Large-eddy simulation of wave turbulent boundary layer over rippled bed

Abstract

Large-eddy simulation (LES) of turbulent boundary layer induced by external oscillatory flow without or with a unidirectional component over rippled bottom is presented. These flows represent prototype turbulent flows of wave–current interactions in the coastal zone. The numerical method is based on a time-splitting scheme for the temporal discretization and a finite-differences approximation on orthogonal grid for the spatial discretization. The immersed boundary (IMB) method is utilized to represent complex boundary shapes, i.e., the rippled bed, on the orthogonal grid. Results are presented for oscillatory flow over ripples of three steepness values and for oscillatory-unidirectional flow of two current magnitudes, including comparisons to available experimental data. In general, the effect of ripple steepness on flow separation, vorticity dynamics, turbulence, wall stress and drag is found to be strong, while the corresponding effect of current relative strength is weak. Finally, the wave friction factor, for the oscillatory flow cases, is computed and, thus, the effective Nikuradse roughness of the rippled bed is evaluated.