Evidence of reverse and intermediate size segregation in dry granular flows down a rough incline

In a dry granular flow, size segregation behave differently for a mixture containing a few large beads with a size ratio (S) above 5 (Thomas, Phys.Rev.E 62,96(2000)). For moderate large S, large beads migrate to an intermediate depth in the bed: this is called intermediate segregation. For the largest S, large beads migrate to the bottom: this is called reverse segregation (in contrast with surface segregation). As the reversal and intermediate depth values depend on the bead fraction, this numerical study mainly uses a single large tracer. Small fractions are also computed showing the link between a tracer behavior and segregation process. For half-filled rotating drum and for rough incline, two and three (3D) dimensional cases are studied. In the tumbler, trajectories of a large tracer show that it reaches a constant depth during the flow. For large S, this depth is intermediate with a progressive sinking when S increases. Largest S correspond to tracers at the bottom of the flow. All 3D simulation are in quantitative agreement with the experiments. In the flow down an incline, a large tracer reaches an equilibrium depth during flow. For large S, its depth is intermediate, inside the bed. For the largest S, its depth is reverse, near the bottom. Results are slightly different for thin or thick flow. For 3D thick flows, the reversal between surface and bottom positions occurs within a short range of S: no tracer stabilizes near mid-height and two reachable intermediate depth layers exist, below the surface and above the bottom. For 3D thin flows, all intermediate depths are reachable, depending on S. The numerical study of larger tracer fractions (5-10%) shows the 3 segregation patterns (surface, intermediate, reverse) corresponding to the 3 types of equilibrium depth. The reversal is smoother than for a single tracer. It happens around S=4.5, in agreement with experiments.Comment: 18 pages, 27 figure

Influence of Rough and Smooth Walls on Macroscale Flows in Tumblers

Walls in discrete element method simulations of granular flows are sometimes modeled as a closely packed monolayer of fixed particles, resulting in a rough wall rather than a geometrically smooth wall. An implicit assumption is that the resulting rough wall differs from a smooth wall only locally at the particle scale. Here we test this assumption by considering the impact of the wall roughness at the periphery of the flowing layer on the flow of monodisperse particles in a rotating spherical tumbler. We find that varying the wall roughness significantly alters average particle trajectories even far from the wall. Rough walls induce greater poleward axial drift of particles near the flowing layer surface, but decrease the curvature of the trajectories. Increasing the volume fill level in the tumbler has little effect on the axial drift for rough walls, but increases the drift while reducing curvature of the particle trajectories for smooth walls. The mechanism for these effects is related to the degree of local slip at the bounding wall, which alters the flowing layer thickness near the walls, affecting the particle trajectories even far from the walls near the equator of the tumbler. Thus, the proper choice of wall conditions is important in the accurate simulation of granular flows, even far from the bounding wall.Comment: 32 pages, 19 figures, regular article, accepted for publication in Physical Review E 200

Why antiplectic metachronal cilia waves are optimal to transport bronchial mucus

International audienceThe coordinated beating of epithelial cilia in human lungs is a fascinating problem from the hydrodynamics perspective. The phase lag between neighboring cilia is able to generate collective cilia motions, known as metachronal waves. Different kinds of waves can occur, antiplectic or symplectic, depending on the direction of the wave with respect to the flow direction. It is shown here, using a coupled lattice Boltzmann-immersed boundary solver, that the key mechanism responsible for their transport efficiency is a blowing-suction effect that displaces the interface between the periciliary liquid and the mucus phase. The contribution of this mechanism on the average flow generated by the cilia is compared to the contribution of the lubrication effect. The results reveal that the interface displacement is the main mechanism responsible for the better efficiency of antiplectic metachronal waves over symplectic ones to transport bronchial mucus. The conclusions drawn here can be extended to any two-layer fluid configuration having different viscosities, and put into motion by cilia-shaped or comb-plate structures, having a back-and-forth motion with phase lags

