We present the study of the non-linear stability of a class of travelling-wave solutions to the compressible pressureless Navier-Stokes system with a singular viscosity. These solutions encode the effect of congestion by connecting a congested left state to an uncongested right state. By using carefully weighted energy estimates we are able to prove the non-linear stability of viscous shock waves to this system under a small zero integral perturbation, which in particular extends previous results that do not handle the case where the viscosity is singular. This is a joint work with Muhammed Ali Mehmood from Imperial College, London.
In this seminar we present a mathematical model to describe the evolution of a city, which is determined by the interaction of two large populations of agents, workers and firms. The map of the city is described by a network with the edges representing at the same time residential areas and communication routes. The two populations compete for space while interacting through the labour market. The resulting model is described by a two population Mean-Field Game system coupled with an Optimal Transport problem. We prove existence and uniqueness of the solution and we provide some numerical tools to develop several numerical simulations. This is a joint work with Fabio Camilli (Sapienza Roma) and Luciano Marzufero (Libera Università di Bolzano).
Le modèle Euler compressible bifluide ne présente pas de difficultés théoriques supplémentaires comparé au cas monofluide. Mais sa résolution numérique est notoirement plus difficile à cause du phénomène d'oscillations de pression à l'interface entre fluides. Nous présentons une approche basée sur un échantillonnage aléatoire "à la Glimm" à l'interface, qui permet de s'affranchir de ce défaut. Le schéma obtenu est applicable à des maillages non structurés, il a d'excellentes propriétés de robustesse et de convergence. Nous l'appliquons à des cas de déferlement.
Pendant l'élongation de l’axe de l'embryon de vertébré, on observe, grâce à l’imagerie live, un phénomène de turbulence cellulaire dans les différents tissus embryonnaires. Nous proposons un modèle mécanique en 2D pour modéliser la croissance des tissus pendant l'élongation de l'embryon, qui permet de retrouver ces flux turbulents à travers un rotationnel non trivial pour les vitesses des tissus. Une autre spécificité de ce modèle est que la ségrégation entre les tissus est assurée par une pression de ségrégation. Après avoir déterminé (formellement) la limite incompressible, nous étudions le comportement qualitatif à la limite et discutons d'un effet fantôme.
Plusieurs modèles physiques permettent de comprendre la dynamique des mélanges de fluides, parmi lesquels figurent les modèles dits de Baer-Nunziato. Les équations aux dérivées partielles associées à ces modèles ressemblent à celles de Navier-Stokes, avec, en plus, de nouveaux termes de relaxation.
Une stratégie pour obtenir ces modèles est l'homogénéisation : à partir d'un mélange mésoscopique, où deux fluides purs satisfaisant les équations de Navier-Stokes compressibles se répartissent l'espace, on effectue un changement d'échelle pour obtenir un mélange macroscopique, où, en chaque point de l'espace, les deux fluides peuvent coexister.
Ce problème relève de l'étude des équations de Navier-Stokes avec des données initiales fortement oscillantes. On commencera donc par expliquer certains résultats dans ce cadre de travail, en dimension un d'espace et sur le tore, d'abord pour des fluides barotropes, puis pour des gaz parfaits non barotropes. On détaillera ensuite les différentes étapes de la démonstration de l'homogénéisation.
Dynamic processes in continuum physics are modeled using time-dependent partial differential equations (PDE), which are based on the conservation of some physical quantities, such as mass, momentum and energy. Depending on the physical phenomenon under consideration, the governing equations can exhibit some mathematical structures like differential constraints, algebraic relations, physical admissible states as well as asymptotic limits and thermodynamics compatibility. An interesting class of mathematical models is provided by symmetric hyperbolic systems that intrinsically imply all the structures listed above. When passing at the discrete level, the exact satisfaction of these structural properties is not automatically guaranteed, thus Structure Preserving numerical schemes have recently emerged with the aim of exactly discretizing at least a subset of these constraints. We will investigate and present some of our research activity carried out in the framework of the development of Structure Preserving schemes, focusing on recent contributions delivered in the last three years. In particular, we will address asymptotic preserving schemes for low Mach flows, div-curl and curl-grad preserving operators for discontinuous Galerkin methods, and a novel geometric and thermodynamically compatible finite volume method for continuum mechanics.
The energy of saline gradients is a very promising source of non-intermittent renewable energy, the exploitation of which is hampered by the lack of economically viable technology. The most investigated harvesting methods rely on selective transport of ions or water molecules through semi-permeable or ion-selective membranes, which demonstrate limited power densities of the order of a few W/m2. While in the last decade single nanofluidic objects such as nanopores of nanotubes have opened up very promising prospects with power density capabilities in the kW or even MW/m2, scale-up efforts face serious issues, as concentration polarization phenomena result in a massive loss of performance.
At the LiPhy we work on a concept of nanofluidic exchanger for power generation from saline gradients, focused on designing a nanoscale flow able to harvest the power at the output of the nanopores. We will present the study of a simple exchanger made of a selective nanoslit fed by a nanofluidic assembly. One specific feature of such an exchanger relies on the non-linear ion fluxes through the nanoslit, according to the so-called 1D Poisson-Nernt Planck equations. Such an elemental brick could be massively parallelized in stackable electricity-generating layers using standard technologies of the semi-conductors industry. We demonstrate here a scheme for rationalizing the choice of the exchanger parameters, taking into account the transport properties at all scales. The simplified numerical resolution of the three-dimensional device shows that net power densities of 300 W/m2 and more can be achieved.
