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

Discharge flow of a granular media from a silo: effect of the packing fraction and of the hopper angle

International audienceSilos are widely used in the industry. While empirical predictions of the flow rate, based on scaling laws, have existed for more than a century (Hagen 1852, translated in [1]-Beverloo et al. [2]), recent advances have be made on the understanding of the control parameters of the flow. In particular, using continuous modeling together with a mu(I) granular rheology seem to be successful in predicting the flow rate for large numbers of beads at the aperture (Staron et al.[3], [4]). Moreover Janda et al.[5] have shown that the packing fraction at the outlet plays an important role when the number of beads at the apeture decreases. Based on these considerations, we have studied experimentally the discharge flow of a granular media from a rectangular silo. We have varied two main parameters: the angle of the hopper, and the bulk packing fraction of the granular material by using bidisperse mixtures. We propose a simple physical model to describe the effect of these parameters, considering a continuous granular media with a dilatancy law at the outlet. This model predicts well the dependance of the flow rate on the hopper angle as well as the dependance of the flow rate on the fine mass fraction of a bidisperse mixture

Charriage de particules dans un écoulement cisaillé

Colloque avec actes et comité de lecture. Internationale.National audienceUn lit de particules soumis à un écoulement de fluide, par exemple le lit d'une rivière, se met en mouvement quand les forces hydrodynamiques deviennent supérieures à une fraction du poids apparent des particules. Nous étudions expérimentalement le transport de particules dans un tube à section rectangulaire. Nous comparons les résultats aux prédictions d'un modèle continu à deux phases, dans lequel nous utilisons une rhéologie granulaire pour la contrainte solide

Investigation of the mobile granular layer in bedload transport by laminar shearing flows

International audienceThe mobile layer of a granular bed composed of spherical particles is experimentally investigated in a laminar rectangular channel flow. Both particle and fluid velocity profiles are obtained using particle image velocimetry for different index-matched combinations of particles and fluid and for a wide range of fluid flow rates above incipient motion. A full three-dimensional investigation of the flow field inside the mobile layer is also provided. These experimental observations are compared to the predictions of a two-phase continuum model having a frictional rheology to describe particle-particle interactions. Different rheological constitutive laws having increasing degrees of sophistication are tested and discussed

Bedload Transport. Part 1 : Two-Phase Model and 3D Numerical Implementation.

GENCI- IDRIS (Grant 2010- 96212)International audienceWe have developed a three dimensional numerical model (Chauchat and Médale, 2010) based on the two-phase modeling having a Newtonian rheology for the fluid phase and Coulombtype friction for the particulate phase which has been developed by Ouriemi et al. (2009a) to study bedload transport in pipe flows. The governing equations are discretized by a finite element scheme and a penalisation method is introduced to cope with the incompressibility constraint. A regularisation technique is used to deal with the visco-plastic behaviour of the granular phase. We have performed three-dimensional computations for bedload transport in rectangular cross-section duct when the bed interface remains fixed. This numerical model which captures the complex coupling between the granular media and the fluid should enable a better understanding of the sediment transport mechanisms in these duct flows (Ouriemi et al., 2009b). The geometry corresponds to the one used in Pailha et al. (2011) where the authors investigate experimentally the behaviour of the flowing granular layer. The presented model and numerical results will be used for a detailed comparison in a near future

Bedload Transport. Part 2: The mobile granular Layer

International audienceWe present a joint theoretical, numerical, and experimental investigation of the mobile granular layer in bedload transport conditions for pipe flows. The theoretical approach uses a twophase model having a Newtonian rheology for the fluid phase and Coulomb-type friction for the particulate phase which has been recently proposed by Ouriemi et al. (2009a). This model has been implemented into a 3D numerical code by Chauchat and Médale (2010) which can describe bedload transport in square and circular cross-section ducts. The experiments are undertaken in a rectangular duct partially filled with transparent spherical particles driven by an index-matched fluid. Direct imaging of the particles and of the fluid in a vertical slice is obtained owing to the addition of fluorescing tracers and to the illumination of the duct by a laser sheet. The main quantity that will be examined and discussed is the velocity field

Conditions d'écoulement d'un lit de particules déposées

Nous présentons les différents régimes d'évolution d'un lit de particules soumis à un écoulement de Poiseuille. Nous avons mis en place un méthode expérimentale reproductible de détermination du seuil de mise en mouvement des particules et nous avons construit un digramme de phase préliminaire d'existence de ces différents régimes

Effondrement granulaire immergé : rôle de la fraction volumique solide initiale

Nous présentons l'étude expérimentale de l'effondrement d'une colonne de grains dans un liquide visqueux. Contrairement au cas sec, le rapport d'aspect initial du tas n'est plus le seul paramètre contrôlant la morphologie du dépôt ni de la dynamique de l'avalanche. Dans le régime visqueux, cette dernière est contrôlée par la fraction volumique solide initiale de la colonne. Nous avons identifié deux régimes principaux selon que le compactage initial est dense ou lâche et les enregistrements de pression sous le tas suggèrent que la dilatance est un mécanisme physique majeur

Liquéfaction des sols sous vagues

Nous étudions expérimentalement le phénomène de liquéfaction des sols sous vagues. Nous utilisons un canal à houle dans lequel nous plaçons un fond sableux initialement lâche et mesurons la pression de pore à différentes profondeurs dans le sable lors du passage des vagues. Le signal de pression de pore moyen présente trois grandes phases d'évolution : une élévation rapide suivie d'une phase de pic ou de plateau puis d'une diminution progressive. Nous interprétons ces résultats par une liquéfaction rapide d'une partie du sol, suivie d'un front de sédimentation. Nous nous intéressons enfin au critère de liquéfaction ainsi qu'à l'épaisseur de sable liquéfiée, en tentant de mettre en évidence les nombres sans dimension pertinents