Programme JEFS 2023

Numerical modelling of Plateau-Rayleigh instability in capillary tubes of fuel cells

* Matthieu Rykner (Université Paris-Saclay, CEA - DES/ISAS/DM2S/SGLS/LCAN)

Elie Saikali (Université Paris-Saclay, CEA - DES/ISAS/DM2S/SGLS/LCAN)

Adrien Bruneton (Université Paris-Saclay, CEA - DES/ISAS/DM2S/SGLS/LCAN)

Benoît Mathieu (CEA - DRT/LITEN/DEHT/SAMA/LMP)

Vadim Nikolayev (Université Paris-Saclay, CEA - DRF/IRAMIS/SPEC/SPHYNX)

Proton Exchange Membrane Fuel Cell (PEMFC) is a promising technology to decarbonize and electrify the transportation and many other energy systems. Electricity is produced by recombining oxygen and hydrogen that are fed via gas flow channels (GFC) in the two opposite sides of the cell. A porous medium separates the GFC from a central membrane where the electrochemical reactions occur. Water should be evacuated via the GFC. However, as the functioning temperature is low (60-80°C), water can be observed both as liquid and vapor, creating a two-phase flow in the millimetric-size GFC. Unwanted formation of liquid plugs (i.e. channel clogging) is then possible due to the Plateau-Rayleigh instability, which leads to species transport limitations, additional pressure losses and fuel cell accelerated degradation. Understanding the conditions of plug formation/evacuation is the aim of this work. The Front-Tracking module of the open-source TRUST-TrioCFD code is used to perform 2D-axisymmetrical simulations of liquid water slug formation in a model problem of a circular capillary tube. A model based on the lubrication approximation has also been developed to validate the numerical results for small Re. This preliminary work will be followed by the development of a 1D macroscopic two-phase model to be coupled to models of other PEMFC components.

Influence du vortex sur la vidange classique de Torricelli

* Aurore Caquas (CEA, Service de Thermo-hydraulique et de Mécanique des Fluides / Unité de Mécanique, ENSTA Paris, Institut Polytechnique de Paris)

Luc Pastur (Unité de Mécanique, ENSTA Paris, Institut Polytechnique de Paris)

Alain Genty (Université Paris-Saclay, CEA, Service de Thermo-hydraulique et de Mécanique des Fluides)

Philippe Gondret (Université Paris-Saclay, CNRS, Laboratoire FAST)

Lorsque nous vidons un récipient, il est très courant de voir un vortex se former. Comment la vidange est-elle modifiée par la présence de ce vortex ? Pour répondre à cette question, nous étudions expérimentalement la vidange d'un réservoir en rotation. Le fluide dans le réservoir est d'abord mis en rotation solide avant que la vidange ne soit déclenchée. Nous montrons que la vitesse de vidange peut être considérablement réduite par le vortex généré par la mise en rotation. Nous démontrons que la loi de vidange habituelle de Torricelli doit être modifiée avec la déformation de surface et que le temps de vidange est principalement régi par un paramètre sans dimension correspondant au rapport entre la taille de la sortie et l'épaisseur de la couche limite d'Ekman. * A. Caquas, L. R. Pastur, A. Genty, and P. Gondret Phys. Rev. Fluids 8, 044702 – Published 25 April 2023

Dynamiques bistables markoviennes en décrochage d'une aile

* Ivan KHARSANSKY ATALLAH (ENSTA PARIS et CentraleSupelec)

Luc PASTUR (ENSTA PARIS)

Romain MONCHAUX (ENSTA PARIS)

Laurent ZIMMER (CentraleSupelec)

