Laboratory measurements on both sides of the air-water interface during early wind-wave generation

Ocean waves play a key role in controlling air-sea exchanges of momentum, energy, and gasses. Several researchers studied theoretically, experimentally, and numerically the influence of surface waves on the wind stress and how wind contributes to wave generation and growth. From an experimental point of view, taking air- or water-side measurements close to the moving wavy surface, is very challenging. And, because of the difference in air and water densities (and viscosity), most existing experimental and numerical studies have not investigated both fluids simultaneously. Here, we present laboratory experiments using a combination of PIV-LIF (Particle Image Velocimetry – Laser Induced Fluorescence) techniques, where air and water velocity are measured simultaneously as close to the surface as O(100 um) in the early stage of wind-wave generation and until a steady wind-wave equilibrium is reached. Our aim is to observe coherence structures in both fluid phases and to link velocity and stresses at the interface. We use a triple decomposition of the velocity signal to partition the contributions of wave- and turbulent-related stresses, and to close momentum and energy balances in the observed control volume.