Convection related phenomena in granular dynamics simulations of vibrated beds

Three-dimensional discrete element simulations are carried out to investigate the behavior of a shallow bed of inelastic, frictional spheres (of uniform diameter d ), which are energized by vertical sinusoidal oscillations of a plane floor at amplitude a and frequency ω=2πf. We investigate the long-term and instantaneous velocity fields as well as the evolution of the pressure tensor. Results show that the onset of convection reported in the literature is not only determined by the floor acceleration, but also the ratio a/d. In a wide bed (L/d∼100) narrow persistent vortices appear near vertical sidewalls, while no distinct pattern is found within the central region. A large sphere within the bed is convected upward to the surface and either “segregates” itself from the bulk, or becomes reentrained, depending on the width of the downward velocity field near the wall relative to the sphere size. An inspection of the bed microstructure reveals internal vortex-like cells spanning its width giving rise to arching observed in recent experiments and other simulations. Computations of the potential constituent of the pressure tensor revealed high values in collision-dominated regions of the bed and a trend that repeated every two oscillations of the floor.