Kinematic simulations as a subgrid-scale model for particle motion: a priori LES of homogeneous isotropic turbulence

Abstract

In this work we investigated the impact of filtering and subgrid-scale modeling on particle settling velocity and collision-related statistics in a turbulent flow. To reduce the complexity of this task we first studied the motion of inertial particles in the low-pass filtered homogeneous and isotropic turbulence, which was subsequently enriched with the subgrid-scale velocity components obtained from a frozen high-pass filtered velocity field. Particular emphasis has been put on the radial distribution function and the radial relative velocity of nearly touching particles both in the presence or absence of the gravitational settling. These statistics are the key input parameters to the kinematic collision kernel which is of crucial importance in determining the collision rate of inertial particles in a turbulent flow. Furthermore, kinematic simulations were selected as a means of enhancing the fluid velocity at particle locations. We analyzed a wide range of Stokes numbers, i.e. a measure of particle inertia, and, in contrast to scientific premises found in the literature, we observed no improvement of particle statistics when the low-pass filtered fluid velocity was enriched with both a synthetic or spectrally-filtered small-scale structures. We discuss the shortages of any frozen-velocity-based subgrid-scale model in predicting both single- and two-point particle statistics. We also indicate that in some cases, particularly concerning the collision rate of particles suspended in homogeneous and isotropic turbulence, subgrid-scale contribution in the particle equation of motion can be neglected.