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Abstract
Evolution of sand ripples in waves and combined wave-current flows is investigated using a numerical model that allows the simulation of coupled flow-bedform evolution in turbulent flows. The numerical model is a large-eddy simulation code which solves the 3-dimensional Navier-Stokes equation with a dynamic mixed subgrid turbulence model on moving generalised curvilinear coordinates. The model allows the calculation of hydrodynamics on a moving bed due to the sediment pick-up and erosion. With a resolved fine grid resolution near the bed, detailed observations of the hydrodynamics in the turbulent boundary layer and the associated sediment transport can be obtained. We investigate the development of sand ripples starting from a flat bed. The model successfully reveals the detailed dynamics of bedform evolution, sediment transport, and flow features during different stages of the ripple development, namely, bedform initiation, rolling grain ripples, and vortex ripples. Furthermore, we investigate sand ripple evolution in waves, waves with weak currents, and waves with strong currents, in which wave and current directions are orthogonal to each other. The present model results are compared with observ ations from the laboratory experiment. Comparison with the sonar images from the laboratory experiment shows good agreement between the model and laboratory results, demonstrating the capability of the present model to capture real features of combined wave-current flow in the laboratory.