Small particles in homogeneous turbulence: Settling velocity enhancement by two-way coupling

The gravitational settling of an initially random suspension of small solid particles in homogeneous turbulence is investigated numerically. The simulations are based on a pseudospectral method to solve the fluid equations combined with a Lagrangian point-particle model for the particulate phase (Eulerian-Lagrangian approach). The focus is on the enhancement of the mean particle settling velocity in a turbulent carrier fluid, as compared to the settling velocity of a single particle in quiescent fluid. Results are presented for both one-way coupling, when the fluid flow is not affected by the presence of the particles, and two-way coupling, when the particles exert a feedback force on the fluid. The first case serves primarily for validation purposes. In the case with two-way coupling, it is shown that the effect of the particles on the carrier fluid involves an additional increase in their mean settling velocity compared to one-way coupling. The underlying physical mechanism is analyzed, revealing that the settling velocity enhancement depends on the particle loading, the Reynolds number, and the dimensionless Stokes settling velocity if the particle Stokes number is about unity. Also, for particle volume fractions Phi(v)greater than or similar to 10(-5), a turbulence modification is observed. Furthermore, a direct comparison with recent experimental studies by Aliseda [J. Fluid Mech. 468, 77 (2002)] and Yang and Shy [J. Fluid Mech. 526, 171 (2005)] is performed for a microscale Reynolds number Re-lambda approximate to 75 of the turbulent carrier flow.