Orateur(e)s :
Walter Boscheri (LAMA); Camille Carvalho, (ICJ); Frédérique Charles, (LJK); Nicolae Cindea, (LMBP); Sue Claret, (LMBP); Baptiste Devyver, (IF); Martin Donati, (IF); Louis Dupaigne, (ICJ); Hugo Eulry, (UMPA); Christophe Lacave, (LAMA); Mickael Nahon, (LJK); Niami Nasr, (ICJ); Pierre-Damien Thizy, (ICJ);
Orateur(e)s :
Walter Boscheri (LAMA); Camille Carvalho, (ICJ); Frédérique Charles, (LJK); Nicolae Cindea, (LMBP); Sue Claret, (LMBP); Baptiste Devyver, (IF); Martin Donati, (IF); Louis Dupaigne, (ICJ); Hugo Eulry, (UMPA); Christophe Lacave, (LAMA); Mickael Nahon, (LJK); Niami Nasr, (ICJ); Pierre-Damien Thizy, (ICJ);
Je présenterai quelques avancées récentes sur la caractérisation des phénomènes de propagation qui apparaissent dans les équations semi-linéaires avec diffusion non locale de type Levy. Récemment différentes dichotomies entre propagation accélérée et propagation à vitesse constante en fonction des paramètres de décroissance du noyau et de l'ordre d'annulation en zéro de la non-linéarité considérée ont été obtenues. Je me concentrerai sur le cas monostable et sur une manière de contourner les difficultés liées au traitement des opérateurs de Levy généraux.
Let ϒ be a generalised cone in Rd. Roughly speaking, Yaglom limit describes the behaviour of the process conditioned not to exit the cone, or, in other words, not to become extinct or not to be absorbed. In the talk we will discuss the existence of this limit for a class of (not necessarily symmetric) α-stable Lévy processes living in the cone ϒ. To this end, we will use the so-called Martin kernel at inifinty - the invariant function for the killed semigroup - to obtain the so-called entrance law from the origin, which we also call the self-similar solution. Using this approach, for the isotropic case we will also obtain the large-time asymptotics for the killed semigroup and provide several examples of our resutts.
When discretizing partial differential equations, one can choose local (finite differences, volumes, elements) or global (spectral) methods. The most common spectral basis is built on trigonometric polynomials, i.e. Fourier series. It constrains the boundary conditions to be periodic and has been an important tool in physics, used for instance to study theoretical scalings of turbulence. While spectral methods show "exponential convergence" for smooth functions, large DNS simulations also become too expensive for e.g. when reaching very large Reynolds numbers. In practice, it is possible to solve a coarser version of the DNS by removing the largest wavenumbers in spectral space (cut-off) and modeling transfers at the smallest (sub-grid) scales instead. The definition of such a model has been an open problem for a long time and classical ones are either too diffusive or unstable. Machine learning started to be an interesting alternative few years ago and people quickly found that learning a model that performs better on a priori (instantaneous) metrics is possible. We have shown that in order to lead to stable simulations in a posteriori tests, the temporal dimension must be taken into account during the learning process. This problem has now been largely explored with periodic boundary conditions, but when it comes to spectral methods with orthogonal polynomials and fixed boundaries, new challenges appear.
In this work in collaboration with T. Iguchi, we show that the Saint-Venant equations in 2D with a partially submerged obstacle is well-posed. To do so, we show that the problem is equivalent to the usual Saint-Venant equations in an external domain, with additionnal non-standard boundary conditions because they are not local in space and time. These conditions do not fit into any category of dissipativity for which the hyperbolic theory is well posed, but we introduce a new class of well-posed hyperbolic boundary problems: that of weakly dissipative boundary conditions. We then show that our system belongs to this class and is therefore well posed.
Among 3-dimensional sets of given Newtonian capacity, which shape minimizes the q-th moment (q>0) of electrostatic equilibrium measure? One readily shows it is the ball. But what if the set is confined to the plane? A centered disk is then the natural minimizer, yet the proof is quite different and involves a cylindrical variant of Baernstein’s star-function. The approach succeeds when 0 <q <= 2. Higher moments (q>2) remain a tantalizing open problem, as do the analogous questions for Riesz equilibrium measures.
Note: this talk does not assume any previous knowledge about capacities.
(Joint work with Carrie Clark, Univ. of Illinois Urbana–Champaign.)
The topic of the talk is existence of weak solutions to the pressureless Navier-Stokes system with nonlocal attraction--repulsion forces. We construct the solutions on the whole three-dimensional space, assuming that the viscosity coefficients are density-dependent. For the nonlocal term it is further assumed that the interaction kernel has the quadratic growth at infinity and almost quadratic singularity at zero. The main point of the construction is the derivation of the analogs of the Bresch--Desjardins and Mellet--Vasseur estimates in the nonlocal setting.