This paper presents quantitative energy and momentum exchanges between fluid and suspended sands via fluid-sand interactions in open channel flows over sandy dunes through comparisons of the sand-laden flow over movable bed and the single phase flows over the fixed bed, on the basis of double camera PIV measurements with an optical selection technique that is capable of measuring simultaneous velocities for the both water and sand phases at the same locations. Since a boundary shear layer formed behind the dunes is constrained to be developed over a movable bed, the turbulent energy on the separated shear layer is much lower than that in a fixed bed experiment. While the drag acting on the sands significantly decelerates the water flow above the shear layer at the inception of sand suspension, in a settling phase, the drag accelerate the water downward. Additional turbulent energy is induced during sand settlement process, which is over five times higher than the fixed bed case. This additional turbulence may disturb the bed surface and enhance re-suspension of the disturbed sands. This recursive energy and momentum exchanges between the sand and water flows via the suspension, diffusion, settlement and re-suspension processes have important roles to characterize the sand-laden flows over the movable sandy dunes.
