Study of the origin of shear zones in quasi-static vertical chute flows by using discrete particle simulations

We perform discrete-particle simulations of vertical chute flows in the quasi-static regime using disk-like particles. The velocity profiles show a plug flow in the central region and shear zones next to the walls approximately 6 particle diameters wide regardless of bin width, as was observed experimentally. The stress distributions are in good agreement with the predictions of the continuum mechanics equations even for small systems (15 and 20 particle diameters wide) as those studied in the present work. Large stress fluctuations in space and time have been observed, these are mainly due to the inhomogeneity of the force network. It is observed in the simulations that the wall friction does not act homogeneously but it is concentrated at certain points on the wall depending on the local arrangement of the packing. Large stress zones or arches appear at these points of the wall. It is the formation and the way these arches collapse that seems to generate the shear zones. Based on this, a simple mechanism to explain the formation of the shear zone is proposed. The simulations have revealed other interesting features of the flow, particularly the presence of macroscopic fluctuations of velocity, in which large blocks of material move together showing sudden accelerations (corresponding to the collapse of the arches) and sudden decelerations (corresponding to the formation of the arches).