Sédimentation de particules paramagnétiques soumises à la gravité et à un champ magnétique externe

Nous nous intéressons à la dynamique et aux effets collectifs de particules en transport turbulent. Les écoulements chargés en particules sont omniprésents dans la nature (écoulements pyroclastiques, transport de polluants ...) et dans de nombreux procédés industriels (mélangeurs diphasique ...). Dans ces exemples, l'écoulement est généralement turbulent. Les interactions entre les particules et l'écoulement sont complexes de part la nature multi-échelles des structures de la turbulence et les diverses caractéristiques des particules. Depuis de nombreuses décennies, la communauté scientifique s'intéresse à ces écoulements, cherchant notamment à établir des modèles prédictifs fiables. Les théories les plus abouties sont les théories inertielles. Celles-ci considèrent les particules comme ponctuelles [1]. La démocratisation des techniques d'imagerie rapide et des moyens numériques ont ouvert la voie à diverses études ayant permis de montrer les limitations prédictives de ces approches idéalisées pour décrire le transport turbulent de particules matérielles réelles et de taille finie. Ceci montre la nécessité de mieux comprendre les couplages entre particules et entre les particules et l'écoulement porteur afin de prendre en compte la multitude des processus physiques sous-jacents (dispersion, effets de l'inertie, rôle de la turbulence, effet de la gravité, formation d'amas et effets collectifs, ...) [2] [3]. Dans un système réel, il est généralement difficile de démêler les effets spécifiques de ces processus. Pour cela, il est nécessaire de développer des études expérimentales permettant d'agir sur l'un ou l'autre de ces effets de couplage. Pour aborder ces problèmes, le travail que nous présentons propose une étude expérimentale préliminaire du rôle des interactions inter-particulaires (s'affranchissant dans un premier temps des effets de la turbulence) : la sédimentation de particules paramagnétiques soumises à la gravité et à un champ magnétique externe dans un fluide initialement au repos.   Le dispositif expérimental est constitué d'une cellule de Hele-Shaw contenant une suspension (eau) de sphères paramagnétiques de 250 um de diamètre et de densité 1,6 placées entre deux bobines de Helmholtz produisant, au choix, un champ homogène ou inhomogène dont l'amplitude peut-être ajustée. L'ensemble des particules forme initialement un paquet dense au fond de la cellule. Celle-ci pivote rapidement induisant la sédimentation des particules.   Nous suivons la déstabilisation du front de particules (analogue à l'instabilité de Rayleigh-Taylor fluide (RT)) par imagerie rapide pour différentes valeurs de l'amplitude du champ externe. Nos premières mesures montrent un ralentissement de la décompaction à mesure que le champ augmente, suggérant un possible effet cohésif résultant de la formation de chaînes de forces entre les particules (par interactions entre dipôles induits). Ceci est compatible avec l'observation qualitative de l'atténuation de la digitation de RT de l'interface, pouvant s'interpréter comme une augmentation de la tension de surface effective du front de particules, induite par la plus forte interaction entre les particules.   Dans des études à venir nous considérons le cas d'un champ magnétique appliqué inhomogène afin d'étudier non seulement les interactions entre particules mais également la possibilité d'agir avec ou contre la gravité. Ceci pourrait ouvrir une voie intéressante pour des études ultérieures visant à démêler les effets de l'inertie et de la gravité dans les écoulements turbulents chargés en particules. Références : [1] M. R. Maxey and J. J. Riley. Equation of motion for a small rigid sphere in a nonuniform flow. Phys. Fluids 26, 883 (1983) [2] Mickaël Bourgoin and Haitao Xu. Focus on dynamics of particles in turbulence. New J. Phys. 16, 085010 (2014) [3] Nauman M. Qureshi, Mickaël Bourgoin, Christophe Baudet, Alain Cartellier, and Yves Gagne. Turbulent transport of material particles : An experimental study of finite size effects. Phys. Rev. Lett. 99, 184502 (2007)

Sedimentation of a suspension of paramagnetic particles in an external magnetic field

International audienceWe investigate the sedimentation of initially packed paramagnetic particles in the presence of a homogeneous external magnetic field, in a Hele-Shaw cell filled with water. Although the magnetic susceptibility of the particles is small and the particle-particle induced magnetic interactions are significantly smaller compared to the gravitational acceleration, we do observe a measurable reduction of the decompaction rate as the amplitude of the applied magnetic field is increased. While induced magnetic dipole-dipole interactions between particles can be either attractive or repulsive depending on the particles relative alignment, our observations reveal an effective overall enhancement of the cohesion of the initial pack of particles due to the induced interactions, very likely promoting internal chain forces in the initial pack of particles. The influence of the magnetic field on the particles once they disperse after being decompacted is on the other hand found to remain marginal

