Measurements of the Turbulent Stress over Wind-Driven Surface Waves in the Wave-Boundary Layer

Wind-driven surface waves and the wave-modulated turbulence on both sides of the interface play an essential role in linking the air and sea. Although the effects of surface waves on the air-sea momentum flux have been the subject of several studies, the current understanding is still insufficient.

We are presenting detailed quantitative velocity measurements over wind waves acquired in the large wind-wave tunnel facility using a combination of particle image velocimetry (PIV) and laser-induced fluorescence (LIF) techniques for wind speeds varying from 0.86 to 16.63 m s^-1. The mean, wave, and turbulent velocity fields were then obtained by a linear triple decomposition in wave-following coordinates. The turbulent stress can be therefore directly calculated. The contribution of the turbulent stress to the total momentum flux, for example, increases with increasing wind speed. The presence of the surface waves leads to wave-phase coherent variations in the turbulent stress within the so-called wave boundary layer. The distribution of the turbulent stress presents a pattern of along-wave asymmetry near the surface with a separation-induced maximum above the downwind of wave crests. Experimental results will be compared to LES results obtained for similar wind and wave conditions.