Breakup of a pendant magnetic drop

International audienceWe report experiments on a millimeter-sized pendant drop of ferrofluid in a horizontal magnetic field. The initial drop size is chosen just below the breakup threshold under gravity. As the magnetic field is increased, the drop tilts in order to align with the direction of the total volume force that is exerted on it: weight plus magnetic force. The breakup is controlled by a generalized Bond number based on this total force and on the radius of the neck of the drop. The evolution of drop shape turns out to be a complex process governed by many parameters such as the angle between the total force and the needle, the drop size relative to the needle radius, and the wettability of the liquid on the needle material. This suggests a certain universality, that a single value of the critical Bond number is found regardless of magnetic fluid properties and whether the force is inclined or not

Physical mechanisms for droplet size and effective viscosity asymmetries in turbulent emulsions

By varying the oil volume fraction, the microscopic droplet size and the macroscopic rheology of emulsions are investigated in a Taylor-Couette (TC) turbulent shear flow. Although here oil and water in the emulsions have almost the same physical properties (density and viscosity), unexpectedly, we find that oil-in-water (O/W) and water-in-oil (W/O) emulsions have very distinct hydrodynamic behaviors, i.e., the system is clearly asymmetric. By looking at the micro-scales, the average droplet diameter hardly changes with the oil volume fraction neither for O/W nor for W/O. However, for W/O it is about 50% larger than that of O/W. At the macro-scales, the effective viscosity of O/W is higher when compared to that of W/O. These asymmetric behaviors can be traced back to the presence of surface-active contaminants in the system. By introducing an oil-soluble surfactant at high concentration, remarkably, we recover the symmetry (droplet size and effective viscosity) between O/W and W/O emulsions. Based on this, we suggest a possible mechanism responsible for the initial asymmetry. Next, we discuss what sets the droplet size in turbulent emulsions. We uncover a -6/5 scaling dependence of the droplet size on the Reynolds number of the flow. Combining the scaling dependence and the droplet Weber number, we conclude that the droplet fragmentation, which determines the droplet size, occurs within the boundary layer and is controlled by the dynamic pressure caused by the gradient of the mean flow, as proposed by Levich (1962), instead of the dynamic pressure due to turbulent fluctuations, as proposed by Kolmogorov (1949). The present findings provide an understanding of both the microscopic droplet formation and the macroscopic rheological behaviors in dynamic emulsification, and connects them

Rheological behaviour and runout of short-lived, fast-moving flows of hot dense suspensions

International audienceThis study aims at extending the previous work of Girolami et al. [11-13] which is still suffering from a lack of reliable predictions for the description of hot, dense suspensions obtained by fluidizing with air a bed of fine solid particles, originally cohesive at room temperature, then released down a rectangular flume. Here, we first present the effective viscosities of the suspensions which increase as a power law of the particle volume fraction Φs. Thus, we show that their rheological behaviour is solely controlled by Φs and its value at packing Φpacking , whatever the material involved in the mixture. Finally, we present a new modeling of the flows runout and highlight that the flows duration is solely controlled by the settling time (i.e. the time necessary for a suspension of a given Φs to settle at a velocity U sed over a distance h exp equals to the expansion height), the initial suspension geometry a, and the key parameter Φs /Φpacking. This prediction allows to distinguish two different flow regimes, with a transition around Φs /Φpacking ≃ 0.85, which seems to be correlated to the variations of the mixture rheology

Sedimentation of gas-fluidized particles with random shape and size

International audienceThis work deals with the fluidization and sedimentation of fine solid particles, of random shape and size, similar to those commonly involved in geophysical mass flows, such as pyroclastic flows. While heated to avoid the effect of moisture and the formation of clusters, particles were first uniformly fluidized by a hot gas flow, up to a high expansion rate, and then sedimented after stopping the gas supply. Three different materials are explored, involving contrasted geometries, each characterized by a specific particle volume fraction at packing Φpack. Within the range of values of the solid volume fraction Φs/Φpack studied here, the dense suspension forms a fully fluidized homogeneous mixture, with no segregation, for which the fluidization and sedimentation velocities are equal. Despite a significant discrepancy between the intrinsic properties of the different materials used, all measured velocities are observed to collapse into a single master curve f(Φs/Φpack) provided that they are normalized by the relevant scaling. Regarding the sedimentation velocity, Φpack turns out to be sufficient to characterize the material made with a random distribution in particle shape and size

On the fluidization/sedimentation velocity of a homogeneous suspension in a low-inertia fluid

The modeling of the fluidization or sedimentation velocity of a suspension of solid particles is revisited by examining experiments conducted in either a liq- uid or a gas. A general expression is found in the case of negligible fluid inertia, i.e. at low Reynolds or Archimedes number. It is built as the product of the velocity of an isolated particle by three non-dimensional corrections that each takes into account a specific physical mechanism. The first correction reflects the variation of the buoyancy with the particle concentration. The second cor- rection describes how the drag force increases with the concentration in case of negligible particle inertia. The third one accounts for the further increase of the drag when the particle inertia is increased. Remarkably, each correction only relies on a single of the three independent non-dimensional groups that control the problem: (1) the particle volume fraction Φs; (2) the ratio Φs/Φpack where Φpack is the bed packing concentration; (3) the Stokes number St0, which characterizes the inertia of the particles and controls their agitation. Moreover, the onset of the instability that separates the homogeneous regime from the heterogeneous one is found to be controlled similarly by the Stokes number. Empirical expressions of the corrections are given, which provide a reliable tool to predict fluidization and sedimentation velocities for all values of the three non-dimensional numbers. The present results emphasize the crucial role of particle inertia, which is often disregarded in previous modeling approaches, such as that of Richardson and Zaki