Direct visualization of the quantum vortex lattice structure, oscillations and destabilization in rotating 4He

Quantum vortices are a core element of superfluid dynamics and elusively hold the keys to our understanding of energy dissipation in these systems. Here we show that we are able to visualize these vortices in the canonical and higher symmetry case of a stationary rotating superfluid bucket. Using direct visualization, we quantitatively verify Feynman’s rule linking the resulting quantum vortex density to the imposed rotational speed. We make the most of this stable configuration by applying an alternative heat flux aligned with the axis of rotation. Moderate amplitudes led to the observation of collective wave mode propagating along the vortices and high amplitudes led to quantum vortex interactions. When increasing the heat flux, this ensemble of regimes defines a path towards quantum turbulence in rotating 4He and sets a baseline to consolidate the descriptions of all quantum-fluids

Formation of glacier tables caused by differential ice melting: field observation and modelling

International audienceAbstract. Glacier tables are structures frequently encountered on temperate glaciers. They consist of a rock supported by a narrow ice foot which forms through differential melting of the ice. In this article, we investigate their formation by following their dynamics on the Mer de Glace (the Alps, France). We report field measurements of four specific glacier tables over the course of several days, as well as snapshot measurements of a field of 80 tables performed on a given day. We develop a simple model accounting for the various mechanisms of the heat transfer on the glacier using local meteorological data, which displays a quantitative agreement with the field measurements. We show that the formation of glacier tables is controlled by the global heat flux received by the rocks, which causes the ice underneath to melt at a rate proportional to the one of the surrounding ice. Under large rocks the ice ablation rate is reduced compared to bare ice, leading to the formation of glacier tables. This thermal insulation effect is due to the warmer surface temperature of rocks compared to the ice, which affects the net long-wave and turbulent fluxes. While the short-wave radiation, which is the main source of heat, is slightly more absorbed by the rocks than the ice, it plays an indirect role in the insulation by inducing a thermal gradient across the rocks which warms them. Under a critical size, however, rocks can enhance ice melting and consequently sink into the ice surface. This happens when the insulation effect is too weak to compensate for a geometrical amplification effect: the external heat fluxes are received on a larger surface than the contact area with the ice. We identified the main parameters controlling the ability of a rock to form a glacier table: the rock thickness, its aspect ratio, and the ratio between the averaged turbulent and short-wave heat fluxes

Stability of a Liquid Jet Impinging on Confined Saturated Sand

International audienceThis work focuses on the dynamics of a liquid jet impacting the surface of a confined, immersed granular bed. Although previous works have considered the erosion process and surface morphology, less attention has been given to the jet hydrodynamics. Based on laboratory experiments, we show that when the liquid jet forms a crater, two situations arise. In the case of weak/no erosion or open craters, the jet is stationary. For vertical or overhanging crater walls, the jet displays a wide range of behaviors, from quasi-periodic oscillations to symmetry breaking and exploration of different states in time. An analysis of the different system states leads to the emergence of a bifurcation diagram depending on a dimensionless parameter, J, comparing the jet impact force to the force necessary to eject a grain. The frequency of the jet oscillations depends on the inertial velocity, the jet dispersion and the ratio between the injector cross-section and the confinement length

Suspension of large inertial particles in a turbulent swirling flow

International audienceWe present experimental observations of the spatial distribution of large inertial particles suspended in a turbulent swirling flow at high Reynolds number. The plastic particles, which are tracked using several high speed cameras, are heavier than the working fluid so that their dynamics results from a competition between gravitational effects and turbulent agitation. We observe two different regimes of suspension. At low rotation rate, particles are strongly confined close to the bottom and are not able to reach the upper region of the tank whatever their size or density. At high rotation rate, particles are loosely confined: small particles become nearly homogeneously distributed while very large objects are preferentially found near the top as if gravity was reversed. We discuss these observations in light of a minimal model of random walk accounting for particle inertia and show that large particles have a stronger probability to remain in the upper part of the flow because they are too large to reach descending flow regions. As a consequence particles exhibit random horizontal motions near the top until they reach the central region where the mean flow vanishes, or until a turbulent fluctuation gets them down