Heaps of sand in flows within a split-bottom Couette cell.

Abstract

In this paper, we study the flow of angular grains in a split-bottom Couette cell. Grains departing from a spherical shape result in collective flow fields that form a heap on the free surface. Here we extend on previous observations in split-bottom cells, exploring a wider range of flows within the inertial regime and finding a richness collection of behaviours. Surface height profiles and velocity profiles are accurately measured with digital image analysis. These measurements allow the characterization of the flow regimes within the cell and the heap morphology. We show that the known flow regimes in split-bottom geometries, like the universal and wall-collapsed regimes, can also be observed in moderately high inertial flows, extending the range for studying universal shear banding. The heap morphology is amplified by the flow inertia, with a partial collapse when the cell comes to a halt. Moreover, at high angular velocities, flows under low confinement will spread radially outwards, while flows under high confinement will develop localized particle ejections. Our results complement the observation of free-surface deformations of flows of nonspherical grains. These observations suggest a need for considering deformable free surface boundary conditions in the simulation of angular grains during shear, with repercussions in the characterization and prediction of natural mass flows.