IMB > Recherche > Séminaires
# Séminaire Calcul Scientifique et Modélisation

Le 23 novembre 2017
à 14:00
*Salle 2*
Pas de séminaire (Modélisation et Analyse des phénomènes dispersifs, 70 ans de J.-C. Saut)

Le 30 novembre 2017
à 11:00
*Salle 2*
Birte Schmidtmann
**Reconstruction Techniques and Riemann Solvers for Finite Volume Methods / Techniques de Reconstructions et Solveurs de Riemann pour les Méthodes de Type Volumes Finis**
We are interested in the numerical solution of hyperbolic conservation laws on the most local compact stencil consisting of only nearest neighbors. In the Finite Volume setting, in order to obtain higher order methods, the main challenge is the reconstruction of the interface values. These are crucial for the definition of the numerical flux functions, also referred to as the Riemann solver of the scheme.Often, the functions of interest contain smooth parts as well as discontinuities. Treating such functions with high-order schemes may lead to undesired oscillations. However, what is required is a solution with sharp discontinuities while maintaining high-order accuracy in smooth regions. One possible way of achieving this is the use of limiter functions in the MUSCL framework which switch the reconstruction to lower order when necessary. Another possibility is the third-order variant of the WENO family, called WENO3.In this work, we will recast both methods in the same framework to demonstrate the relation between Finite Volume limiter functions and the way WENO3 performs limiting. We present a new limiter function, which contains a decision criterion that is able to distinguish between discontinuities and smooth extrema. Our newly-developed limiter function does not require an artificial parameter, instead, it uses only information of the initial condition.We compare our insights with the formulation of the weight-functions in WENO3. The weights contain a parameter ε, which was originally introduced to avoid the division by zero. However, we will show that ε has a significant influence on the behavior of the reconstruction and relating the WENO3 weights to our decision criterion allows us to give a clarifying interpretation.In a second part, we will review some well-known Riemann solvers and introduce a family of incomplete Riemann solvers which avoid solving the eigensystem. Nevertheless, these solvers still reproduce all waves with less dissipation than other methods such as HLL and FORCE, requiring only an estimate of the globally fastest wave speeds in both directions. Therefore, the new family of Riemann solvers is particularly efficient for large systems of conservation laws when no explicit expression for the eigensystem is available.Joint work with: M. Torrilhon (RWTH Aachen University), B. Seibold (Temple University, Philadelphia), Rémi Abgrall (University of Zurich), Pawel Buchmüller (Universität Düsseldorf )

Le 7 décembre 2017
à 14:00
*Salle 2*
Simon Labarthe
**A mathematical model of the human gut microbiota in its environment**
The human gut harbors a complex bacterial community that maintains a symbiotic relationship with its host. An increasing number of studies highlight its implication in the maintain of the host's health, but also in various disorders such as inflammatory bowel disease, allergic or metabolic disorders. We propose to integrate in the same model different micriobiological or biophysical informations related to the microbiota structure and functions and to the gut environment. A population dynamics model of functional microbial populations involved in fibre degradation is coupled to a fluid mechanic model of the intestinal fluids. This model is simplified through asymptotic analysis and is used to study the mechanisms that impact the spatial structure of the gut microbiota.

Le 21 décembre 2017
à 14:00
*Salle 2*
Tommaso Taddei
**Model order reduction methods for Data Assimilation: simulation-based approaches for state estimation, and damage identification**
I present work toward the development of Model Order Reduction (MOR) techniques to integrate (i) parameterized mathematical models, and (ii) experimental observations, for prediction of engineering Quantities of Interest (QOIs). More in detail, I present two Simulation-Based approaches — the PBDW approach to state estimation, and the SBC approach for damage identification — that map observations to accurate estimates of the QOI, without estimating the parameters of the model. PBDW and SBC rely on recent advances in MOR to speed up computations in the limit of many model evaluations, and/or to compress prior knowledge about the system coming from the parameterized model into low-dimensional and more manageable forms. In the last part of the talk, motivated by the extension of PBDW and SBC to Fluid Mechanics problems, I present a MOR technique for long-time integration of parameterized turbulent flows. The approach corrects the standard Galerkin formulation by incorporating prior information about the attractor, and relies on an a posteriori error indicator to estimate the error in mean flow prediction.

Le 11 janvier 2018
à 14:00
*Salle 2*
Luca Gerardo Giorda

Le 25 janvier 2018
à 14:00
*Salle 2*
Elie Bretin

Le 8 février 2018
à 14:00
*Salle 2*
Yannick Privat

Le 1er mars 2018
à 14:00
*Salle 2*
Juliette Venel

Le 15 mars 2018
à 14:00
*Salle 2*
Mădălina Petcu

Le 29 mars 2018
à 14:00
*Salle 2*
Andrés Castillo

Le 12 avril 2018
à 14:00
*Salle 2*
Laurent Seppecher

Le 26 avril 2018
à 14:00
*Salle 2*
Michel Bergmann

Le 17 mai 2018
à 14:00
*Salle 2*
Créneau disponible

Le 31 mai 2018
à 14:00
*Salle 2*
Créneau disponible

Le 14 juin 2018
à 14:00
*Salle 2*
Créneau disponible

Le 28 juin 2018
à 14:00
*Salle 2*
Créneau disponible

Responsables : Annabelle Collin et Maria Kazolea. En alternance avec le séminaire Image Optimisation et Probabilités.