Dynamics of transient eddy above rolling-grain ripples

This study deals with the flow motion over the so-called rolling-grain ripples which are generated by water oscillations above a sand bed. We focus our efforts on quantifying by means of laboratory experiments and numerical calculations the morphology and the dynamics of transient flow patterns. We report, for the first time, on the formation of an unsteady pattern with closed streamlines (we call it "eddy") above rolling-grain ripples using flow visualizations and particle image velocimetry measurements. This structure appears in the ripple trough during flow reversal and scales with the ripple wavelength. The experimental results are in qualitative agreement with the perturbative flow solution calculated by Vittori in 1989. Even if the relative ripple amplitude is not small in the experiment the perturbative expansion at the first order gives an accurate description of the flow dynamics.