Nous étudions expérimentalement la dynamique de décrochage d'un profil symétrique mince (NACA0012) pour des nombres de Reynolds Re allant de 50 000 à 110 000. Nous confirmons que des oscillations à basse fréquence peuvent être observées dans la gamme la plus basse des nombres de Reynolds, comme cela a été décrit dans plusieurs études numériques et expérimentales. Nous avons découvert qu'au-delà d'une valeur critique du nombre de Reynolds, la dynamique de l'aile devient bi-stable intermittente et remplace les oscillations à basse fréquence. Dans ce nouveau régime, la boucle d'hystérésis attendue n'est pas observée en raison de la dynamique intermittente, qui relie les deux branches de solutions pour un angle d'attaque fixe. À notre connaissance, cette dynamique bi-stable intermittente n'a encore jamais été décrite dans la dynamique de décrochage d'une aile. Elle est caractérisée par des transitions aléatoires sans mémoire entre les états d'écoulements complètement attaché et complètement détaché ; elle peut être décrite par la théorie des chaînes de Markov continues et des événements rares.

Turbulence d'ondes internes dans un fluide stratifié, avec et sans modes propres du domaine fluide

* N. Lanchon (FAST, Université Paris-Saclay & CNRS)

D.O. Mora (FAST, Université Paris-Saclay & CNRS)

E. Monsalve (FAST, Université Paris-Saclay & CNRS)

P.-P. Cortet (FAST, Université Paris-Saclay & CNRS)

L’une des caractéristiques des modèles océaniques globaux est l'utilisation de paramétrisations rendant compte de la dynamique aux « petites échelles », auxquelles il est attendu que la stratification du fluide et les ondes internes associées jouent un rôle crucial. La théorie de la turbulence faible d’ondes internes apparaît comme une piste séduisante pour décrire cette dynamique océanique à petite échelle qui pourrait ouvrir la voie à des améliorations des modèles océaniques s’appuyant sur une théorie physique. Néanmoins, son développement se heurte à des difficultés analytiques et la pertinence des prédictions reste incertaine. Les expériences favorisant l'émergence d'un écoulement dans le régime de turbulence d'ondes internes sont donc d'un grand intérêt pour guider les travaux théoriques. Nous présenterons des expériences visant à atteindre ce régime qui est resté jusqu’ici globalement inaccessible en laboratoire. Ref : N. Lanchon, D.O. Mora, E. Monsalve, P.-P. Cortet, Physical Review Fluids 8 054802 (2023)

Approximation différentielle pour la turbulence 2D

* Pierre Morel (Université Paris-Saclay, Laboratoire de Physique des Plasmas)

Özgür D. Gürcan (CNRS, Laboratoire de Physique des Plasmas)

Un modèle d’approximation différentielle des transferts turbulents d’énergie est proposé pour la turbulence 2D incompressible, couplant équation de Navier-Stokes (en formulation potentielle), et scalaire passif advecté par l’écoulement potentiel [1]. Ces équations sont appelées équations de Hasegawa-Wakatani dans le cadre des plasmas fortement magnétisés. En prenant la limite continue des modèles en couches, ou en nous basant sur des arguments dimensionnels, nous dérivons une approximation différentielle très générale, pouvant s’appliquer à la dynamique de n’importe quel champ scalaire, et dont la troncature de Navier-Stokes est vue comme un cas particulier.

Structure Dominated Two-Dimensional Turbulence : Formation, Dynamics and Interactions of Dipole Vortices

* Özgür D. Gürcan (Laboratoire de Physique des Plasmas, CNRS)

