(380am) Fluid-Particle Drag in Low-Reynolds-Number Bidisperse Suspensions

A second-order immersed boundary-lattice Boltzmann method (IB-LBM) is used to study the drag force of freely evolving bidisperse suspensions at low Reynolds numbers. The total solid volume fraction of the suspension is varied between 0.1 and 0.3, and the particle size ratio is from 1.2:1 to 2:1. The main characteristics of freely evolving suspensions, compared to fixed beds that were reported in the literature, is that the two dispersed phases have different slip velocities relative to the fluid phase. Hence, in freely evolving bidisperse systems, the drag force on each particle phase is normalized by the Stokes drag force based on their corresponding slip velocities. It is found that the drag force in freely evolving bidisperse suspensions is significantly different from previous results based on computations of systems with fixed particles. The drag force on small particles on average decreases much slower than that in fixed beds as the particle size ratio increases. The difference of the drag force on the small and large particles is profoundly smaller than that predicted using previous relations obtained from fixed-bed simulations. Based on the present simulation results, new relations for the drag forces on both small and large particles are proposed for bidisperse suspensions. The effects of the slip velocities of the small and large particles are explicitly accounted for in the new relations.