Impact of particle friction and stiffness on silo discharge*

The silo discharge process was studied experimentally and numerically, varying the particle stiffness and friction. We obtained that lowering the friction coefficient of soft grains (relative deformation in the order of 10%) leads to a gradual change in the discharge curve: the flow rate becomes filling height dependent, decreasing during the discharge process. Hard grains with a maximum relative deformation of 0.05% showed a similar but less pronounced trend. The force acting at the silo bottom revealed a gradual transition from Janssen to hydrostatic-like behavior when decreasing friction. In addition, we computed the macroscopic density, velocity, and stress tensor fields by applying coarse-graining to the DEM simulation data. The analysis of the pressure fields revealed significant differences in the spatial stress transmission, when examining soft or hard particles. We proposed a phenomenological formulation based on the momentum balance that predicts the linear decrease of the flow rate with decreasing filling height for soft low-friction particles.