Long surface wave instability in dense granular flows

In this paper we present an experimental study of the long surface wave instability that can develop when a granular material flows down a rough inclined plane. The threshold and the dispersion relation of the instability are precisely measured by imposing a controlled perturbation at the entrance of the flow and measuring its evolution along the slope. The results are compared with the prediction of a linear stability analysis conducted in the framework of the depth-averaged or Saint-Venant equations. We show that when the friction law proposed in Pouliquen (1999a) is introduced in the Saint-Venant equations, the theory is able to predict quantitatively the stability threshold and the phase velocity of the waves but fails in predicting the observed cutoff frequency. The instability is shown to be of the same nature as the long wave instability observed in classical fluids but with characteristics that can dramatically differ due to the specificity of the granular rheology.Comment: 29 pages, 20 figures, to be published in Journal of Fluid Mechanic

Longitudinal Vortices in Granular Flows

We present a new instability observed in rapid granular flows down rough inclined planes. For high inclinations and flow rates, the free surface of the flow experiences a regular deformation in the transverse direction. Measurements of the surface velocities imply that this instability is associated with the formation of longitudinal vortices in the granular flow. From the experimental observations, we propose a mechanism for the longitudinal vortex formation based on the concept of granular temperature.Comment: 4 pages, 4 figure

Velocity correlations in dense granular flows

Velocity fluctuations of grains flowing down a rough inclined plane are experimentally studied. The grains at the free surface exhibit fluctuating motions, which are correlated over few grains diameters. The characteristic correlation length is shown to depend on the inclination of the plane and not on the thickness of the flowing layer. This result strongly supports the idea that dense granular flows are controlled by a characteristic length larger than the particle diameter

The influence of surface roughness on the rheology of immersed and dry frictional spheres

Pressure-imposed rheometry is used to examine the influence of surface roughness on the rheology of immersed and dry frictional spheres in the dense regime. The quasi-static value of the effective friction coefficient is not significantly affected by particle roughness while the critical volume fraction at jamming decreases with increasing roughness. These values are found to be similar in immersed and dry conditions. Rescaling the volume fraction by the maximum volume fraction leads to collapses of rheological data on master curves. The asymptotic behaviors are examined close to the jamming transition

Louis Guillaume, Gaston Bachelard , une amitié féconde

On présente ici l’itinéraire d’amitié en poésie du philosophe français Gaston Bachelard et du poète Louis Guillaume. Les influences et les échanges possibles entre deux êtres habités par le fair poétique ainsi que d’autres réflexions sur l’écriture se dégagent de la lecture attentive du journal de Louis Guillaume (1935-1971) et de la correspondance qu’ils ont entretenue entre 1951 et 1962.Se presenta aquí la trayectoria de amistad en poesía del filósofo francés Gaston Bachelard con el poeta Louis Guillaume. Las influencias y los posibles intercambios entre dos seres habitados por el hecho poético así como otras reflexiones sobre la escritura se desprenden de la lectura atenta del diario de Louis Guillaume (1935-1971) y de la correspondencia que ambos mantuvieron entre 1951 y 1962