Suspension of sediment particles over a ripple due to turbulent convection under unsteady flow conditions

Abstract

We analyzed the motions of small sediment particles over a sinusoidal ripple due to an unsteady turbulent boundary layer flow using Large Eddy Simulation. The motions of sediment particles are described in terms of the Lagrangian framework as it is helpful in studying the structure of sediment suspension in detail. Strong coherent vortical structures are well developed along the upslope of the ripple surface during the accelerating flow phase, which effectively drag the particles to the ripple crest. At the maximum flow rate and at the decelerating flow phase, a cloud of vortical structures is developed vertically in the lee area of the ripple. Sediment particles render strong dispersion in the vertical direction when they are captured by these turbulent vortices, causing convective sediment flux that cannot be explained by the mean flows. The convective sediment suspension is strongest at the time of flow deceleration, while over a flat bed at the time of flow reversal. This observation suggests that bed form effect should be considered in modeling convective sediment flux.