| dc.description.abstract | This thesis examines the mixing behaviors of granular materials subjected to external vertical vibration and sheared force. Three-dimensional discrete element computer simulation is used to study the velocity distribution, convective flow, diffusive motion and granular mixing in both vibrated granular bed and sheared granular flow.
A series of systematic validation tests for the DEM program, including six tests of simple collision system and a test of macroscopic phenomenon. The conservation laws of momentum and kinetic energy are used to verify tests of collisions between particles or between particles and boundaries. Also, the simulation result of symmetric convection rolls are compared with the experimental result. With frictional sidewalls, the convection flow is a very important phenomenon in the vibrated granular bed. The influence of vibrating conditions, including vibration acceleration and frequency, on the formation of symmetric convection flow is investigated in this work. In order to characterize the convective flow and the diffusive motion of granular materials, the dimensionless convection flow rate, Jconv, and the vertical self-diffusion coefficient, Dyy, are defined, respectively. The Péclet number, Pe, is employed to characterize the ratio of the convective flow to the diffusive motion in vertical direction. The role of Pe in the formation of symmetric convection flow is discussed in detail. Moreover, the top-bottom initial loading pattern of two groups of glass beads with different colors is employed to investigate mixing behavior of granular materials. The well-known Lacey index is employed as the mixing degree, M, to quantify the mixing quality. The mixing rate is calculated from a least-square fit using the time evolution of M. The simulation results demonstrate that the mixing rate increases with increasing Dyy in exponential relation. In order to characterize the effect of electrostatic force on the granular flow, the Electrostatic Number Es is defined as the ratio of the electrostatic force to the particle weight. The simulation results demonstrate that the granular temperatures increase linearly with the increasing Es number. Meanwhile, the mixing rate constants increase with the increasing Es number in power law relations. In the simulation of sheared granular flow, the initial loading of identical glass beads with different colors is also arranged in a top-bottom loading pattern. The transverse mixing of particles is caused by the moving bottom wall with a constant velocity along the x direction. The mixing layer thicknesses are compared with the calculations from a simple diffusion equation using the data of apparent self-diffusion coefficients obtained from the simulation measurements. The calculations and simulation results showed good agreements, demonstrating that the mixing process of granular materials occurred through the diffusion mechanism. | en_US |