupling FV and FE approaches for solving the variable density Navier-Stokes equation in dimension 2. The code is developed around the GetFem++ and the Bamg softwares. It allows in particular mesh refinement strategies so that very relevant simulations can be reached (as the falling droplet with very high density ratios, see for example . The current version of the code consider the additional terms in the Kazhikhov-Smagulov model.

Here is the self-assessment of the team effort following the grid provided by Inria (see :
http://

We have developed a set of numerical codes, written in Scilab, to compute the solutions of the system coupling the Euler equations to the radiation through energy exchanges, in the non equilibrium regime. This covers several situations in the hierarchy of asymptotic problems. The code treats the one-dimensional framework. In particular the code can be used to investigate radiative shocks profiles. The main advantage of our numerical codes is that they do not require any refinement near the singularities. The numerical tests show a very good agreement with the theoretical predictions.

Here is the self-assessment of the team effort following the grid provided by Inria (see :
http://

We have developed a numerical code, written in Scilab, to compute the solutions of the two-phase flows equations describing particles interacting with a fluid through friction forces. The code treats one-dimensional situation and is well adapted to describe gravity driven flows in either bubbling or flowing regimes. In particular, it can be used to describe the evolution of pollutants in the atmosphere. The numerical strategy, based on a asymptotic-based scheme, is described in details in .

Here is the self-assessment of the team effort following the grid provided by Inria (see :
http://

As a byproduct of the review paper
, a user-friendly interface is offered

SPARCS is the code developed by Thales Alenia Space for the simulation of the charge phenomena the spacecrafts are subject to. The current version of the code, according to the PhD thesis of O. Chanrion and M. Chane-Yook performed in collaboration with the team Caiman at Sophia Antipolis, is specialized to geostationary atmospheres. The model consists in the stationary Vlasov-Poisson system, but where instationary effects are taken into account with the boundary condition for the electric field. We participate, in particular through the post doc of N. Vauchelet, to the elaboration of an improved version of the code which includes parallization optimized procedures, the modeling of the natural difference of potential between different dielectric surfaces of the spacecraft, as well as the possible presence of devices emitting charged particles.

The analysis of multi-scale phenomena and asymptotic problems aiming at identifying the influence of microscopic scales on the macroscopic observations is a hot topic in the team. Results have been obtained concerning the derivation of effective law describing the behavior of a particle interacting with a thermal bath or a set of oscillators. This work, which combines modeling efforts, analysis and large computations, is the object of a longstanding collaboration with P. Parris (Missouri-Rolla) and is the heart of the PhD thesis of B. Aguer , . More recently, S. De Bièvre and P. Parris have described the approach to equilibrium in such systems . Transport in a related quantum mechanical model was studied in . Some long time effective behavior of related models has been obtained in .

At the same time, M. Rousset is working on the numerical simulation of stochastically perturbed Molecular Dynamics. The main goal is to handle in the same simulation the fastest time scales (the oscillations of molecular bindings), and the slowest time scales (the so-called reaction coordinates). Recently, a monograph has been published which summarized standard and state-of-the-art free energy calculations, that are used to accelerate slow variables in MD simulations. In analysis of constrained dynamics is proposed, with associated numerical schemes. In , a new method has been proposed which drastically slows down the fast frequencies with a penalty and accelerates simulations, while conserving the statistical behavior of molecular systems.

The convergence analysis of numerical schemes for conservation laws with unstructured meshes with an original proof based on probabilistic argument is a striking result due to F. Lagoutière with F. Delarue, . More generally, we refer to for an overview of F. Lagoutière's works.

J.-F. Coulombel has studied the stability of finite difference approximations of hyperbolic systems with boundary conditions. This series of works, part of which is a collaboration with A. Gloria, generalizes and simplifies previous results by Gustafsson, Kreiss, Tadmor, Wu and others. In collaboration with O. Guès and M. Williams, J.-F. Coulombel has also studied the justification of geometric optics for hyperbolic boundary value problems. The results describe the reflection of highly oscillating wave trains on a boundary. Eventually, J.-F. Coulombel has studied with S. Benzoni and N. Tzvetkov well-posedness issues for some nonlocal versions of Burgers equation.

Results on active and passive control on the 3D Ahmed body in the 25
^{o}rear-window configuration were obtained in
,
,
. Moreover, results on the pressure wave generated by high-speed
trains entering tunnels were published in
.