Two dimensional turbulence in geophysical fluids and plasma physics tends to be spotty, intermittent and rich in large scale structures such as coherent vortices or zonal flows, due to various mechanisms of self organization. Nonlinear solutions that rely on the vanishing of nonlinearity, especially the dipole vortex solution, stand out as key aspects of this structure dominated turbulence state. Using numerical simulations, it is demonstrated that an initial condition with a small number of high intensity turbulent patches, evolves towards a state dominated by coherent structures, and in particular dipole vortices, as each patch is organized into a finite number of dipole vortices that are ejected from this initially active region. In order to study the details of this process, an initial condition of two Gaussian peaks of the stream function is considered, and it was shown to result in a Chaplygin-Lamb dipole if the peaks have the same amplitude, or a Flierl-Stern-Whitehead dipole that rotates in the direction implied by the excess of vorticity if they do not. Analytical estimates for the velocity, the radius and the radius of curvature of the resulting dipole vortex is given in terms of the peaks and widths of the initial conditions. These are then verified by a detailed comparison of the analytical form of the vorticity of the dipole vortex and its numerical realization. It is argued that since these coherent structures are spared from the strong shear forces normally exerted by the nonlinearities, and can coexist with other localized solutions, or large scale flow patterns, they provide the backbone of the structure dominated or ``sporadic'' turbulent state in two dimensions, on top of which other structures, waves and instabilities can develop. In order to elucidate these, a number of collision scenarios are considered. It is also shown that a simple two point vortex approximation to a dipole vortex seems to be appropriate for describing their evolution far from each-other, or for computing head on collisions between two or more dipole vortices, but not in the case of close or grazing collisions or their interaction with a nontrivial large scale flow.

Mesures 2D de température dans un écoulement proche paroi avec particules luminescentes

Guangtao Xuan (Université de Magdebourg, Allemagne)

Luming Fan (Conseil National de la Recherche du Canada, Ottawa, US)

Frank Beyrau (Université de Magdebourg, Allemagne)

* Benoît Fond (Departement d'Aerodynamique, Aeroelasticité et Acoustique, ONERA)

Nous présentons ici une méthode de mesure basée sur des particules solides luminescentes pour des mesures 2D de température dans des écoulements en proche paroi avec une résolution de l’ordre du micron. Des particules sub-micrométriques sont ensemencées dans l’écoulement, et sous excitation par laser UV, ces particules forment des images luminescentes sur deux caméras équipées d’une lentille macro sur un champ de vue de plusieurs centimètres. Chacune des caméras est équipée d’un filtre spectral distinct afin d’exploiter le changement de couleur de la luminescence avec la température, de telle sorte que le ratio des intensités dans les deux gammes spectrales est une fonction de la température. En ajustant les images des particules avec des fonctions gaussiennes 2D, chaque particule est localisée dans un plan 2D avec une résolution de 0.2 pixels, correspondant à quelques microns. Nous montrerons que par ces ajustements de courbes, il est possible de séparer l’émission des particules de la lumière provenant de surfaces environnantes comme la paroi dans une couche limite. Comme démonstration, des mesures sont présentées dans une couche limite formée par un jet chaud débouchant le long d’une paroi froide. La température peut être alors mesurée à une distance de 40 microns de la paroi, et le profil de température est comparé au profil théorique de couche limite laminaire pour validation.

Effet de la mobilité sur la convergence du modèle Cahn-Hilliard/Navier-Stokes dans le cas de l'instabilité de Rayleigh-Taylor

Raphael Zanella (Laboratoire PMC, École Polytechnique)

* Hervé HENRY (Laboratoire PMC, École Polytechnique)

Les modèles d'interface diffuse comme le modèle de Cahn Hilliard permettent de simuler simplement des écoulements multiphasiques. Ici par une étude numérique dans le cas de l'instabilité de Rayleigh-Taylor en lien avec des calculs asymptotiques et une interprétation physique nous montrons que le choix des paramètres du modèle (épaisseur d''interface et mobilité) doit être fait suivant certaines règles afin d'assurer une convergence optimale.

