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

Abstract

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. Plain Language SummaryIn shallow coastal regions, shoaling waves produce oscillatory bottom flows, which create sand ripples on a sandy seabed. The acceleration skewness of the oscillatory flow is a result of the nonlinear feature of the overlying water waves, and it can differentiate the two half-cycles of the oscillatory flow and therefore leading to a net (cycle-averaged) sediment transport. In this study, we use an oscillatory water tunnel and particle image velocimetry to investigate the oscillatory boundary layer flow over vortex ripples with a focus on the effect of acceleration skewness. Our measurements provide detailed descriptions of the formation-ejection process of coherent vortices, which is produced by the flow separation behind the ripple crest. Some useful observations on flow turbulence are also provided. The results showed that acceleration skewness can substantially differentiate the two coherent vortices developed within the two half-cycles. The offshore-side vortex can carry more turbulence over the ripple crest than the onshore-side vortex, which possibly leads to some net onshore sediment transport rate. We also showed that the ripple-averaged flow is fairly close to that over a flat bed, so flat-bed boundary layer model can be possibly used for rippled bed with some parameterizations.

Large Eddy Simulation of three‐dimensional flow structures over wave‐generated ripples

“…The vortex dynamics and the occurrence of the secondary vortex are accurately simulated and consistent with Figure 4 in Önder and Yuan (2019). The overall vorticity field is also in good agreement with the laboratory experiments performed by Yuan and Wang (2019).…”

A Numerical Study of Onshore Ripple Migration Using a Eulerian Two‐phase Model

“…Therefore, boundary layer flow, sediment transport, ripple geometry, and migration are all interrelated processes. A more thorough analysis of the vorticity dynamic and vortex formation‐ejection over full‐scale ripples can be found in the recent work of Yuan and Wang (2019). By analyzing velocity, vorticity, and swirling length (Zhou et al., 1999) simultaneously, they revealed the similar pair‐vortex pattern previously only observed in direct numerical simulation (Blondeaux & Vittori, 1991; Önder & Yuan, 2019).…”

Improvement of the Full-Range Equation for Wave Boundary Layer Thickness

“…In recent years, experimental studies have been carried out for boundary layers in an acceleration-skewed oscillatory flow (e.g., Suntoyo et al [17], van der A et al [34], and Yuan and Wang [35]). The equations proposed herein for a sinusoidal motion can be applied to acceleration-skewed oscillatory waves as well by introducing an equivalent sinusoidal orbital amplitude, a c , for a m as proposed by Suntoyo et al [17] and van der A et al [34].…”

“…The equations proposed herein for a sinusoidal motion can be applied to acceleration-skewed oscillatory waves as well by introducing an equivalent sinusoidal orbital amplitude, a c , for a m as proposed by Suntoyo et al [17] and van der A et al [34]. In recent years, experimental studies have been carried out for boundary layers in an acceleration-skewed oscillatory flow (e.g., Suntoyo et al [17], van der A et al [34], and Yuan and Wang [35]). The equations proposed herein for a sinusoidal motion can be applied to accelerationskewed oscillatory waves as well by introducing an equivalent sinusoidal orbital amplitude, ac, for am as proposed by Suntoyo et al [17] and van der A et al [34].…”