Settling of heavy particles in a turbulent Stokes layer: Numerical simulations

The influence of the turbulence structure on the motion of a small heavy sphere settling in a turbulent Stokes layer is numerically investigated for moderately high values of the flow Reynolds number. A one-way coupling numerical scheme is implemented in order to evaluate the forces instantaneously acting on the particle and calculate particle trajectories. The flow field is obtained through a 3D direct numerical simulation of an oscillatory flow over a flat smooth unbounded wall. We compare results based on an averaged formulation for the hydrodynamic force acting on the particle with results based on an instantaneous formulation. The main results of our analysis, performed on a limited number of sample computations, may be summarized as follows. Firstly, for sufficiently small particles, the settling speed based on the averaged flow field turns out to be not markedly different from the settling speed based on the instantaneous flow field. Secondly, particle trajectories may be qualitatively different in the two cases, especially where turbulent events are fairly intense and the hydrodynamic force on the particle may take values up to twice as large as gravity: as a result, under the latter conditions, re-suspension may occur. Thirdly, lift is invariably found to be negligible in the present simulations.