CONVECTION RELATED PHENOMENA IN GRANULAR DYNAMICS SIMULATIONS OF VIBRATED BEDS

Three-dimensional discrete element simulations are carried out to inve stigate the behavior of a shallow bed of inelastic, frictional spheres (of uniform diameter d), which are energized by vertical sinusoidal o scillations of a plane floor at amplitude a and frequency omega=2 pi f . We investigate the long-term and instantaneous velocity fields as we ll as the evolution of the pressure tenser. Results show that the onse t of convection reported in the literature is not only determined by t he floor acceleration, but also the ratio a/d. In a wide bed (L/d simi lar to 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 inter nal vortex-like cells spanning its width giving rise to arching observ ed in recent experiments and other simulations. Computations of the po tential constituent of the pressure tensor revealed high values in col lision-dominated regions of the bed and a trend that repeated every tw o oscillations of the floor. (C) 1997 American Institute of Physics. [ S1070-6631(97)02912-7].