(La Société Française de Thermique a mis en place le prix Biot-Fourier qui récompense la meilleure communication scientifique lors de son congrès annuel.)

Mickaël Le Bohec received the Biot-Fourier prize of the French Heat Transfer Society

Title : “Model reduction of radiative heat transfer in buildings by a hierarchical radiosity method”

Texte : In buildings, radiative heat transfers can be idealized by the radiosity equation. This one needs the evaluation of geometrics couplings between the elements of the scene called form factors or view factors. It’s generally hard to get them, especially when there are obstructions. Beyond the evaluation of those factors, the algebraic system is difficult to solve because each surface interacts, usually, with all the others and because the number of nodes required for the description of a complexe scene is important. We present a resolution method which refines the mesh of the scene while constructing a multi scale representation of form factors between its elements, in order to avoid having to compute all the transfers at the finest resolution. This drastically reduces the computation time and allows to use this method in an industrial development process.

Credits : This work was supervised by Denis Lemonnier (Senior Researcher, CNRS) and Didier Saury (Professor of University, P’ Institute) and funded by the ANRT and the Groupe Atlantic.

Label : This solution was obtained in 52 min while several days would be necessary with a full matrix method.

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Séminaire régulier IUSTI – 24 juin 2019 – 11h salle 250

Direct Numerical Simulation of Pore-Scale Turbulence: Multiscale Statistics and Upscaling

Sourabh V. Apte – Oregon State Univ., ÉU

Turbulent flows in porous media occur in a wide variety of applications, from catalysis in packed beds to heat exchange in nuclear reactor vessels. In this talk, we will review some of the recent developments in characterizing turbulence in porous media and using the data to develop volume averaged, upscaled models. The porescale flow physics, which are important to properties such as bulk mixing performance and permeability, are investigated in detail using direct numerical simulation (DNS) through a periodic face centered cubic (FCC) unit cell, covering inertial through fully turbulent regime. This low porosity arrangement of spheres is characterized by rapid flow expansions and contractions, and results an early onset to turbulence. The simulations are performed using a fictitious domain approach [Apte et al, J. Comp. Physics 2009], which uses non-body conforming Cartesian grids, with very high resolution. Results are used to investigate the structure of turbulence in the Eulerian and Lagrangian frames, the distribution and budget of turbulent kinetic energy, and the characteristics of vorticity and helicity distribution in complex packed beds. In addition, Lagrangian statistics of scale dependent curvature angle are calculated by tracking a large number of fluid particle trajectories and compared with homogeneous, isotropic turbulence to understand the effects of flow confinement on turbulence in porous media. A Monte-Carlo based stochastic model to predict the long-time behavior of curvature angles is developed and shown to correctly predict the asymptotic behavior as obtained from DNS. Finally, the porescale data is used to close a spatially-averaged upscaled model. A Darcy-Forchheimer type law is derived, and a prior computation of the permeability and Forchheimer coefficient are presented and compared with existing data.

Séminaire régulier IUSTI – 14 juin 2019 – 11h salle 250

Une nouvelle approche pour la modélisation des vagues côtières

Gaël Richard – LEGI, Grenoble

La présentation portera sur un nouveau modèle de vagues côtières dérivé en moyennant sur la profondeur les équations de la simulation des grandes structures de la turbulence. Les grandes échelles de la turbulence sont explicitement résolues et prises en compte par un tenseur d’enstrophie alors que les petites échelles sont modélisées par une hypothèse de viscosité turbulente. Les propriétés dispersives sont analogues à celles des équations de Serre-Green-Naghdi et peuvent être améliorées par les mêmes techniques. La résolution numérique repose sur une reformulation asymptotiquement équivalente des équations permettant la mise en oeuvre de la méthode à diagonale constante et un schéma de type Galerkin discontinu. Les tests de validation du modèle les plus récents seront présentés, notamment le comportement de l’énergie turbulente dans des cas de déferlement glissant et plongeant ou encore des simulations numériques de propagation de trains de vagues aléatoires.

Séminaire régulier IUSTI – 17 mai 2019 – 11h salle 250

Models for Radiative Heat Transfer and Radiation–Turbulence Interactions in Combustion Systems

Michael Modest – UC Merced, ÉU

Predicting radiative transfer rates in high-temperature reacting flows is an extremely difficult task, because the radiative transfer equation (RTE) is a six-dimensional integro-differential equation, and is exacerbated by the fact that radiative properties in reacting flows undergo extreme fluctuations across the electromagnetic spectrum. The problem encounters additional levels of difficulty in high-temperature plasmas, as found in the hypersonic shock layer in front of spacecraft entering an atmosphere, leading to thermochemical nonequilibrium, and in the presence of turbulence, leading to highly-nonlinear interaction terms. This talk will concentrate on spectral modeling of radiation from combustion gases and plasmas. The nature of radiative properties of high-temperature gases and plasmas will be reviewed, followed by a discussion of traditional as well as modern spectral methods, with emphasis on recent work carried out by the speaker in the field of combustion. Finally, some application examples will be presented.

