Discharge of elongated grains in silos under rotational shear

TL;DR

This study numerically investigates how rotating bottoms in silos affect the discharge of elongated grains, revealing complex dependencies of flow rate on shear, orifice size, and grain orientation, with implications for controlling granular flow.

Contribution

It introduces a detailed numerical analysis of elongated grain discharge under rotational shear, highlighting the transition from funnel to mass flow and the effects of shear on flow rate and grain orientation.

Findings

Flow rate decreases up to 70% with initial shear, then modestly increases with more shear.

Flow behavior differs between spheres and rods, with rods following an exponential trend at small apertures.

External shear influences packing fraction, grain orientation, and flow pattern transition.

Abstract

The discharge of elongated particles from a silo with rotating bottom is investigated numerically. The introduction of a slight transverse shear reduces the flow rate $Q$ by up to 70% compared to stationary bottom, but the flow rate shows a modest increase by further increasing the external shear. Focusing on the dependency of flow rate $Q$ on orifice diameter $D$, the spheres and rods show two distinct trends. For rods, in the small aperture limit $Q$ seems to follow an exponential trend, deviating from the classical power-law dependence. These macroscopic observations are in good agreement with our earlier experimental findings [Phys. Rev. E $\textbf{103}$, 062905 (2021)]. With the help of the coarse-graining methodology we obtain the spatial distribution of the macroscopic density, velocity, kinetic pressure, and orientation fields. This allows us detecting a transition from funnel to mass flow pattern, caused by the external shear. Additionally, averaging these fields in the region of the orifice reveals that the strong initial decrease in $Q$ is mostly attributed to changes in the flow velocity, while the weakly increasing trend at higher rotation rates is related to increasing packing fraction. Similar analysis of the grain orientation at the orifice suggests a correlation of the flow rate magnitude with the vertical orientation and the packing fraction at the orifice with the order of the grains. Lastly, the vertical profile of mean acceleration at the center of the silo denotes that the region where the acceleration is not negligible shrinks significantly due to the strong perturbation induced by the moving wall.