Etude hydrodynamique de l’écoulement traversant un réseau aléatoire de sphères fixes

Dans de nombreuses applications des inclusions (bulles, gouttes ou particules) sont injectées dans un liquide pour mettre des produits en contact. Sous l'effet de la gravité, le mouvement des inclusions agite le liquide. Nous avons conçu un modèle expérimental capable de reproduire cette agitation à l'aide d'un écoulement à travers un réseau aléatoire de sphères fixes. Nous avons déterminé les propriétés statistiques et spectrales des fluctuations et montré qu'elles étaient représentatives de l'agitation dans un essaim de bulles. Par une décomposition des fluctuations en une contribution correspondant aux inhomogénéités spatiales et une correspondant aux fluctuations temporelles, nous avons jeté un éclairage nouveau sur la pseudo-turbulence des écoulements diphasiques. ABSTRACT : In many applications particles (bubbles, drops or particles) are injected into a liquid for enhancing the contact between the phases. The buoyancy-induced motions of the particles causes an intense agitation of the liquid. We designed an original experimental setup able to reproduce this agitation by means of a flow through a random array of fixed spheres. We determined the statistical and spectral properties of the fluctuations and showed that they were similar to those observed in a swarm of rising bubbles. By decomposing the fluctuations in a contribution corresponding to the spatial inhomogeneities and another corresponding to the time fluctuations, we shed light on the nature of pseudo-turbulence in two phase flow

Attenuation of the wake of a sphere in an intense incident turbulence with large length scales

We report an investigation of the wake of a sphere immersed in a uniform turbulent flow for sphere Reynolds numbers ranging from 100 to 1000. An original experimental setup has been designed to generate a uniform flow convecting an isotropic turbulence. At variance with previous works, the integral length scale of the turbulence is of the same order as the sphere diameter and the turbulence intensity is large. In consequence, the most intense turbulent eddies are capable of influencing the flow in the close vicinity of the sphere. Except in the attached region downstream of the sphere where the perturbation of the mean velocity is larger than the standard deviation of the incident turbulence, the flow is controlled by the incident turbulence. The distortion of the turbulence while the flow goes round the sphere leads to an increase in the longitudinal fluctuation and a decrease in the transversal one. The attenuation of the transversal fluctuations is still significant at 30 radii downstream of the sphere whereas the longitudinal fluctuations relax more rapidly toward the incident value. The more striking result however concerns the evolution of the mean velocity defect with the distance x from the sphere. It decays as x−2 and scales with the standard deviation of the incident turbulence instead of scaling with the mean incident velocity

Velocity fluctuations generated by the flow through a random array of spheres: a model of bubble-induced agitation

This work reports an experimental investigation of the flow through a random array of fixed solid spheres. The volume fraction of the spheres is 2%, and the Reynolds number Re based on the sphere diameter and the average flow velocity is varied from 120 to 1040. Using time and spatial averaging, the fluctuations have been decomposed into two contributions of different natures: a spatial fluctuation that accounts for the strong inhomogeneity of the flow around each sphere, and a time fluctuation that comes from the instability of the flow at large enough Reynolds numbers. The evolutions of these two contributions with the Reynolds number are different, so that their relative importance varies. However, when each is normalized by using its own variance and the integral length scales of the fluctuations, their spectra and probability density functions (PDFs) are almost independent of Re. The spatial fluctuation mostly comes from the velocity deficit in the wakes of the spheres, and is thus dominated by scales larger than one or two sphere diameters. It is found to be responsible for the asymmetry of the PDFs of the vertical fluctuations and of the major part of the anisotropy level between the vertical and the horizontal components of the fluctuations. The time fluctuation dominates at scales smaller than the integral length scale. It is isotropic and its PDFs, well described by an exponential distribution, are non-Gaussian. The spectra of the spatial and the time fluctuations both show an evolution as the power −3 of the wavenumber, but not exactly in the same subrange. All these properties are found in remarkable agreement with the results of both experimental investigations and large eddy simulations (LES) of a homogeneous bubble swarm. This confirms that the main mechanism responsible for the production of bubble-induced fluctuations is the interaction of the velocity disturbances caused by obstacles immersed in a flow and that the structure of this agitation is weakly dependent on the precise nature of the obstacles. The understanding and the modelling of the agitation generated by the motion of a dispersed phase, such as the bubble-induced agitation, therefore require one to distinguish between the roles of these two contributions