Subcritical dynamics of axisymmetric rotor-stator flow

* Artur Gesla (Sorbonne Université, LISN-CNRS, Université Paris-Saclay)

Laurent Martin Witkowski (Univ Lyon, Univ Claude Bernard Lyon 1, CNRS, Ecole Centrale de Lyon, INSA Lyon, LMFA, UMR5509)

Yohann Duguet (LISN-CNRS, Université Paris-Saclay)

Patrick Le Quéré (LISN-CNRS, Université Paris-Saclay)

Rotor-stator flows have been studied extensively in the past. There have been many experimental observations of coexistence of both circular rolls and spiral arms [1, 2]. The origin of the latter is well understood [3] while that of the former is not. We revisit this flow using numerical simulation and dynamical systems tools. For lower values of Re, at least three flow regimes are identified - base flow, turbulent state and an edge state separating the two. This edge state features several incommensurate frequencies, involves inertial waves, and, contrary to expectations, does not originate directly from the Hopf bifurcation point. [1] L. Schouveiler, P. Le Gal, and M. P. Chauve, Phys. Fluids, vol. 10, pp. 2695–2697, 1998. [2] G. Gauthier, P. Gondret, and M. Rabaud, J. Fluid Mech., vol. 386, pp. 105–126, 1999. [3] A. Y. Gelfgat, Fluid Dyn. Res., vol. 47, p. 035502(14), 2015

Earth's Sphericity and the Dynamics of Subduction

* Neil M. Ribe (CNRS, Universite Paris-Saclay)

Alexander Chamolly (Institut Pasteur, Paris)

The dynamics of subduction on Earth are strongly influenced by the sphericity of the tectonic plates. To understand this influence, we study an axisymmetric model of a dense viscous spherical shell sinking in a less viscous ambient mantle. Scaling analysis of the model reveals two key dimensionless parameters: a "flexural stiffness" that determines which viscosity controls the rate of subduction, and a "dynamical sphericity number" (DSN) that measures the importance of spherical geometry. The DSN is proportional to the cotangent of the angular radius of the shell, implying the paradoxical result that the dynamical effect of sphericity is greater for small plates than for large ones. We confirm the scaling analysis using boundary-element numerical solutions, which reveal that sphericity has an especially strong influence on the hoop (longitudinal normal) stress. This result has implications for the interpretation of the moment tensors that encode the magnitude and orientation of the fault slip that generates great earthquakes.

Building a Boiling-Flow Multiphase CFD Framework for Nuclear Reactor Conditions

* Corentin Reiss (Université Paris-Saclay, CEA, Service de Thermo-hydraulique et de Mécanique des Fluides)

Antoine Gerschenfeld (Université Paris-Saclay, CEA, Service de Thermo-hydraulique et de Mécanique des Fluides)

Catherine Colin (Institut de Mécanique des Fluides de Toulouse, Université de Toulouse, CNRS, INPT, UPS)

Multiphase CFD's predictions for boiling flow are limited by available models for interfacial heat transfer and area. Both terms are greatly interdependent. Much research has relied upon adiabatic experiments on bubbly flow to determine the contribution of coalescence and breakup terms on the interfacial area independently of heat transfer. These models are often applied to boiling flows. We develop a two-fluid Euler-Euler CFD framework based on the PolyMAC numerical scheme in CEA's open-source TrioCFD code. We implement a k-omega turbulence model, along with an original adaptive wall law treatment. Interfacial momentum closure terms are selected and validated using bubbly adiabatic experiments on vertical flows. The local experimental bubble diameter is enforced to limit interactions with interfacial area closures. We simulate the Debora experiment, an ascending boiling freon flow in a tube, and again use the experimental diameter. The long-term goal is to run simulations using independently selected coalescence-fragmentation and heat transfer closures.

