Numerical simulation of the production of three-dimensional sediment dunes

Knowledge of the relationship between sediment motion and flow conditions is fundamental to our understanding of three-dimensional sediment dune development in river and coastal environments. In this study, numerical simulations were performed on a mobile flat sand bed. The simulation results provide important insights into the coupling between migrating bedforms and turbulent stratified flow in the open channel. The formation of micro sand waves can be divided into three stages. First, the initial defects appear on the bed at the beginning of the process and are closely correlated with the instantaneous flow velocity just before the bed is destabilized. Second, the defects in areas of high instantaneous flow velocity are washed away, while the defects in areas of low instantaneous flow velocity grow in length and height due to sediment deposition. Finally, a constant wake zone where sediment continues to accumulate forms downstream of the micro sand wave. Despite the formation of micro waves, the near-bed flow velocity and turbulent structures play important roles as sand passes from upstream dune crests to downstream ones. The high flow velocity breaks O-shaped dune crests and drives excess sand to the downstream dune crests. The near-bed vortices usually occur at the stoss sides of the dunes, and most are elongated in the spanwise direction.