Direct Numerical Simulation of Turbulent Flow over Sandy Rippled Beds

The presence of ripples on the seafloor affects the turbulent dynamics of bottom boundary layer (BBL) flow. The difference in the roughness length scales of a planar and rippled sand bed produces quantifiable differences in the turbulent BBL. A complete understanding of the effect of suspended sediment concentration on turbulence modulation is currently unknown. We use mixture theory to implement a three-dimensional BBL model that simulates the coupled interaction between the fluid and sediment. The mixture theory approach treats the fluid-sediment mixture as a single continuum with effective properties that parametrize the fluid-sediment and sediment-sediment interactions. We compare two-dimensional and three-dimensional simulations with existing laboratory measurements of fluid velocity and sediment concentration over rippled sand beds. We find that the vortex dynamics over sand ripples are highly three-dimensional. Two-dimensional flow simulations are inadequate for the numerical modeling of turbulent flow over sand ripples. We also find that suspended sediment concentration influences the production of turbulence; therefore, accurate simulation of turbulent flow over sandy beds must include an adequate description of fluid-sediment interactions.