DEM modeling of the dilute-to-dense transition of granular flow in silos

The dilute-to-dense transition of granular flow in silo has been investigated by researchers, yet a detailed particle-scale description of this transition is still lacking. In this study, three-dimensional (3D) discrete element method (DEM) simulations are performed to investigate the dynamics of particles during this transition process through adjusting the inflow rate at the upper inlet. Based on Voronoi tessellation, the concept of caged particles is introduced to characterize the microscopic packing details of particles inside the silo. The analyses of probability density functions (PDFs) of vertical velocity and local packing fraction of particle in the proximity of outlet suggest that the dilute flow regime can be divided into two sub-stages (stage I and stage II). And the transition from stage I to stage II is manifested by the formation of persistent connection between the left and right clusters made of caged particles. It is found that for the dilute-to-dense transition process under fixed inflow rate condition, there exist two well defined critical packing heights that scale with the outlet size. Dynamic buildup and collapse of contact force network astride the whole outlet take place when the packing height of particle assembly at the centerline is in between these two critical heights. The simulation results thus suggest that the formation of a persistent contact force network spanning the whole outlet signals the transition of flow state from the dilute to dense.