Numerical simulations of random suspensions at finite Reynolds numbers

The sedimentation of solid spherical particles in an initially quiescent fluid is investigated through numerical simulations. Under conditions of Stokes flow, the results provide good agreement with experiments for the mean settling velocity while the velocity fluctuation levels grow with the size of the system in accordance with theoretical predictions for homogeneous suspensions. New results on finite Reynolds number suspensions are presented that illustrate the role of wake-induced interactions between particles. A significant reduction in the average settling velocity is explained by an enhancement of a wake-induced scattering process. Anisotropy in the fluid flow is evident and the evolution of velocity correlations is investigated for various particulate Reynolds numbers. Increases in Reynolds number tend to decrease the integral length scale of the fluctuating flow field and reduce significantly the Lagrangian time scale of the velocity fluctuations. Analysis of the evolution of fluctuation levels when the domain width grows indicates that inertial screening dramatically reduces the divergence.