Direct numerical simulation of particle dispersion in homogeneous turbulent shear flows

In a recent paper [Phys. Fluids 12, 2906 (2000)] we presented the results of our direct numerical simulations (DNS) study of the two-way coupling effects of solid particles dispersed in a homogeneous turbulent shear flow. The objective of that study was to describe the physical mechanism associated with two-way coupling. Here, we present a DNS study of the dispersion of small spherical particles and fluid points in a homogeneous turbulent shear flow with one-way coupling. The effects of varying shear number, particle inertia, gravity magnitude, and direction on particle dispersion were studied. Our results show that the particle mean square displacement in the streamwise direction proportional tot(2) for short times, whereas for large times, the time exponent exceeds 3. The larger the particle inertia, the larger is the exponent. is at least one order of magnitude larger than the mean dispersion in the two lateral directions. This dispersion anisotropy is due to the dominance of the spanwise vortex layers which are elongated and inclined toward the streamwise direction, thus suppressing the velocity fluctuations in the lateral directions. These layers are generated from the inclined longitudinal vortex tubes by the action of the mean shear [Kida and Tanaka (1994)]. Preferential accumulation of particles in homogeneous shear flows at zero gravity is maximum when tau (p) equals the Kolmogorov time scale. (C) 2001 American Institute of Physics.