Numerical modeling of accidental release of liquid hydrogen

* Y Liang (Centre Borelli, ENS Paris-Saclay)

Y Qu (Centre Borelli, ENS Paris-Saclay)

J M Ghidaglia (Centre Borelli, ENS Paris-Saclay)

L Liu (Technical Institute of Physics and Chemistry of the Chinese Academy of Sciences)

N Peng (Technical Institute of Physics and Chemistry of the Chinese Academy of Sciences)

Liquid hydrogen (LH2), generated from renewable energy sources, is now a major business to industries around the world. The wide use of LH2 transportation and storage systems also raises fresh concerns over their safety. Control of the hazards is the key ingredient for ensuring the safety of hydrogen systems and thus becomes the main content of the safety requirements for storage and transportation of LH2 standards. In order to provide an efficient and accurate for investigating accidental release scenarios involving cryogenic liquids such as LH2, a research project has been carried out to develop a 3D numerical model to simulate large-scale LH2 pool releases to examine the hydrogen behaviors and predict the LH2 pool size, downwind hazardous distance, and flammable mass of the hydrogen-air clouds formed for different environmental conditions and release scenarios. This paper presents our ongoing work of developing a new methodology for 3D multiphase study in thermal and thermodynamics by using an open-source CFD code, Open Foam. In order to validate this model, simulations are evaluated with the comparison with the NASA LH2 experimental data set [1]. The schematic diagram of NASA’s LH2 release experiment is shown in Fig. 1. LH2 flows downward from the release point. After the heating of the air and the ground, part of the liquid hydrogen evaporates rapidly to form gaseous hydrogen, and part of it forms an LH2 pool on the ground. The hydrogen vaporized into a gaseous state and mixed with air. The early stage of diffusion is the diffusion of heavy gas, and then as the temperature rises, it becomes a buoyant mixed gas, which moves downwind and upward under the action of wind and buoyancy. This process is a complex process involving heat transfer, phase transition between multiphase, diffusion and mixing of multi-species, etc. Based on the standard solver of the open-source software Open FOAM, a new solver is developed by us. Based on the standard solver rhoReactingBuoyantFoam, we refer to the file structure of icoReactingMultiphaseInterFoam and add the VOF and phase transition models. The solver we wrote that is a compressible solver that can solve continuity, momentum, energy, composition, and VOF multiphase flow equations and supports phase transition models. This solver was used to solve the LH2 leakage process. Fig.2 is the concentration contour of the LH2 leak calculated using the new solver. The detailed settings of this simulation, including boundary conditions, discrete format, solution method, geometry and mesh, etc. are explained in detail. The simulation and experiment were compared in detail, the most critical is the comparison with NASA's sixth experiment at the hydrogen concentration at the 20 s. Through various comparisons, the validity of the simulation has been fully verified. Overall, the model-observation comparison of the hydrogen concentration distribution shows a good agreement.

Reynolds Stress Anisotropy Tensor Predictions using Neural Networks

* Jiayi Cai (CEA-SACLAY, DES/ISAS/DM2S/STMF/LMSF | LISN-CNRS)

Pierre-Emmanuel Angeli (CEA-SACLAY, DES/ISAS/DM2S/STMF/LMSF)

Jean-Marc Martinez (CEA-SACLAY, DES/ISAS/DM2S/SGLS/LIAD)

Guillaume Damblin (CEA-SACLAY, DES/ISAS/DM2S/SGLS/LIAD)

Didier Lucor (LISN-CNRS)

Reynolds-Averaged Navier-Stokes (RANS) based turbulence modeling is the most widely-used approach for engineering interests due to its high cost-effectiveness. Even though, despite researchers’ continued focus, the RANS approach still suffers from a reliable closure model for the Reynolds stress anisotropy tensor. In recent years, advances in computing power have opened up a new way to tackle this problem with the aid of machine learning techniques. The main objective of the present work is to fully predict the Reynolds stress anisotropy tensor of plane channel flows and square duct flows by employing neural networks. Both interpolation and extrapolation predictive capability will be examined upon our testing flow configurations.