We are interested in two-phase flows involving a dense and a disperse phase. These models lead to interesting mathematical questions, . We develop new asymptotic preserving methods for fluid/particles flows . This approach follows the scheme we developed for radiative transfer equations .

We obtained several results of asymptotic analysis concerning either kinetic or macroscopic models for charge transport, see , and (and we also refer to the related work ). Through the collaboration with Thales we proposed in a method to compute numerically the boundary conditions for the Euler system derived from the BGK equation in the hydrodynamic limit. In particular this method is based on an analysis of boundary layers. In we extend these techniques to the coupled Euler Poisson system.

The linear or nonlinear Schrödinger equation with potential is one of the basic equations of quantum mechanics and it arises in many areas of physical and technological interest, e.g. in quantum semiconductors, in electromagnetic wave propagation, and in seismic migration. The Schrödinger equation is the lowest order one-way approximation (paraxial wave equation) to the Helmholtz equation and is called Fresnel equation in optics, or standard parabolic equation in underwater acoustics. The solution of the equation is defined on an unbounded domain. If one wants to solve such a whole space evolution problem numerically, one has to restrict the computational domain by introducing artificial boundary conditions. So, the objective is to approximate the exact solution of the whole-space problem, restricted to a finite computational domain. A review article was written this year to describe and compare the different current approaches of constructing and discretizing the transparent boundary conditions in one and two dimensions. However, these approaches are limited to the linear case (or nonlinear with the classical cubic nonlinearity: an article written was dedicated to this case this year ) and constant potentials. Therefore, in collaboration with X. Antoine (IECN Nancy and Inria Lorraine), we proposed to P. Klein to study, in her PhD thesis, the case of the Schrödinger equation with variable potentials. The study of the non-stationary one-dimensional case has already led to one publication and some preliminary results in the stationary case are really promising. These cases are relevant since for example the equations appear in the Bose Einstein condensate with a quadratic potential.

This problem is obviously not limited to the Schrödinger equation and new developments are in progress on the Korteweg de Vries equation with M. Ehrhardt. This equation is more difficult to study due to its third order derivative in space.

Dispersive equations, such as the Schrödinger equation are also considered as boundary-value problems. For example, in , G.D. studies the long time asymptotics of the solutions of linear Schrodinger equations considered as initial-boundary value problems on the half-line and on bounded intervals when the boundary data are periodic functions of time. G.D. obtains theoretical results using a transformation method introduced by T. Fokas and provides several numerical experiments to support them.

The Diffusion Monte-Carlo method is a powerful strategy used by chemists to estimate the groundstate energy of a N-body Schrödinger Hamiltonian with high accuracy.However, the method suffers from two major limitations :

The quantity of physical interest is more the energy variation with respect to a parameter than the energy itself.

The case of Fermions (as electrons of atoms) relies on constraining the random walkers in some nodal pocket (which imposes skew-symmetry on the distribution of walkers) which is only approximately known (the Fixed Node Approximation).

We have three types of results regarding the homogenization theory and its applications.

The first series of results is related to nonlinear elasticity. In , A. Gloria has proved the convergence of a discrete model for rubber towards a nonlinear elasticity theory in collaboration with R. Alicandro and M. Cicalese. The numerical simulation of the model has been addressed within the ARC Disco by A. Gloria, P. La Tallec and M. Vidrascu (project team MACS). Comparisons with mechanical experiments are promising, and related inverse problems have been addressed in the post-doc of M. de Buhan. Two publications are in preparation. Related theoretical results in homogenizaton of nonlinear elasticity models have been obtained by A. Gloria and S. Neukamm (MPI Leipzig) in .

A second type of results concerns a quantitative theory of stochastic homogenization of discrete linear elliptic equations. A breakthrough has been obtained by A. Gloria and F. Otto (MPI Leipzig) in and , who gave the first optimal variance estimate of the energy density of the corrector field for stochastic discrete elliptic equations. The proof makes extensive use of a spectral gap estimate and of deep elliptic regularity theory, bringing in fact the probabilistic arguments to a minimum. This analysis has enabled A. Gloria to propose efficient numerical homogenization methods, both in the discrete and continuous settings , . In , A. Gloria and J. C. Mourrat has pushed the approach forward and introduced new approximation formulas for the homogenized coefficient. In they have considered a more probabilistic approach and given a complete error analysis of a Monte-Carlo approximation of the homogenized coefficients in the discrete case. Work in progress concerns the generalization of the results on discrete elliptic equations to the continuous case.

