Settling characteristics of bidisperse dilute suspension in the vortex shedding regime

Abstract

In a fully periodic domain, monodisperse particles form clusters while settling in stagnant fluids at high Reynolds numbers (Re > 250) and dilute suspensions (solid volume fraction less than 1%). This is due to the entrapment of particles in the wakes developed by upstream particles. In this paper, this phenomenon is investigated for suspensions containing particles of different sizes that shed vortices during settling. To model the particle-fluid and particle-particle interactions, the immersed boundary method and discrete element method are used, respectively. Initially, the particles are randomly distributed in the computational domain and allowed to settle under the action of gravity. The gravitational force acting on the particles is adjusted to obtain the desired Reynolds number. The total solid volume fraction used in the simulations is about 0.1%, and the settling Reynolds number, which is based on the Sauter mean diameter, ranges from 250 to 450. Two particle diameter ratios (i.e., diameter of larger particles to smaller particles) of 2:1 and 3:1 are studied. For each particle diameter ratio, the mass fraction for each particle size varies from 0.2 to 0.8. For comparison, simulations of monodisperse particles settling under similar conditions are also conducted, and the average settling velocity, particle velocity fluctuations, and particle microstructures are studied. The simulation results show that, in the case of bidisperse particles, the settling characteristics are dominated by the larger-sized particles. Finally, the physics behind the studied anomalies is discussed in detail.