Laboratory Study of the Flow Structure Above and Below Wind-Generated Waves, With/Without Surfactants

Air-sea exchange processes across the Sea surface Micro Layer (SML) depend on submillimeter-scale dynamics directly above and below the sea surface. Understanding these processes requires an in-depth knowledge of the small-scale surface waves and coupled air-water dynamics. Monolayers are known to have a dampening effect on surface gravity-capillary waves, which in turn strongly impacts the fluxes and exchange processes across the SML. Yet the feedback mechanisms involved are not fully understood.

A novel imaging system combining LIF surface detection (Laser Induced Fluorescence) and PIV (Particle Image Velocimetry) techniques was developed and installed at a fetch of 15.5m at the large, 26-m long, 1.5-m high, 1-m wide, wind-wave tank of the University of Hamburg (Germany), a facility specially designed for the study of the effect of surfactants on wind-wave dynamics. Wind speeds ranging from 4 m/s to 6 m/s were generated; experiments were run with and without surfactants (oleyl alcohol).

Water-side and air-side PIV-LIF systems, with a relatively large, 50cm-wide field of view, provided high resolution, instantaneous snapshots of flow dynamics and surface elevation for several waves within a group. The high resolution LIF imagery (55µm/pixel) was able to resolve parasitic capillary waves downwind of gravity wave crests. The PIV measurements show intense airflow separation events past micro-scale breaking waves in slick-free conditions, with enhanced turbulence under wave crests. In the presence of surfactants, capillary waves largely disappear, and non-separated sheltering events dominate. The role of surfactants on the partitioning of the momentum flux between viscous, turbulent, and wave-coherent contributions will be discussed.