The third direction of research concerns the periodic homogenization of a coupled elliptic/parabolic system arising in the modeling of nuclear waste storage. This work is in collaboration with the French agency ANDRA. A. Gloria, T. Goudon, and S. Krell have made a complete theoretical analysis of the problem, derived effective equations, and devised an efficient method to solve the effective problem numerically, based on the reduced basis approach. A publication is in preparation. This subject will be continued with the arrival of Z. Habibi.

The interest of the team in developing efficient numerical methods preserving the asymptotic behavior of kinetic equations modelling the radiative transfer phenomena in the diffusive regime has recently brought two new innovative schemes. On the one hand, the moment closure equations proposed in involve non local terms that lead to the introduction of specific numerical approximations. On the other hand, a new scheme based on the projective integration procedure due to Gear and Kevrekidis was proved very efficient theoritically and numerically.

The team has also obtained a rigorous justification of the formation of so-called Zeldovich spikes in radiative shock profiles . As long as the amplitude of the wave is sufficiently large, the temperature profile displays an overshoot. Such phenomenon does not occur for small amplitude waves, nor for viscous shock profiles.

First, the generalization of in the case of the full convection-reaction-diffusion case lead to robust estimators, for which the dependance of the constant in the data are explicitly given . Then, we succeeded in obtaining some "asymptotic exact estimators" for the Reissner-Mindlin system in order to provide an accurate bound of the error, while keeping a reasonable computational cost , . Finally, we obtained some residual-based a posteriori error estimators for the Maxwell system in its vectorial and scalar potential formulation , .

On the one hand we have optimized and proposed a parallel version of the code SPARCS which is designed to compute the electric potential on the spacecraft and around it. On the other hand we have proposed alternative hydrodynamic models and a method to derive adapted boundary conditions for the macroscopic quantities in order to reproduce phenomena due to the kinetic ones.

We are starting a collaborative program with the CEA (Direction des Applications Militaires) devoted to the modeling and simulation of plasmas arising in Inertial Confinement Fusion devices. We are concerned with the derivation of the so-called nonlocal models, which are variations around the standard diffusion models.

The goal of the program relies on the determination of effective coefficients for models of transfer of solutes in porous media. The problem appeals to the homogenization of convection-diffusion equations. We wish to participate to the research efforts for the numerical simulation of the disposal of radioactive waste. This work is the object of a collaboration with S. Krell and A. Gloria.

Ch. Besse and I. Lacroix-Violet are members of the new 4-years ANR "blanche" project MICROWAVE. Ch. Besse is the North node coordinator. The scientific subjects deal with artificial boundary conditions for dispersive equations, electromagnetism and high frequency regimes in acoustic simulations. This ANR project concerns the development of new numerical methods for wave propagation problems using tools from microlocal analysis. It focuses on microlocal analysis and numerical methods for acoustic and electromagnetic wave scattering and microlocal analysis and numerical methods for Schrödinger-type equations.

Ch. Besse has obtained a 4-years ANR grant, from the Cosinus proposal, for the project IODISSEE. P. Lafitte and C. Yang, also members of the EPI Simpaf, are involved in this project. The project IODISSEE also involves a team of mathematicians from Toulouse, a physicist team from Versailles and the Thales group. It deals with the elaboration of a physical model for helping the industrial partner for the new generation of Galileo satellites. For the last decade, satellite positioning devices became one of the most interesting means of navigation for the displacement of the goods and the people. The only current solution is based on the constellation of satellites Navstar GPS American system. Originally developed for military applications, its use was released under the Clinton administration. However, in order to guarantee its autonomy, Europe decided to launch a competitor program known as Galileo. Galileo system differs from the GPS thanks to its capability to provide real time integrity information to the user. In order to guarantee the stability of this system, it is fundamental to take into account the various problems which can affect the mission and to identify all the potential sources of system unavailability. One of the main source of data unavailability that has been identified is the phenomena of ionospheric scintillations. Indeed scintillation causes radio frequency signal amplitude fades and phase variations as satellite signals pass through the ionosphere. Such effects may induce loss of lock or cycle slips on ranging signals broadcast by Galileo satellites making them totally useless for accurate integrity information determination. Scintillations are clearly identified like a source of disturbances. They appear as the turbulent aspect of a larger disturbance of the ionospheric plasma density which have the shape of a plasma bubble. The difficulty of their modeling is due to the lacks of in situ measurements with regard to them. However, some measurements recently acquired during the mission of satellite DEMETER make possible on the one hand the validation of the models existing but also, using techniques of data-models coupling, to reinforce them. The object of this proposal is therefore to provide a physical model making it possible to anticipate the attenuation of the signals during their propagation within the disturbed Earth ionosphere.

