An Experimental Investigation of Acceleration‐Skewed Oscillatory Flow Over Vortex Ripples

A full‐scale experimental study of acceleration‐skewed oscillatory flow over vortex ripples, which is an approximation of wave‐driven flow over a rippled seabed, was conducted in an oscillatory water tunnel. Three acceleration‐skewed‐flow tests and a reference sinusoidal‐flow tests were involved in this study. In each test, two‐dimensional vortex ripples were generated from a coarse‐sand movable bed, and the flow field was measured with a particle image velocimetry. The dominant feature of phase‐averaged flow is the generation of onshore (offshore) coherent vortices during the onshore (offshore) half‐cycle. The acceleration skewness expedites the offshore‐onshore flow reversal and therefore speeds up the ejection of offshore vortex, while the opposite occurs for the onshore vortex. The ripple‐averaged flow is similar to the acceleration‐skewed turbulent oscillatory flow over a flat bed in terms of leading Fourier harmonics, boundary layer thickness, and cycle‐averaged flow, suggesting a possible analogy between the two. From the aspect of flow turbulence, acceleration skewness elongates the offshore accelerating quarter, so the turbulence embedded in the offshore vortex can be better developed. This, together with a quick ejection process, allows the offshore vortex to carry a significant amount of residual turbulence to the onshore‐side ripple flank, which may even enhance the production of turbulence within the onshore half‐cycle. Some results on pressure field and form drag are also presented in this paper.