Sound generation on bubble coalescence following detachment

A system in which bubbles coalesced on formation was used to probe one mechanism by which bubbles create sound. The aim was to determine in which situations sound is produced and to predict its amplitude. A set of carefully co-ordinated high-speed video and acoustic timeseries showed that needle-formed bubbles generated loud bubble-acoustic emissions at the instant of coalescence of secondary bubbles with the primary bubble. As the air flow rate increased, the size and number of secondary bubbles increased, and the sound amplitude also increased. On coalescence, the sound pressure always rose initially. A dimensionless scaling found that the sound amplitude emitted scaled with the volume of the secondary bubble. This scaling was shown to be consistent with the sound-emission mechanism being the equalization of pressures in the coalescing bubbles. The trend in amplitude with bubble production rate was well predicted by the scaling

Dynamique d'un nuage de bulles homogène confiné

National audienceDe nombreuses applications industrielles mettent en jeu des écoulements à bulles dans des échangeurs de masse et de chaleur ou des réacteurs. Les mouvements des bulles génèrent de l'agitation dans le liquide qui, en retour, influence la distribution spatiale des bulles et leur vitesse. La compréhension générique de ce problème de couplage inverse total est fondamentale mais délicate. Des travaux expérimentaux dédiés dans des configurations d'écoulements bien définies sont donc nécessaires pour atteindre cet objectif. Ce travail explore la dynamique d'un nuage de bulles en ascension à grand nombre de Reynolds dans une cellule de Hele-Shaw ([1]). Cette configuration apporte une contribution à une compréhension générale car elle permet d'étudier l'agitation générée par des bulles à grand nombre de Reynolds possédant des sillages instables tout en empêchant, par les effets de confinement, la production de turbulence. La comparaison avec la dynamique de nuages de bulles non confinés ([2]) est également éclairante. Par ailleurs, la détection des interfaces est considérablement facilitée par le confinement: une description complète et précise de la répartition spatiale et de la dynamique des bulles peut être ici obtenue directement par ombroscopie avec une seule caméra. De même, la mesure par PIV du champ de vitesse du liquide intégré dans l'épaisseur de la cellule permet de caractériser de manière pertinente la dynamique du liquide ([3]) (Fig.1-a). La dynamique des deux phases a ainsi été explorée pour des fractions volumiques de gaz α comprises entre 1% et 14% dans un régime où l'inertie est importante (Re≈500). Les bulles étudiées possèdent un sillage instable avec des lâchers tourbillonnaires réguliers et suivent une trajectoire ascendante oscillante tout en gardant une forme elliptique constante. Le frottement aux parois impose néanmoins une décroissance très forte des sillages ([4]). Les résultats montrent que l'on peut expliquer les statistiques associées au mouvement des bulles dans le nuage à partir de deux mécanismes élémentaires: (i) les oscillations induites par le sillage associées aux lâchers tourbillonnaires et (ii) la forte perturbation de vitesse localisée à l'arrière des bulles. Le mécanisme dominant dans la direction verticale est l'entrainement dans le sillage alors que celui qui régit la dynamique des bulles dans la direction horizontale est associé aux oscillations générées par les sillages dont l'intensité est indépendante de α (Fig.1-b). L'auto-dispersion des bulles a également été étudiée. Elle peut être caractérisée par des coefficients de dispersion qui évoluent linéairement avec α.En ce qui concerne l'agitation dans le liquide, comme en écoulement non confiné, les deux composantes des fluctuations de vitesse évoluent proportionnellement à αn avec ici αn=0.38 et 0.46 dans les directions horizontales et verticales respectivement. Le spectre spatial des fluctuations de vitesse évolue, sur une gamme de nombres d'ondes k bien définie, proportionnellement à k-³. Dans cette configuration où la turbulence ne peut se développer, cette évolution s'explique très clairement par la superposition linéaire de perturbations de vitesses aléatoires ([5]), il s'agit donc d'un effet statistique associé aux passages de perturbations convectées par les bulles

Drop breakup modelling in turbulent flows

This paper deals with drop and bubble break-up modelling in turbulent flows. We consider the case where the drop/bubble slip velocity is smaller than or of the order of the turbulent velocity scales, or when the drop/bubble deformation is mainly caused by the turbulent stress (atomisation is not addressed here). The deformation of a drop is caused by continuous interactions with turbulent vortices; the drop responds to these interactions by performing shape-oscillations and breaks up when its deformation reaches a critical value. Following these observations, we use a model of forced oscillator that describes the drop deformation dynamics in the flow to predict its break-up probability. Such a model requires a characterization of the shape-oscillation dynamics of the drop. As this dynamics is theoretically known only under restrictive conditions (without gravity, surfactants), CFD two-phase flow simulations, based on the Level-Set and Ghost Fluid methods, are used to determine the interface dynamics in more complex situations: deformation of a drop in the presence of gravity, bubble-vortex interactions. Results are compared with experimental data. The perspectives to apply this model to breakup in emulsification processes are also discussed

Turbulence effect on clustering in bubble swarms using DNS and front-tracking

International audienceWe investigate cluster's formation in bubbles' swarms using direct numerical simulations and the Front-Tracking method for bubbles' motions. Void fractions of 3 % and 6 % are simulated with and without a background forcing turbulence. The bubbles' concentration is measured using Voronoi volumes computed from the bubbles' mass centres. We show a clear effect of turbulence on the probability density function of Voronoi volumes as it increases the probability of extreme values. Common scalings based on standard deviation and analytical functions are investigated and show only a partial fit with our data suggesting a different cluster mechanism

Experimental study of mass transfer in a dense bubble swarm

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