This study deals with the flow motion over the so-called rol領(lǐng)-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 rol領(lǐng)-grain ripples using flow visualizations and particle image velocimetry(PIV) 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. 德國LaVision PIV/PLIF粒子成像測速場儀 Imager SX PIV相機(jī) 用于粒子成像測速(PIV)的熒光示蹤粒子