Scale-free channeling patterns near the onset of erosion of sheared granular beds

Erosion shapes our landscape and occurs when a sufficient shear stress is exerted by a fluid on a sedimented layer. What controls erosion at a microscopic level remains debated, especially near the threshold forcing where it stops. Here we study experimentally the collective dynamics of the moving particles, using a set-up where the system spontaneously evolves toward the erosion onset. We find that the spatial organization of the erosion flux is heterogeneous in space, and occurs along channels of local flux $\sigma$ whose distribution displays scaling near threshold and follows $P(\sigma)\sim J/\sigma$, where $J$ is the mean erosion flux. Channels are strongly correlated in the direction of forcing but not in the transverse direction. We show that these results quantitatively agree with a model where the dynamics is governed by the competition of disorder (which channels mobile particles) and particle interactions (which reduces channeling). These observations support that for laminar flows, erosion is a dynamical phase transition which shares similarity with the plastic depinning transition occurring in dirty superconductors. The methodology we introduce here could be applied to probe these systems as well.Comment: 8 pages, 6 figure

Falling Jets of Particles in Viscous Fluids

This fluid dynamics video presents experiments and simulations of gravity-driven particulate jets in viscous fluids at low Reynolds number. An initially straight jet is shown to develop varicose modulations of its diameter as it sediments under the action of gravity. While this instability is qualitatively reminiscent of the classical Rayleigh-Plateau instability for immiscible fluids, its mechanism has yet to be understood as neither inertia nor surface tension play a role in the case of a dilute suspension at Re=0.Comment: Accompanies video submission to APS DFD 2008 Gallery of Fluid Motio

Rheology of dense suspensions of non colloidal particles

Dense suspensions are materials with broad applications both in industrial processes (e.g. waste disposal, concrete, drilling muds, metalworking chip transport, and food processing) and in natural phenomena (e.g. flows of slurries, debris, and lava). Despite its long research history and its practical relevance, the mechanics of dense suspensions remain poorly understood. The major difficulty is that the grains interact both by hydrodynamic interactions through the liquid and by mechanical contact. These systems thus belong to an intermediate regime between pure suspensions and granular flows. We show that we can unify suspension and granular rheology under a common framework by transferring the frictional approach of dry granular media to wet suspensions of spherical particles. We also discuss non-Newtonian behavior such as normal-stress differences and shear-induced migration. Beyond the classical problem of dense suspension of hard spheres which is far from being completely resolved, there are also entirely novel avenues of study concerning more complex mixtures of particles and fluids such as those involving other types of particles (e.g. fibers) or non-Newtonian fluids that we will also address

Sedimentation of small particles: how can such a simple problem be so difficult?

International audienceAlthough sedimentation can be considered as one of the simplest examples of suspension flow, much remains unknown about the fundamental properties of sedimenting suspensions. The problem that one encounters lies in the long range nature of the multibody hydrodynamic interactions between particles. This will be illustrated for sedimenting suspensions of spheres, of non-spherical particles such as fibers, and for sedimenting clouds of particles. To cite this article: É. Guazzelli, C. R. Mecanique 334 (2006). Résumé Sédimentation de petites particules : comment un problème si simple peut-il être si compliqué ? La sédimentation de particules à bas nombre de Reynolds peut être considérée comme un des exemples les plus simples d'écoulement de suspension. Et pourtant ce problème est compliqué à cause de la dominance des interactions hydrodynamiques multicorps à longues portées. Trois situations illustreront cette difficulté : la sédimentation d'une suspension de sphères, de particules anisotropes (des fibres) et d'un nuage sphérique de particules. Pour citer cet article : É. Guazzelli, C. R. Mecanique 334 (2006)

Inertial effects on fibers settling in a vortical flow

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Fluctuations and Instability in Sedimentation

International audienceThis review concentrates on the fluctuations of the velocities of sedimenting spheres, and on the structural instability of a suspension of settling fibers. For many years, theoretical estimates and numerical simulations predicted the fluctuations of the velocities of spheres to increase with the size of the container, whereas experiments found no such variation. Two ideas have increased our understanding. First, the correlation length of the velocity fluctuations was found experimentally to be 20 interparticle separations. Second , in dilute suspensions, a vertical variation in the concentration due to the spreading of the front with the clear fluid can inhibit the velocity fluctuations. In a very dilute regime, a homogeneous suspension of fibers suffers a spontaneous instability in which fast descending fiber-rich columns are separated by rising fiber-sparse columns. In a semidilute regime, the settling is hindered, more so than for spheres

Des particules qui se la coulent douce...

Formation d’un tore, déstabilisation en gouttelettes, puis cascade auto-similaire : la sédimentation d’un nuage de particules dans un fluide visqueux est spectaculaire. Comment un système conceptuellement aussi simple peut-il développer une telle richesse de comportements ? La longue portée des interactions hydrodynamiques entre particules et le comportement collectif qu’elles entraînent (du type problème à N-corps) sont les composantes essentielles pour comprendre cette dynamique