Observation of star-shaped surface gravity waves.

We report a new type of standing gravity wave of large amplitude, having alternatively the shape of a star and of a polygon. This wave is observed by means of a laboratory experiment by vertically vibrating a tank. The symmetry of the star (i.

Propagation of acoustic waves in a one-dimensional array of noncohesive cylinders.

By means of a photoelastic method, we access the visualization of acoustic waves propagating in a one-dimensional array of noncohesive cylinders. As pointed by Nesterenko in the case of spherical grains [V. F.

New standing solitary waves in water.

By means of the parametric excitation of water waves in a Hele-Shaw cell, we report the existence of two new types of highly localized, standing surface waves of large amplitude. They are, respectively, of odd and even symmetry. Both standing waves oscillate subharmonically with the forcing frequency.

Response of a noncohesive packing of grains to a localized force: deviation from continuum elasticity.

In order to characterize the mechanical behavior of grain piles, we investigate the response of a noncohesive two-dimensional packing of cylinders submitted to a localized force. By means of image processing, we obtain an accurate measurement of the individual grain displacements in the reversible regime of deformation. The measured displacement field deviates unambiguously from the predictions of linear elasticity and of other theoretical descriptions commonly used to model the behavior of cohesionless soils in civil engineering or in soil mechanics.

Some remarks on the rheology of dense granular flows.

The paper reviews some peculiar properties exhibited by granular flows. We emphasize the inability of kinetic theory and of Bagnold's heuristic approach to describe the rapid regime of densely packed flows, characterized by the breakdown of the binary collision picture and by multibody long-lasting contacts. We suggest that deformation waves through the continuous paths of contacts can be effective to transport momentum and energy through the bulk, in a time very short compared to the inverse of the shear rate.

Dense, rapid flows of inelastic grains under gravity.

The standard hydrodynamic description does not apply to the rapid flow regime of inelastic grains in the dense limit. Emphasizing the role of inelastic loss and collapse, we propose a new approach relying on a nonlocal dissipation scheme. Our model succeeds in accounting qualitatively and quantitatively for the linear profile of velocity found in experiments on dense gravity-driven flows.

Development of grain avalanches.

We present a new theoretical approach aimed to describe the dynamical properties of avalanches. Starting from simple microscopic considerations, we propose an analytical derivation accounting for the avalanche growth as a function of time. The solution resembles very closely recent experimental observations.

Dynamics of grain avalanches.

We study the nucleation and the growth of avalanches in a model experimental system consisting of a bidimensional packing of noncohesive grains positioned in a rotating drum. We show that the avalanche mass increases linearly in time, and that the growth rate is governed by the velocities of the two up and down fronts. The upper front is shown to propagate upwards with a velocity which is equal to the averaged velocity of the flowing grains, whereas the velocity of the downslope propagating front is approximately equal to twice the avalanche velocity.

Stress transmission through textured granular packings.

We propose a theoretical approach aimed to describe the stress transmission through granular pilings. According to our framework, the stress transmission obeys a diffusive process in random isotropic packings, while it follows a local convection-diffusion equation in textured packings characterized by an anisotropic distribution of the contact orientations. In the latter case, the direction of the convection depends on the angular distribution function of the contact orientations.

Stress transmission through a model system of cohesionless elastic grains.

Understanding the mechanical properties of granular materials is important for applications in civil and chemical engineering, geophysical sciences and the food industry, as well as for the control or prevention of avalanches and landslides. Unlike continuous media, granular materials lack cohesion, and cannot resist tensile stresses. Current descriptions of the mechanical properties of collections of cohesionless grains have relied either on elasto-plastic models classically used in civil engineering, or on a recent model involving hyperbolic equations.