Dynamics of non-spherical particles resting on a flat surface in a viscous fluid

Abstract

The resting dynamics of non-spherical particles on a flat surface can be considered the last phase in settling a particle, which has yet to be fully investigated. This last phase for the non-spherical particle is numerically investigated, for the first time, using a sharp-interface immersed boundary method coupled with a kinematic-based collision model. The collision model guarantees a realistic, stable/settled position of non-spherical-shaped particles, contrary to alternative models that implement a repulsive penalty force. In the simulations, a single particle is released with a constant velocity downwards close to the wall until the collision occurs. Hydrodynamic moments alter the settling dynamics depending on the Reynolds number (Re) by opposing the gravity-driven motion of particles. It was observed that the settling trajectories/angles were generally not affected for each particle, but their rate of change, i.e., angular velocities, reduced as the Reynolds number decreased. A simplified model for the hydrodynamic moment was explored based on a modified Stokes drag moment for spherical particles, which includes a shape factor Kn for relating non-spherical particles to spherical ones. It was found that using the projected area of non-spherical particles provided the best overall scaling to find their equivalent spheres because it provided the lowest Kn values. In addition, Kn was found to deviate from the constant theoretical value because of the build-up pressure between the particle and the wall which changed with Re. A linear relation between the mean Kn and Re was found to be a good approximation. This work demonstrates how particle-resolved simulations can provide the data required for developing simplified models for non-spherical particles.