Entrainment and suspension of sediments into a turbulent flow over ripples

We have analysed the trajectories of small particles released into a turbulent flow over a wavy wall in order to clarify the role of coherent structures in controlling the suspension and entrainment of sediments when ripples are present. LES was used to simulate the turbulent flow, while the motion of individual particles was calculated in a Lagrangian framework considering the effects of drag, lift and added mass. The results show that the process of suspension can be divided into two phases: the formation of a cloud in the lee of the ripples and the subsequent ejection of the particles across the shear layer capping the sediment cloud. Strong vortical structures on the upslope are shown to control the amounts of sediment entrained along the upslope, which feed the cloud. The implications for modelling are discussed.