Etude des phénomènes de percement dans le cadre de la fusion par confinement inertiel

* Laurent Masse, CEA/DAM/DIF

Heliodore Bernard-Brunel, CEA/DAM/DIF

Ce travail s'inscrit dans le cadre des travaux, actuellement en cours au CEA/DAM, visant à obtenir l'allumage de réactions de fusion thermonucléaire au moyen de lasers de fortes puissance. On parle dans cette configuration de fusion par confinement inertiel (FCI). La FCI consiste à irradier la surface d'une capsule sphérique contenant un mélange fusible par un puissant rayonnement. L'ablation de la matière externe de la capsule engendre de formidables pressions provocant l'implosion de l'édifice. La capsule, violemment accélérée, devient le siège d'instabilité hydrodynamique du type " Rayleigh-Taylor " pouvant compromettre l'uniformité de l'implosion et l'intégrité de la capsule elle-même. Si la simulation directe de l’ensemble des phénomènes est nécessaire il est aussi avantageux de disposer de codes d’études s’attachant à ne simuler qu’une partie du problème. Dans ce contexte, un code basé sur des méthodes intégrales, a été développé pour étudier la croissance des instabilités hydrodynamiques. La précision et l’efficacité de ces méthodes permettent d’obtenir des solutions de référence à faible coût et ainsi de générer des bases de données pouvant être utilisées pour mettre en œuvre des algorithmes de machine learning. Nous présentons ici d’une part des résultats expérimentaux de croissance d’instabilité de type Rayleigh-Taylor ainsi que la méthode que nous développons pour les interpréter.

Towards modeling the impact of the aspect ratio in an energy model describing the transient flow boiling crisis at high subcooling

* Elie ROUMET (CEA | CNAM LAFSET)

Raksmy NOP (CEA)

Marie-Christine DULUC (CNAM LAFSET)

Nicolas DORVILLE (CEA)

Boiling crisis under transient heat input is a phenomenon of major interest for the safety of research nuclear reactors. Recently, a model for highly subcooled flow was developed by Nop et al. (2021), which assumes the existence of a homogeneous mantle exchanging with the heating wall and the bulk flow until its saturation. We present an extension of this approach, taking into account the increase of the bulk temperature during the transient. We demonstrate that, if the assumption of an isothermal bulk is valid under the experimental conditions of Nop et al. (2021), it is no longer the case for configurations with high aspect ratios (L/D h ), and thus for the application of the model to reactor geometries.

Alloy composition and electrical potential in liquid metal batteries

* Sabrina Bénard (Université Paris-Saclay ,LISN, CNRS)

Wietze Herreman (Université Paris-Saclay, FAST, CNRS)

Loïc Cappanera (Department of Mathematics, University of Houston)

Caroline Nore (Université Paris-Saclay, LISN, CNRS)

In liquid metal batteries, the mixing of the alloy phase is a real issue to optimize the battery and limit the drop of the electrical potential. Most of the studies on this topic have forgotten that the electrical potential influences directly the alloy composition, through a jump in electrical potential at the electrolyte-alloy interface. This jump has been first taken into account by Weber et al. (2019), but they did not show the impact of the jump on the alloy composition. During this presentation, we show how the jump in potential affects the alloy composition, and how wrong it is to neglect it in the model.

Self-Organisation of phoretic suspensions in shear flows

* Prathmesh Vinze (LadHyX,CNRS-Ecole Polytechnique,Institut de Paris)

Sebastien Michelin (LadHyX,CNRS-Ecole Polytechnique,Institut de Paris)

