Settling and clustering of particles of moderate mass density in turbulence

Abstract

We present a numerical study of settling and clustering of small inertial particles in homogeneous and isotropic turbulence, in the dilute regime in which particles do not interact with each other or affect the fluid. Particles are denser than the fluid, but not in the limit of being much heavier than the displaced fluid. At fixed Reynolds and Stokes numbers, we vary the fluid-to-particle mass ratio and the gravitational acceleration. The effect of varying one or the other is similar but not quite the same. We report nonmonotonic behavior of the particles' velocity skewness and kurtosis with the second parameter, and an associated anomalous behavior of the settling velocity when compared to the free-fall Stokes velocity, including some cases of loitering. Clustering increases for increasing gravitational acceleration and for decreasing fluid-to-particle mass ratio.