Sillage d'une sphère dans un écoulement turbulent

Ce travail est une étude expérimentale du sillage d'une sphère dans une turbulence intense dont les échelles sont proches du diamètre de la sphère pour des nombres de Reynolds particulaires allant de 100 à 1000. Les mesures du champ de vitesse par anémométrie laser conduisent à distinguer deux régions de l'écoulement. A moins de 6 rayons l'écoulement est semblable à celui d'une sphère dans un écoulement laminaire. Au-delà, la structure du sillage présente les propriétés suivantes : décroissance du déficit de vitesse moyenne en z à la puissance -2 , existence d'une région centrale près de l'axe où la turbulence bien qu'uniforme reste fortement anisotrope, niveau de fluctuation imposé par la turbulence externe. Ceci conduit au résultat remarquable que le déficit de vitesse moyenne normalisé par la fluctuation de vitesse de la turbulence incidente est indépendant du nombre de Reynolds

Wake attenuation in large Reynolds number dispersed two-phase flows

The dynamics of high Reynolds number-dispersed two-phase flow strongly depends on the wakes generated behind the moving bodies that constitute the dispersed phase. The length of these wakes is considerably reduced compared with those developing behind isolated bodies. In this paper, this wake attenuation is studied from several complementary experimental investigations with the aim of determining how it depends on the body Reynolds number and the volume fraction a. It is first shown that the wakes inside a homogeneous swarm of rising bubbles decay exponentially with a characteristic length that scales as the ratio of the bubble diameter d to the drag coefficient Cd, and surprisingly does not depend on a for 10K2%a%10K1. The attenuation of the wakes in a fixed array of spheres randomly distributed in space (aZ2!10K2) is observed to be stronger than that of the wake of an isolated sphere in a turbulent incident flow, but similar to that of bubbles within a homogeneous swarm. It thus appears that the wakes in dispersed two-phase flows are controlled by multi-body interactions, which cause a much faster decay than turbulent fluctuations having the same energy and integral length scale. Decomposition of velocity fluctuations into a contribution related to temporal variations and that associated to the random character of the body positions is proposed as a perspective for studying the mechanisms responsible for multi-body interactions

Étude hydrodynamique de l'écoulement traversant un réseau aléatoire de sphères fixes

Dans de nombreuses applications, des inclusions (bulles, gouttes ou particules) sont injectées dans un liquide pour mettre des produits en contact. Sous l'effet de la gravité, le mouvement des inclusions agite le liquide. Nous avons conçu un modèle expérimental capable de reproduire cette agitation à l'aide d'un écoulement à travers un réseau aléatoire de sphères fixes. Nous avons déterminé les propriétés statistiques et spectrales des fluctuations et montré qu'elles étaient représentatives de l'agitation dans un essaim de bulles. Par une décomposition des fluctuations en une contribution correspondant aux inhomogénéités spatiales et une correspondant aux fluctuations temporelles, nous avons jeté un éclairage nouveau sur la pseudo-turbulence des écoulements diphasiques.In many applications, particles (bubbles, drops or particles) are injected into a liquid for enhancing the contact between the phases. The buoyancy-induced motions of the particles causes an intense agitation of the liquid. We designed an original experimental setup able to reproduce this agitation by means of a flow through a random array of fixed spheres. We determined the statistical and spectral properties of the fluctuations and showed that they were similar to those observed in a swarm of rising bubbles. By decomposing the fluctuations in a contribution corresponding to the spatial inhomogeneities and another corresponding to the time fluctuations, we shed light on the nature of pseudo-turbulence in two phase flows.TOULOUSE-ENSEEIHT (315552331) / SudocSudocFranceF