A Janus phoretic particle converts the chemical energy present in its environment to mechanical energy to self-propel. These active particles modify and respond to their hydrodynamic and chemical environment, thus allowing them to respond to an external flow and interact hydro-dynamically and chemically with other particles. These interactions are known to lead to non-trivial collective behaviour within such biological or synthetic active suspensions (e.g., cluster formation of phoretic particles or bacterial swarming). Recent experiments and analysis have demonstrated that the response of active suspensions to shear flow is non-trivial and can, in fact, lead to a significant reduction in viscosity due to the injection of energy at the microscopic scale. In this work, we numerically analyse the response to the shear of a dilute and confined suspension of chemotactic phoretic particles using a continuum model of a dilute phoretic suspension. We show that a 1D transient steady distribution driven by the effect of confinement is a common feature for the confinement and shear rate intensities considered. This 1D state is stable for strong confinement and thus observed in the long-term dynamics for small channel widths. For wider channels, the transient state is unstable to streamwise perturbations due to the chemotactic instability, leading to particles' aggregates forming along the channel's walls. The relative arrangements of the aggregates on the two opposite walls are determined by the relative influence of shear intensity and chemotaxis and critically condition the suspension's dynamics and particle-induced flow. In the second step, we investigate the feedback effect on the flow of the self-organised suspension and, more specifically, on the effective viscosity of the suspension. We show that the induced flow and, consequently, its rheological behaviour strongly depend on the self-organisation regime. The most substantial reduction in viscosity is achieved for a weak shear case with the induced flow consisting of clockwise rotating vortices. The clockwise rotating vortices entrain the plate, thus reducing the required force at the plates to maintain a constant velocity.

Multiscale modeling of active dissolution in bacteria suspensions and swarms

* Maxime Theillard (University of California, Merced)

Autonomous collective motion of interacting agents in far-from-equilibrium settings is fundamental to many biological and engineering systems. Striking biological examples of such collectives are bacterial suspensions and swarms. These dense active fluids are comprised of dense motile cells interacting via the ambient medium, with each other and with boundaries. In many cases, these biological fluids may include non-motile cells or particles that together form an active two-species or a two-phase system. Here we present a new computational method for modeling such multiphase active fluids under geometric confinement. Starting from the Fokker Planck description, we formulate a continuum multiscale mean-field theory wherein the state of each phase (species) is represented by the first three orientational moments of the corresponding probability distribution function. The spatiotemporal evolution of this system is coupled hydrodynamically to the velocity and pressure fields in the suspending fluid subject to appropriate boundary conditions at the confining boundaries. The resulting nonlinear system is solved using parallel hybrid level-set-based discretization on adaptive Cartesian grids for high computational efficiency and maximal flexibility in the confinement geometry. Computational explorations demonstrate that we can reproduce emergent collective patterns observed in confined dense bacteria suspensions and predict essential features of experimentally observed dissolution dynamics in two-phase bacteria swarms. Our work lays the foundation for a systematic characterization of multi-phase natural systems such as bacterial colonies and biomimetic synthetic systems such as active colloidal suspensions or field-activated interacting programmable matter.

Mouvement phototactique des microalgues peut remplacer l'agitation mécanique dans un photobioreacteur

German Dario MARTINEZ (FAST)

Behnam Taidi (LGPM)

* Mojtaba Jarrahi (FAST)

Nous avons étudié la réponse phototactique de la microalgue C. reinhardtii pour déclencher une agitation d’origine biologique dans un photo-bioréacteur. Nous avons stimulé les microalgues en les faisant nager en directions opposées suivant des cycles de 40 minutes (20’ vers la droite, 20’ vers la gauche). Ce mouvement oscillatoire a perduré plusieurs jours tout en maintenant une amplitude comparable à la largeur du photo bioréacteur. La phototaxie a permis aux algues de rester en suspension; ainsi, il a été possible de d’agiter la culture d’une façon non conventionnelle seulement par contrôle de la lumière et sans apport d’énergie mécanique externe.

Active Viscous Fingering

* C. Douarche (FAST)

A. Ganesh (FAST)

H. Auradou (FAST)

Adding swimming bacteria to a liquid lowers its viscosity. We study how this phenomenon can lead to the formation of viscous Saffman–Taylor fingers which occurs when a less viscous fluid is injected into a more viscous fluid. To prove that active fluids can induce this instability, we injected bacterial suspensions into a Hele-Shaw cell containing the suspending fluid. The velocity, length, and width of the Saffman-Taylor fingers observed are identical to those obtained with a pair of Newtonian fluids with different viscosities. Our study determines the onset conditions of the Saffman-Taylor instability from an active fluid.