We study the gravity driven evolution of a step composed of non-cohesive steel beads using high-speed imaging and particle tracking. The step is initially held together with electromagnets, and released when the current is switched off. This evolution has been described by Boutreux & de Gennes (1997) using the BCRE model with boundary conditions for the granular step. They predict that the step relaxes to an angle which is less than the angle of repose.

We find the angle of repose to be constant as a function of height which obtained by pouring beads slowly in the rectangular box. 

The initial failure of the pile occurs at the surface and the depth of the flow first increases and then decreases as the pile relaxes. The final angle of inclination is lower than the angle of repose of the grains, and the rate of change of the surface inclination reaches a maximum well before angle of repose is reached contrary to the prediction of a recent convective-diffusion model.

A time sequence of the Velocity fields of the particles during the relaxation after the electromagnets are switched off at t = 0 s. Velocity fields are found by direct particle tracking. (a) t = 0.04 s, (b) t = 0.085 s, (c) t = 0.125 s, and (d) t = 0.175 s, (e) t = 0.25 s, (f) t = 0.45 s. The direction and length of the vectors corresponds to the velocity of the particle at that location. Note that the speed of the particles appear to decrease smoothly with depth, and the depth of flow increases initially and then decreases. The initial pile is observed to deform rapidly initially and then comes to rest with the free surface inclined at angle to the horizontal. It can be noted that the surface is clearly nonlinear at early times, but appears somewhat linear at the end.

Contact information:
Arshad Kudrolli, akudrolli@clarku.edu