Experimental study of coherent structures and bed shear stress on a smooth bed under a solitary wave

Abstract

The coherent structures induced by a solitary wave on a smooth bed were studied in a wave flume. The solitary wave was near the point of incipient breaking and the wave Reynolds number was in the intermittent turbulent regime. The velocity field inside the bottom boundary layer was measured using a Particle Image Velocimetry (PIV) system. The bed shear stress was obtained from the measured velocities in the viscous sublayer by employing the Newton's law of viscosity. It was found that the generation and maintenance of turbulent coherent structures in a solitary wave boundary layer was like that observed in oscillatory flow boundary layers at similar wave Reynolds numbers, a process that was first described as the bursting phenomenon in the wall boundary layer of steady flows. The effect of coherent structures on the instantaneous velocity profile and bed shear stress was examined. Ejections and sweeps were found to play a crucial role in producing large bed shear stress fluctuations. Quadrant analysis and probability distribution plots of the bed shear stress showed that sweep (Q4) events were responsible for producing instantaneous bed shear stress values as high as four to five times the standard deviation above the mean, while ejection (Q2) events produced bed shear stress values mostly below the mean. Both types of events were more numerous than outward interaction (Q1) and inward interaction (Q3). It was also found that a high proportion of the measured velocity profiles that did not follow the law of the wall were associated with large vortices in the boundary layer. It was shown that the instantaneous velocities did not conform to a law-of-the-wall velocity profile unless the measured velocity field was averaged over an area large compared to the vortices.