M. Rousset is involved in the ANR MEGAS. The main scientific subject is numerical methods in Molecular Dynamics simulation.

J.-F. Coulombel has obtained a 4-years ANR grant "young researcher", for the project INTOCS. In addition to the coordinator, two other members of the EPI SIMPAF are involved in this project: P. Lafitte et F. Lagoutière. The main scientific subject of the project is the interaction of compressible waves, and more precisely the propagation of high frequency oscillations in hyperbolic boundary value problems. One of the physical motivations is the "Mach stems" formation in reacting gas flows.

SIMPAF is involved in the project led by E. Sonnendrucker from EPI Calvi, which aims at fostering the national research effort in mathematics and computer science towards the simulation of large magnetic confinement devices, like ITER. The project Fusion is currently under evaluation.

In 2006, under an initiative of J.-P. Chehab, the SIMPAF team has initiated a collaborating program “3+3 Méditerranée” funded by INRIA. This program, devoted to Modelling, Analysis and Simulation of Hydrodynamic Waves, is the continuation of the MASOH project. To be more specific, the project focuses on water waves modelled by dispersive PDEs (Korteweg-De Vries, Benjamin-Ono, KP and Nonlinear Schrödinger equations). The goal is to elaborate efficient multilevel numerical schemes that will be able to help in the understanding of finite time blow up or the asymptotic smoothing effects due to damping.

As a consequence, four PhD theses were started co-advised by SIMPAF's members.

Emna Ezzoug, from Monastir, advised by E. Zahrouni, J. Laminie and O. Goubet, started in July 2006;

Ibtissem Damergi, from Monastir, by advised E. Zahrouni, Ch. Besse and O. Goubet, started in July 2006;

Salim Amr Salim Djabir, from Marrakesh, advised by M. Abounouh and J.-P. Chehab, started in January 2007;

Maithem Trabelsi, from Tunis advised by E. E. Zahrouni and J.-P. Chehab, started in September 2007.

A. Gloria is the leader of the ARC project DISCO. The main objective is the design, mathematical analysis, numerical analysis, and numerical simulation of discrete models for rubber. The participants are F. Lequeux (polymer physics, ESPCI), P. Le Tallec (mechanics, Ecole polytechnique), F. Otto (mathematics, MPI Leipzig), M. Vidrascu (scientific computing, INRIA Paris-Rocquencourt), and M. de Buhan (postdoc until September 2011). A workshop was organized in January 2011.

A. Gloria is involved in the 4-year ANR project “young researcher” AMAM, led by V. Millot (Paris 7). The aim of the project is to develop mathematical tools for the analysis of multiscale problems in material sciences (PDEs and variational methods). The fields of interest are primarily micromagnetics, dislocations, fatigue in nonlinear elasticity, and homogenization.

The main goal of the project is the mathematical analysis and the simulation of miscible or immiscible fluids. In this context, C. Calgaro visited the University of Monastir (Tunisia). As a consequence, one PhD thesis started in october 2010 : Meriem Ezzoug, from Tunis, advised by E. Zahrouni and C. Calgaro.

C. Calgaro is in charge of the communication of the Math. Department and she is in charge of the relation between the University of Lille and high schools. Accordingly, she organizes various events like “Les Métiers des maths”, “Stage de maths en seconde” and “Demain l'université”. With the help of the Communication Department of INRIA, C. Calgaro, E. Creusé and T. Goudon have produced a documentary fiction (in French) for a general audience on how research in applied mathematics is being done. The title is "Avis de recherche".

F. Lagoutière organized CEMRACS 2011, and A. Gloria and Y. Penel proposed and supervised two research projects at CEMRACS 2011.

T. Goudon and P. Lafitte are members of the jury of the national hiring committee of the “Agrégation de mathématiques”.

S. De Bièvre is in charge of the Doctoral formation in Applied Mathematics at the University of Lille.

C. Besse and E. Creusé are involved in the project of a new International Master degree at the University of Lille devoted to Scientific Computing.

Members of the team are involved in MSc degrees at USTL (C. Calgaro, S. De Bièvre, G. Dujardin, A. Gloria, P. Lafitte, M. Rousset).