Velocity profile of granular flow inside a flat-bottomed silo: The influence of front and rear walls

The discharge behavior of granular assembly inside a thin flat-bottomed silo is investigated by performing three-dimensional (3D) Discrete Element Method (DEM) simulations. The influence of boundary conditions in the thickness direction is evaluated by applying both wall conditions with varying friction coefficients and periodic boundary (PB) conditions. It is found that the introduction of frictional front and rear walls leads to the stick–slip motion of discharged particles and upward propagation of velocity waves, manifested by the formation of bubble-like sub-flow zones. In contrast, the adoption of periodic boundary condition in the thickness direction results in the resonant movement of particles. Statistical analyses indicate that the different modes of particle motion can be ascribed to the different topological features of particle contact networks. While the direction distribution of particle contacts in PB case is approximately isotropic, in the wall case, it exhibits notable anisotropy and a significant amount of particle contacts orient toward the frictional front and rear walls. The DEM simulation results demonstrate that under the conditions considered here, the horizontal profile of the particle vertical velocity in the wall case follows the Gaussian type, whereas that in the PB case presents a clear flattened feature. Analysis of the transient horizontal distributions of particle vertical velocity suggests that the Gaussian-type velocity profile can be attributed to the continuous emergence and updrift of the bubble-like sub-flowing zones.