Séminaire régulier IUSTI – 3 mai 2019 – 11h salle 250

Water waves: a unique medium to study exotic wave physics

Sander Wildeman – ESPCI, Paris

“[Water waves] that are easily seen by everyone and which are usually used as an example of waves in elementary courses […] are the worst possible example […]; they have all the complications that waves can have,” Richard Feynman remarked in his famous lectures. In this talk I will show that if one stays away from some of these complications, water waves can offer a versatile playground to study elementary and exotic wave physics. First I will discuss a new digital Schlieren technique which allows for (almost) effortless real-time quantitative measurement of surface waves. I will show several studies where this technique has recently been applied in our lab: from measuring the wave field around bouncing oil droplets, to visualizing the wake behind supervelocity rotating wave sources. Finally, I will discuss how one can steer water waves using strong electric fields, opening up the possibility to simulate wave propagation through exotic materials with space and time varying refractive index in a way that would be hard to achieve with other types of waves (such as light or sound).

Séminaire régulier IUSTI – 5 avril 2019 – 11h salle 250

Morphing soft structures with instabilities

Joël Marthelot – IUSTI

Fracture and buckling of slender structures are typically regarded as a first step towards failure. Instead, we envision mechanical and interfacial instabilities in structures as opportunities for scalable, reversible, and robust mechanisms that are first to be predictively understood, and then harvested for function. I will first show how delamination and fracture cooperate in thin films leading to the propagation of robust fracture patterns that offer opportunities to use cracks as a tool to design surfaces at small scales. I will then focus on thin elastic shells, where periodic dimpled patterns are observed when the shell is constrained from within by a rigid mandrel. We find that the geometry of the system is central in setting the surface morphology. This prominence of geometry suggests a scalable, and tunable mechanism for reversible shape-morphing of spherical shells. Finally, I will move on to interfacial instabilities in liquid elastomeric coating, harnessing Rayleigh-Taylor instabilities to spontaneously fabricate solid structures at the materials level.

Séminaire régulier IUSTI – 22 mars 2019 – 11h salle 250

Écoulements et transferts dans la microcirculation sanguine cérébrale

Sylvie Lorthois – IMFT, Toulouse

Après une introduction montrant le rôle central de la microcirculation cérébrale dans le fonctionnement, et certains dysfonctionnements, du cerveau, je présenterai l’architecture du réseau microvasculaire cérébral, et montrerai qu’il est la superposition de deux types de structures : une structure capillaire maillée, homogène au-dessus d’une longueur de coupure correspondant à la longueur caractéristique des vaisseaux capillaires (~50 µm), et des structures arborescentes fractales, composées des artères et des veines. M’appuyant sur ces résultats, je présenterai les approches que nous développons pour l’étude des écoulements sanguins et/ou des transferts de masse à différentes échelles, dont la plupart s’inspirent de méthodologies développées pour l’étude d’écoulements multiphasiques ou réactifs en milieux poreux. Enfin, je présenterai quelques perspectives en lien avec le rôle de la microcirculation cérébrale dans les maladies neuro-dégénératives.

Séminaire régulier IUSTI – 8 mars 2019 – 11h salle 250

Homogénéisation asymptotique par couche limite de surfaces à résonance interne en acoustique et en élastodynamique

Logan Schwan – LAUM, Le Mans

La présentation s’intéresse aux surfaces ou interfaces structurées à résonance interne dans le domaine de l’acoustique ou de l’élastodynamique. Ces surfaces sont constituées de la répétition 2D-périodique d’une cellule unitaire surfacique ou volumique comportant des éléments résonants. Ces résonances peuvent être de nature diffusive (système parabolique) ou elasto-inertielle (système hyperbolique). Sous sollicitation de grande longueur d’onde (séparation d’échelles), la description macroscopique effective de telles surfaces est déterminée ici par la théorie de l’homogénéisation asymptotique adaptée à l’analyse des couches limites se développant à leur voisinage. En particulier, des effets non-conventionnels de dépolarisation d’ondes mécaniques, d’absorption totale, d’effets mémoire ou de comportement non-local seront prédits théoriquement et mis en évidence expérimentalement ou numériquement.

Séminaire exceptionnel – 7 mars 2019 – 10h salle 250

First-passage time of non-markovian walkers and instability in the actin cortex

Nicolas Levernier – Univ. Genève, Suisse

My presentation will consist in the presentation of two independent studies. The first one deals with first-passage times (FPT) of non-markovian walkers. How long does it take for a random walker to find a target ? This question appears naturally in many context and at many scales. After a brief summary of existing results, which essentially concern Markovian (meaning memory-less) random walks, I will present the theory we have developed to deal with non-markovian random walks. I will show that, strikingly, the mean first-passage time for such a walk is fully determined by the trajectory of the walker after the FPT. Finally, I will show some application of this theory to known examples, such as the Fractional Brownian Motion. The second part will present recent works of my postdoc, and deals with a simple model of actin cortex. The actin cortex is a thin layer of actin polymers, created on the inner face of the cell membrane, and vital for the cell. I will show that describing the cortex as an extended object (that is, not using thin layer approaches), an interesting instability appear. This instability can lead directly to spatio-temporal chaos in the actin density, a feature that cannot be accounted in effective one-dimensional approaches.

Séminaire régulier IUSTI – 8 févr. 2019 – 11h salle 250

Why does actomyosin contract?

MArtin Lenz – LPTMS, Paris

The motion of living cells is in large part due to the interaction of semi-flexible actin filaments (F-actin) and myosin molecular motors, which induce the relative sliding of F-actin. It is often assumed that this simple sliding is sufficient to account for all actomyosin-based motion. While this is correct in our highly organized striated muscle, we question the application of this dogma to less ordered actomyosin systems, thus reexamining a cornerstone of our understanding of cellular motion.

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