Juillet 2018 à Marseille : 16th International Workshop on the Physics of Compressible Turbulent Mixing (IWPCTM)
Site web : http://www.iwpctm16.fr/
Evènement organisé par les membres de l’axe de Recherche ECOCI
Juillet 2018 à Marseille : 16th International Workshop on the Physics of Compressible Turbulent Mixing (IWPCTM)
Séminaire IUSTI – 18 oct. 2019 – 11h salle 250
Viscoplastic or yield stress materials can behave like solids or fluids. Such materials, if not sufficiently stressed, behave like an elastic solid, but once the stress exceeds a critical value (the yield stress), the material deforms like a viscous fluid (typically with a nonlinear viscosity). I will discuss the effect of the yield stress on spreading droplets. I use experiments, asymptotic solutions, and numerical simulations to explain the dynamics and final shape of the droplets. Later, I will show how one can externally control the shape of a droplet, using temperature. For that, I will first present the rheological properties of a thermo-responsive material that undergoes sol(Newtonian)-gel(yield stress) transition upon heating. Then, I show the final diameter of a thermo-responsive droplet can be controlled by simply changing the surface temperature. In the same part of the thesis, we introduce a novel experimental method based on optical coherence tomography to identify the solidified region inside a droplet. Eventually, I will briefly discuss the other applications of viscoplastic droplets.
Séminaire exceptionnel – 15 oct. 2019 – 15h salle 259
Séminaire fédération – 4 oct. 2019 – 11h Amphithéâtre Fermi
Yves Couder and Emmanuel Fort discovered that droplets walking on a vibrating fluid bath exhibit several features previously thought to be exclusive to the microscopic, quantum realm. These walking droplets propel themselves by virtue of a resonant interaction with their own wave field, and so represent the first macroscopic realization of a pilot-wave system of the form proposed for microscopic quantum dynamics by Louis de Broglie in the 1920s. New experimental and theoretical results allow us to rationalize the emergence of quantum-like behavior in this hydrodynamic pilot-wave system in a number of settings, and explore its potential and limitations as a quantum analog.
Séminaire IUSTI – 20 sept. 2019 – 11h salle 250
Colloidal suspensions are ubiquitous in our daily life. Micrometric particles dispersed in a solvent are indeed present in common liquids such as paints, inks or even food products. We will discuss the properties of those colloidal suspensions from their liquid phase to solid deposits after drying. First, colloidal suspensions exhibit a wide range of rheological behaviors from shear-thinning to yield stress fluids. We will focus on the shear-thickening transition as dense suspensions experience a dramatic increase in viscosity above a critical shear-stress. By modifying the physico-chemistry of the particles, we can tune this rheological transition and thus understand the interactions involved in this behavior. Increasing concentration can also be noticed during drying when solvent evaporates: particles finally form a solid deposit. After drying, a drop of a colloidal suspension leads to a variety of patterns from coffee-stain to more homogeneous coatings in paintings. We will discuss the effect of the initial concentration of particles on the drying pattern and on the subsequent mechanical instabilities such as cracks propagation. Finally, after the whole drying of the colloidal suspension, coatings are achieved. Depending of the nature of the particles, we can tune the wettability of the substrate up to superhydrophobic solid. We will briefly discuss how such a water-repellent substrate can allow levitation of liquids.
Séminaire IUSTI – 6 sept. 2019 – 11h salle 250
In this talk I will present the results of two independent projects regarding the physics of phytoplankton. The first project deals with a very large class of non-motile phytoplankton called diatoms. Despite lacking motility we’ll see how these elongated unicellular organisms manage to encounter each other when sinking in quiescent fluids thanks to a hydrodynamically-driven density instability (instability well-studied at the IUSTI lab a few years back!). Our results pave the way for further investigating the yet poorly understood sexual reproduction that these globally-dominant organisms must occasionally go through to ensure the survival of the population. The second project is about the self-generated flow-fields that swimming microorganisms produce when swimming near surfaces. As opposed to previous theoretical predictions, we’ll show how the presence of no-slip boundaries enhances the diversity of microbial flow-fields compared to the bulk situation. Such results should have a large impact in e.g. our understanding of the emergence of collective behaviour of active suspensions in confined situations.
Séminaire exceptionnel – 2 sept. 2019 – 11h salle 252
Depuis 2004 le dispositif Apprentis Chercheurs permet à des collégiens et lycéens associés en binômes d’être accueillis régulièrement tout au long de l’année scolaire dans des laboratoires de recherche de leur quartier. Ils y mènent un projet scientifique, s’initient à la recherche et découvrent l’écosystème de la recherche, en immersion.
Au mois de juin ils présentent leurs travaux lors d’un congrès regroupant à Marseille tous les Apprentis Chercheurs Marseillais.
Cette année, le congrès apprentis Chercheurs s’est tenu à l’Institut de Microbiologie de la Méditerranée (IMM) en présence de Monsieur Nicolas CLAIRE, Vice Président Délégué à la Culture Scientifique et à la Recherche d’Aix-Marseille Université. Voir le programme ici
Le public était au rendez-vous (près de 120 personnes), les présentations de haute volée, les parents fiers de leurs enfants. L’intervention de deux anciennes apprenties (promotion 2017-2018) était pleine de maturité et de recul. Et tout cela dans l’ambiance chaleureuse et conviviale qui caractérise ce congrès.
L’IUSTI, participant depuis la première année, a présenté des travaux intitulés : “Les ondes de choc peuvent-elles êtres les antibiotiques de demain ? “
Outre le quotidien La Provence, un journaliste de Science et Avenir était également présent en “repérage” : il souhaite faire l’année prochaine un reportage, avec visite en labo, pour leur future revue Science et Avenir Junior…
Séminaire exceptionnel – 4 juil. 2019 – 11h salle 250
What do a guitar string and a balloon have in common? They are both floppy unless rigidified by geometric incompatibility. The same kind of rigidity transition in under-constrained materials has more recently been discussed in the context of disordered biopolymer networks like collagen and models for biological tissues. Here we show that these materials exhibit generic elastic behavior close to the rigidity transition, which is independent of the microscopic structure and the disorder in the system. Phrasing the condition of geometric incompatibility in terms of a minimal length function, we obtain analytic expressions for the elastic stresses and moduli. We numerically verify our findings by simulations of under-constrained spring networks as well as 2D and 3D vertex models for dense biological tissues. For instance, we analytically show that the ratio of the excess shear modulus to the shear stress is inversely proportional to the critical shear strain with a prefactor of three, which we expect to be a general hallmark of rigidity in under-constrained materials induced by geometric incompatibility. This could also be used in experiments to distinguish whether strain-stiffening as observed for instance in biopolymer networks arises from nonlinear characteristics of the microscopic material components or from effects of geometric incompatibility.
(La Société Française de Thermique a mis en place le prix Biot-Fourier qui récompense la meilleure communication scientifique lors de son congrès annuel.)
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.PDF Embedder requires a url attribute
Séminaire régulier IUSTI – 24 juin 2019 – 11h salle 250
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.