Turbulent scales under wind-driven waves

The coupling between surface waves and the wind-driven shear leads to more complex dynamics compared to classic boundary layer turbulence. In particular, the velocity fluctuations are believed to be greatly enhanced by sub-surface Langmuir circulation: counter-rotating elongated vortices roughly aligned in the direction of wind and waves, believed to play an important role in the mixing of the near-surface layer and heat/gas transfer across the interface. We investigate the generation and evolution of Langmuir circulations in a combined wind and water tunnel. This features an 8-m long test section with a 0.8-m air space above 0.6-m deep water. Particle image velocimetry characterizes the water flow, while the free surface is imaged by laser-induced fluorescence. The wind inception creates a shear layer that deepens by viscous diffusion. This process continues monotonically until the inception of Langmuir vortices, which quickly mix horizontal momentum across the surface layer, accelerating its growth rate while decelerating the surface. Both size and depth of these structures increase with wind intensity. By two-point statistics, we identify the vortex length scales and track their evolution to a steady state: after being generated adjacent to the surface, the Langmuir vortices grow and sink up to a depth of about half the dominant wavelength.