Settling and collision of spheroidal particles with an offset mass centre in a quiescent fluid

Abstract

Gravitational sedimentation and the collision of particles play key roles in various natural and engineering processes. In practice, particles are often non-spherical in shape with non-uniform mass distribution. In this study we investigate how the mass eccentricity influences the settling and gravitational collision of non-spherical particles in a quiescent fluid. Firstly, we theoretically analyse the effect of mass centre offset on the settling motion of a single spheroid under the low-Reynolds-number assumption. We find that the competition of fluid-inertia torque and gravitational torque determines the terminal settling mode of the spheroid. As the mass centre offset increases from zero to a critical value, the orientation of a settling spheroid undergoes a transition from the broad-side-on to narrow-side-on alignment. With an intermediate mass centre offset, the settling spheroid prefers an oblique orientation with a horizontal drift. Secondly, we investigate the gravitational collision rate of settling spheroids. With the change of particle orientation, the collision kernel exhibits a non-monotonic variation with a maximum when particles settle with an intermediate oblique orientation. Therefore, adjusting the mass centre offset to alter particle orientation can indirectly affect the collision rate of settling spheroidal particles. In summary, our findings reveal the significance of the mass eccentricity on particle dynamics in fluid flows, and suggest a potential approach for manipulating the settling motion and collision rate of non-spherical particles by adjusting their mass centre position.