Measurements of sub-surface turbulence beneath laboratory wind-driven waves*

We explore the characteristics of the turbulence beneath young wind-driven surface waves. We leverage a wind-wave tunnel in which the ratio between the air-side friction velocity and wave phase speed varies between 0.8 and 1.5. Specifically, we perform simultaneous particle image velocimetry (PIV), which provides the sub-surface velocity field, and planar laser-induced fluorescence (PLIF), which provides the overhead water surface position and enables the inference of the wave-induced orbital velocity. In one experimental configuration, two-dimensional PIV and PLIF are performed in a streamwise vertical plane, resolving the wind-induced drift and the two primary components of the orbital motion; in a second configuration, three-component PIV is performed in a vertical plane normal to the flow, resolving the wave-coupled structures characteristic of the wind drift layer.

 

Analyzed together, the results allow for calculations of kinetic energy exchanges throughout the sub-surface boundary layer and between its turbulent, orbital, and time-averaged components. The surface elevation data is further employed to investigate the characteristics of the turbulence beneath different portions of a wave, revealing differences in the turbulence structure beneath trough and crest regions.