Slow axial drift in three-dimensional granular tumbler flow

Models of monodisperse particle flow in partially filled three-dimensional tumblers often assume that flow along the axis of rotation is negligible. We test this assumption, for spherical and double cone tumblers, using experiments and discrete element method simulations. Cross sections through the particle bed of a spherical tumbler show that, after a few rotations, a colored band of particles initially perpendicular to the axis of rotation deforms: particles near the surface drift toward the pole, while particles deeper in the flowing layer drift toward the equator. Tracking of mm-sized surface particles in tumblers with diameters of 8-14 cm shows particle axial displacements of one to two particle diameters, corresponding to axial drift that is 1-3% of the tumbler diameter, per pass through the flowing layer. The surface axial drift in both double cone and spherical tumblers is zero at the equator, increases moving away from the equator, and then decreases near the poles. Comparing results for the two tumbler geometries shows that wall slope causes axial drift, while drift speed increases with equatorial diameter. The dependence of axial drift on axial position for each tumbler geometry is similar when both are normalized by their respective maximum values

Écoulement granulaire et ségrégation en tambour tournant lisse ou rugueux

La ségrégation axiale en tambour tournant sphérique est étudiée à travers l'influence du taux de remplissage et de la rugosité des parois. L'étude des écoulements monodisperses a mis en évidence la courbure des trajectoires des particules et l'existence de cellules de convection. Nous montrons comment ces deux caractéristiques, couplées à la ségrégation amène à une organisation axiale en trois bandes petites/grandes/petites ou grandes/petites/grandes selon le phénomène dominant

Profils de vitesse des écoulements granulaires

Nous présentons une étude expérimentale et numérique de l'écoulement sur plan incliné de milieux granulaires monodisperse et bidisperse ségrégé. Après présentation des profils de vitesse théoriques attendus, et ceux observés expérimentalement et numériquement, que l'augmentation de la vitesse d'écoulement par la présence de grandes particules à la surface a lieu sur les écoulements de faibles épaisseurs. Dès que l'écoulement est plus épais, on retrouve numériquement la superposition des deux profils monodisperses, contrairement au cas expérimental

Comparaison de différentes méthodes numériques pour l'étude de la collision dipôle-paroi

Nous présentons une confrontation entre différentes méthodes numériques pour résoudre les équation de Navier-Stokes : Volumes Finis, Différences Finies, Fourier, Tchebyshev, Gaz de Boltzmann, Ondelettes, ... pour l'étude de la collision entre un dipôle et une paroi non glissante. La précision obtenue en fonction de la résolution, l'ordre des méthodes, les temps de calculs requis seront présentés pour toutes les méthodes

Instabilité dans les écoulements granulaires ségrégés

National audienceL'écoulement sur plan incliné d'un milieux granulaire sec constitué de deux types de particules est étu-dié. Les particules présentent un rapport de taille et de densité (les grandes particules sont plus denses). Pour un système initialement bi-couche (particules denses en surface), une instabilité de Rayleigh-Taylor se développe pendant l'écoulement. Dans le cas d'un système initialement homogène, la ségrégation va induire la formation d'une couche de particules grandes et denses à la surface qui se déstabilisera dans un second temps. Dans les deux cas, l'écoulement granulaire converge vers un système de bandes alternées avec des cellules de recirculation analogues aux cellules de convection de Rayleigh-Bénard. Abstract : Dry granular flow made of two types of particle over a rough incline is studied. Particles have a size and a density ratio (large particles are denser). When the system is initially made of two layers (dense particles above), a Rayleigh-Taylor instability develops during the flow. In the case of an initially homogeneous system, the granular segregation will lead to the formation of a layer of dense and large particles at the surface which will destabilise in a second time. For both cases, the granular flow evolves toward a pattern of alternating bands with recirculation cells analogous to Rayleigh-Bénard convection cells

A polydisperse sedimentation and polydisperse packing model

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Self-Induced Rayleigh-Taylor Instability in Segregating Dry Granular Flows

International audienceDry granular material flowing on rough inclines can experience a self-induced Rayleigh-Taylor (RT) instability followed by the spontaneous emergence of convection cells. For this to happen, particles are different in size and density, the larger particles are the denser but still segregate toward the surface. When the flow is, as usual, initially made of two layers, dense particles above, a Rayleigh-Taylor instability develops during the flow. When the flow is initially made of one homogeneous layer mixture, the granular segregation leads to the formation of an unstable layer of large-dense particles at the surface which subsequently destabilizes in a RT plume pattern. The unstable density gradient has been only induced by the motion of the granular matter. This self-induced Rayleigh-Taylor instability and the two-layer RT instability are studied using two different methods, experiments and simulations. At last, contrarily to the usual fluid behavior where the RT instability relaxes into two superimposed stable layers of fluid, the granular flow evolves to a pattern of alternated bands corresponing to recirculation cells analogous to Rayleigh-Bénard convection cells where segregation sustains the convective motion