Laboratory measurements of mean and turbulence velocities and shear stresses through the wave roller in strong plunging waves

Abstract

In this paper we present experimental measurements of mean and turbulent velocities in a strongly plunging wave. Measurement of the fluid velocities over the entire water column and through the outer and inner surf zones were obtained by particle and bubble image velocimetry. The velocity flow fields were analyzed to estimate mean and turbulent velocity by Reynolds decomposition. The mean velocities were used to estimate the forward and reverse mass fluxes, from which an estimate of the average fluid density in crest of the waves is obtained. In addition we provide details of the wave and turbulent shear stresses. The changes in characteristics of all these quantities as the wave propagates from the break point into the outer and inner surf zones are highlighted. We provide observations of both time dependent and phase averaged features of velocities, turbulence kinetic energy (TKE), and the wave and turbulence shear stresses over the wave cycle. The average relative fluid density in the wave crests was found to have an upper bound between 20 % in the outer surf zone to 30 % in the inner surf zone. This translates to a lower bound for the void fraction of approximately 0.8 and 0.7 in the outer and inner zones, respectively. The measured wave shear stress was found to be of similar magnitudes as the turbulence shear stress. The wave shear stress also changed sign depending on the horizontal position along the surf zone and on the relative phases between the horizontal and vertical velocities. For this particular wave conditions the measured Froude scaled TKE was found to be higher than those previously reported in the literature. Near the bottom, Froude scaled TKE values of approximately 0.1 was observed, while peak values of 0.25 were measured above the trough level.