Discharge Rate of a Silo with Rotating Bottom*

Discharge of spherical grains from a silo was investigated numerically with the bottom plate rotating around the vertical axis of the cylindrical silo. We observe that the flow rate exhibits an increasing trend for small, and a nonmonotonic trend for larger orifices as a function of rotation speed, which was also recently found in experiments. Our study sheds light on the cause of this peculiar behavior. Using a coarse-graining technique, we compute the macroscopic density, momentum, and the stress tensor fields. These results show conclusively that changes in the discharge process are directly related to changes in the flow pattern from funnel flow to mass flow. In addition, by analyzing the spatial distribution of the kinetic stress, we find that for small orifices increasing rotational shear enhances the mean kinetic pressure and the system dilatancy. This reduces the stability of the arches, and, consequently, the flow rate increases monotonically. For large orifices, however, we detected that the kinetic stress changes nonmonotonically, which might explain the nonmonotonic behavior of the flow rate when varying the rotational shear.