Large-eddy simulation of sediment transport in oscillatory flow over wavy terrain

In this work, the three-dimensional and time-dependent sediment transport patterns over ripples have been simulated and investigated with a Large Eddy Simulation code. The volume-filtered Navier Stokes equations and an advection diffusion equation have been solved for the flow and concentration fields, respectively. The flows investigated include both laboratory-scale channel flows with Reynolds numbers [Re] of 2000 and field scale flows in waves and a current with Re ∼ 650,000. Our aim is to study the detailed sediment dynamics that occur over ripples in channel flows, waves, and waves with a current at small time [O(seconds)] and spatial [O(meters)] scales.; The first simulations were of channel flow over laboratory scale ripples and of pure oscillatory flow over prototypical field scale vortex ripples. In the former case, Görtler vortices were shown to play an important role in sediment transport. In the latter, the well-known sediment transport pattern over vortex ripples—that of entrapment of sediment by the lee spanwise vortex and entrainment into the outer flow by its ejection during flow reversal—is fully described as a three dimensional process. However, in order to establish that the code was both qualitatively and quantitatively accurate, further simulations of field experiments were performed. Simulations and measurements of the near-bed profiles of the three components of velocity and sediment concentration were compared at the location of the probe. The results of these simulations prove that the code is capable of predicting sediment concentration profiles in the 0.36 m above the sea floor to within a factor of 2–3 and also of reproducing the near-bed suspension events. Simulations with two particle sizes were performed [with diameters of 125μm and 165μm] and results indicate the need to implement multiple size classes into the code. Those with the smaller particle size reproduced observed suspension patterns but overpredicted concentrations, whereas those with the larger particle size did not reproduce as many of the suspension events but underpredicted sediment concentrations.; Overall, the simulations provide considerable insight into the mechanisms responsible for entraining sediment away from the boundary and into the outer flow.