  In the present study, we investigate the settling of prolate spheroidal
particles in a quiescent fluid by means of particle-resolved direct numerical
simulations. By varying the particle volume fraction (phi) from 0.1% to 10%, we
observe a non-monotonic variation of particle mean settling velocity with a
local maximum at phi=1%. To explain this finding, we examine the spatial
distribution and orientation of dispersed particles. The particle-pairs
statistics show the tendency of particles to form column-like microsturctures,
revealing the attraction and entrapment of paritcles in wake regions. This
effect results in the formation of large-scale particle clusters at
intermediate particle volume fractions. The most intense particle clustering at
phi=1%, quantified by the standard deviation of Voronoi volume, induces a swarm
effect and substantially enhances the particle settling rate by around 25%.
While, in the very dilute suspension at phi=0.1%, the particle clustering is
attenuated due to the long inter-particle distance. In dense suspensions,
however, the crowded particle arrangement disrupts particle wakes, breaks
particle microstructures and inhibits the particle clustering. Consequently,
hindrance effect dominates and reduces the settling speed. In contrast to
particle spatial distribution, the particle orientation plays a less important
role in the mean settling velocity. At last, we examine the collision rate of
settling particles by computing the collision kernel. As phi increases, the
collision kernel decreases monotonically, which is primarily attributed to the
decrease of radial distribution function. However, the radial relative velocity
is almost a constant with the change of phi. These results reveal the
importance of hydrodynamic interactions, including the wake-flow-induced
attractions and the lubrication effect, in collisions among settling spheroids.
