Turbulence structure and fluid–particle interaction in sediment-laden flows over developing sand dunes

In this study, we conducted the fluid–particle simultaneous measurements in order to reveal the fluid–particle interaction mechanism in the developing stages of sand dunes. We measured the instantaneous velocities over the growing sand dunes by the use of both the discriminator particle-image velocimetory (D-PIV) and the discriminator particle-tracking velocimetory (D-PTV). In this D-PIV&PTV system, fluid tracers and sediment particles are discriminated accurately by their occupied particle sizes, and thus, the particle velocity Up and fluid velocity Uf can be measured simultaneously. It was found from the present measurements that turbulence intensities in the trough area became larger after the generation of the reverse flow, i.e., Uf < 0. There are two kinds of coherent eddies behind dunes, i.e., one is convected along the shear layer behind dune and the other is lifted up from the reattachment point due to the kolk-boil vortex.