Turbulent flows over rough surfaces are encountered in many engineering and geophysical applications. Flows of this nature, due to their increasing technological interests, have been a subject of rigorous investigation in recent years. Of the particular interest to the oceanographic community is the study of turbulent oscillatory flow over rough surfaces, representative of sediment-bed in a coastal environment. In particular, formulation of predictive criteria for onset of sediment motion requires detailed knowledge of the dynamics of the near-bed turbulence structure and resultant variations in the magnitudes and time-scales of the destabilizing drag and lift forces on sediment grains. The primary objectives of this work are (i) to quantify, using high-fidelity numerical experiments, sediment grain-turbulence interactions and (ii) provide data on the temporal variations in the magnitude of drag and lift forces on sediment grains, the time-scales associated with these variations, and their correlations with the near-bed turbulence in an oscillatory flow environment.
To this end, particle-resolved direct numerical simulations (DNS) are performed to investigate the behavior of an oscillatory flow field over a bed of closely packed fixed spherical particles for a range of Reynolds numbers in transitional and rough turbulent flow regime. Presence of roughness leads to a substantial modification of the underlying boundary layer mechanism resulting in increased bed shear stress, reduction in the near-bed anisotropy, modification of the near-bed sweep and ejection motions along with marked changes in turbulent energy transport mechanisms. Characterization of such resulting flow field is performed by studying statistical descriptions of the near-bed turbulence for different roughness parameters. A double-averaging technique is employed to reveal spatial inhomogeneities at the roughness scale that provide alternate paths of energy transport in the turbulent kinetic energy (TKE) budget. Spatio-temporal characteristics of unsteady particle forces by studying their spatial distribution, temporal auto-correlations, frequency spectra as well as cross-correlations with near-bed turbulent flow variables are reported. Intermittency in the forces by means of impulse is also investigated.