Flow of granular material through an upright 2D silo with a tilted orifice: effects of vertically offset bottom boundaries

The flow of granular materials from silos is of great importance in industry and for understanding multiphase flow of particulate matter. Recent works have demonstrated that the flow rate of the material can be modified by tilting the entire silo with respect to gravity. Here, we demonstrate in novel experiments of quasi-2D silo discharge that the flow rate can also be reduced by adjusting the relative height between each edge of the silo orifice without rotating the entire container. We show that the flow rate decreases with orifice angle, but that this decrease is not as strong with angle as when the entire container is tilted. However, with a simple modification, a grain-sized ridge set on each side of the orifice, the flow rate collapses with prior tilted-silo results. Moreover, for steep angles and in the absence of a ridge, the material starts to form a horizontal channel of flow along the lower orifice boundary. These findings suggest that the slip zone of grains along the bottom boundary plays an important role when the symmetry of flow is broken. Indeed, we find that the stagnant zones next to the lower and upper orifice edges exhibit asymmetric creep speeds with increasing tilt angle. We successfully capture these findings by extending a previous model of tilted silo discharge [Kozlowski et al., Granular Matter 25 (2023) 19], accounting now for stagnant zone creep.