Study of instantaneous flow behind a single fixed ripple

Two-component velocity measurement using Particle Image Velocimetry (PIV) and flow visualizations have been performed to investigate the different hydrodynamic mechanisms process over a fixed single ripple. Flow patterns over ripple are studied for five Reynolds number based on ripple height (Re = 125, 300, 500, 600 and 700). Results show that the unsteady flow is reflected in vortex pattern generation over the ripple. While for Re < 500, a recirculation zone and a longer length of reattachment point are observed. When Re > 500, instantaneous flow visualization shows an intense vortex interaction with or without coalescence associated with a decrease of the length of the reattachment point. Furthermore, quadrant decomposition of the instantaneous Reynolds shear stress has been calculated to analyse the contribution of the four events in shear stress generation. In addition of a spatial cycle dominance of the sweeping and ejection events near bed, inward interaction events seem to contribute also to this cyclic process and to the flow features.