Global-in-time solutions for the isothermal Matovich-Pearson equations

In this paper we study the Matovich-Pearson equations describing the process of glass fiber drawing. These equations may be viewed as a 1D-reduction of the incompressible Navier-Stokes equations including free boundary, valid for the drawing of a long and thin glass fiber. We concentrate on the isothermal case without surface tension. Then the Matovich-Pearson equations represent a nonlinearly coupled system of an elliptic equation for the axial velocity and a hyperbolic transport equation for the fluid cross-sectional area. We first prove existence of a local solution, and, after constructing appropriate barrier functions, we deduce that the fluid radius is always strictly positive and that the local solution remains in the same regularity class. To the best of our knowledge, this is the first global existence and uniqueness result for this important system of equations

Linear stability and positivity results for a generalized size-structured Daphnia model with inflow§

We employ semigroup and spectral methods to analyze the linear stability of positive stationary solutions of a generalized size-structured Daphnia model. Using the regularity properties of the governing semigroup, we are able to formulate a general stability condition which permits an intuitively clear interpretation in a special case of model ingredients. Moreover, we derive a comprehensive instability criterion that reduces to an elegant instability condition for the classical Daphnia population model in terms of the inherent net reproduction rate of Daphnia individuals

Asymptotic analysis of a size-structured cannibalism model with infinite dimensional environmental feedback

In this work we consider a size-structured cannibalism model with the model ingredients (fertility, growth, and mortality rate) depending on size (ranging over an infinite domain) and on a general function of the standing population (environmental feedback). Our focus is on the asymptotic behavior of the system, in particular on the effect of cannibalism on the long-term dynamics. To this end, we formally linearize the system about steady state and establish conditions in terms of the model ingredients which yield uniform exponential stability of the governing linear semigroup. We also show how the point spectrum of the linearized semigroup generator can be characterized in the special case of a separable attack rate and establish a general instability result. Further spectral analysis allows us to give conditions for asynchronous exponential growth of the linear semigroup

Asymptotic behavior of size-structured populations via juvenile-adult interaction

In this work a size structured juvenile-adult population model is considered. The linearized dynamical behavior of stationary solutions is analyzed using semigroup and spectral methods. The regularity of the governing linear semigroup allows to derive biologically meaningful conditions for the linear stability of stationary solutions. The main emphasis in this work is on juvenile-adult interaction and resulting consequences for the dynamics of the system. In addition, we investigate numerically the effect of a non-zero population inflow, due to an external source of newborns, on the dynamical behavior of the system in a special case of model ingredients

Particles and fields in fluid turbulence

The understanding of fluid turbulence has considerably progressed in recent years. The application of the methods of statistical mechanics to the description of the motion of fluid particles, i.e. to the Lagrangian dynamics, has led to a new quantitative theory of intermittency in turbulent transport. The first analytical description of anomalous scaling laws in turbulence has been obtained. The underlying physical mechanism reveals the role of statistical integrals of motion in non-equilibrium systems. For turbulent transport, the statistical conservation laws are hidden in the evolution of groups of fluid particles and arise from the competition between the expansion of a group and the change of its geometry. By breaking the scale-invariance symmetry, the statistically conserved quantities lead to the observed anomalous scaling of transported fields. Lagrangian methods also shed new light on some practical issues, such as mixing and turbulent magnetic dynamo.Comment: 165 pages, review article for Rev. Mod. Phy

Mathematical Biology

Mathematical biology is a fast growing field of research, which on one hand side faces challenges resulting from the enormous amount of data provided by experimentalists in the recent years, on the other hand new mathematical methods may have to be developed to meet the demand for explanation and prediction on how specific biological systems function

Commande des Systèmes Hyperboliques décrits par des Equations aux Dérivées Partielles

This work is part, from a theoretical point of view, of the control of systems describedby partial differential equations (PDE). The other aspect is the application of those resultsto real process’s applications.Great developments have been done on modelization technics, identification and thecontrol for systems in finite dimension since a long time. Those technics have reached amaturity level, and are applied to numerous applications. Nevertheless, the developmentof advanced technologies have for consequence to increase the size of the controlledmodels, rising which is the symbol of the passage from finite dimension system to infinitedimension system.In recent decades, a real work on the development of infinite dimension tools hasemerged. This work originally dedicated to rather academic cases are being extendedtoday to practical cases.My work has found its place at this level : since 10 years I am interested in stabilityproblems and in the development of controls for systems described by hyperbolic PDE.For this I use mathematical structures such as semigroups, "natural" invariants like theRiemann invariants, energy structures like the Hamiltonian functional, or by the extensionof existing results in finite dimension to the infinite dimension such for the LMI (LinearMatrices Inequalities) to LOI (Linear Operator Inequalities).All these theoretical results have no interest if they are not applied, at least that’sthe goal I would like to maintain. To this end, all results have been developed on realprocesses : irrigation channels, navigable waterways, extrusion process, and more tocome. The issue of water described by the shallow water equations is a central examplein my work, but this is simply because I have access to benchmarks allowing me tovalidate the developed approaches.All of my works has been published internationally, but also broadcasted on lessonsfrom doctoral schools, in training of masters students and PhD students.Ce travail s’inscrit, d’un point de vue théorique, dans le domaine du contrôle dessystèmes décrits par des équations aux dérivées partielles (EDP). L’autre versant de cetravail est l’application concrète à des procédés.Un grand effort de développement des techniques de modélisation, d’identification etde commande a été réalisé pour les systèmes de dimension finie depuis des années.Ces techniques ont atteint un certain degré de maturité et sont utilisées dans de nombreusesapplications. Néanmoins, les développements des technologies de pointes ontentrainé une hausse considérable de la taille des modèles de commande, hausse quiest le reflet dans beaucoup de cas, du passage de la commande d’un vrai système dedimension finie vers un système de dimension infinie.Depuis quelques décennies, un réel travail de développement des outils en dimensioninfinie a donc vu le jour. Ces travaux initialement dédiés à des cas plutôt académiquesse voient aujourd’hui étendus à des cas pratiques.Mes travaux se posent à ce niveau : depuis 10 ans je m’intéresse aux problèmes destabilité et au développement de commandes de systèmes décrits par des EDP hyperboliques.Pour cela, j’utilise des structures mathématiques telles que les semigroupes,des invariants "naturels" comme ceux de Riemann, des structures énergétiques commeles Hamiltoniens, ou par l’extension de résultats existants en dimension finie à la dimensioninfinie comme pour les LMI (linear matrices inequalities) en LOI (linear operatorinequalities).Tous ces résultats théoriques n’ont d’interêt que s’ils sont appliqués, du moins c’estl’objectif que je souhaite maintenir. A cette fin, tous les résultats ont été développés surde réels process : les canaux d’irrigation, les voies naviguables, l’extrusion, et d’autres àvenir. La problématique de l’eau décrite par les équations de Saint-Venant est certe unexemple central dans mon travail, mais cela est dû simplement au fait que j’ai accès àdes bancs d’essais me permettant de valider les approches développées.L’ensemble de mes travaux a été publié au niveau international, mais aussi diffusé enlocal lors d’enseignements auprès d’écoles doctorales, lors d’encadrement de mastersrecherche et de thésards