Numerical Investigation of Coupled Boundary Layers Air-Sea Transfer (CBLAST) at Small Scales

We investigate numerically the mechanisms of air-sea coupling at low wind speeds at small spatial scales. Simulations of the air and water turbulent flows are performed with coupled free-surface boundary conditions. We consider two canonical problems: coupled air-water Couette flows with passive scalars and unsteady spilling breakers, to obtain useful insights into the structures and dynamics of turbulent flows in the vicinity of the air-sea interface. It is found that the flow structures at the air-water interface are mainly controlled by the underneath water motions. Surface features such as low-speed streaks and streamwise vortices are highly correlated with the coherent turbulence structures on the waterside. While the turbulent boundary layer on the airside resembles the boundary layer near a solid wall, the boundary layer on the waterside is qualitatively distinct from those near a solid wall or a shear-free free surface. It is also found that, in the presence of steep and spilling-breaking surface waves, there exists substantial vortex flux at the air-water interface. Analysis of the turbulent kinetic energy (TKE) budget shows that energy production is largest near the interface. On the airside, dissipation increases towards the interface and reaches a maximum at the interface; turbulence velocity fluctuations transport TKE from the bulk region to the near-surface region. On the waterside, as the interface is approached, dissipation decreases first and then increases; turbulence transport removes part of TKE from the near-surface region and put it at the deep region. Viscous diffusion is only significant very close to the interface, while pressure transport is found to be much smaller than other processes. In the presence of spilling breaking waves, the surface vorticity is generated by two mechanisms: surface parallel velocity with a sharp change in interface curvature and work due to surface tension forces. It is found that the inviscid energy transport associated with surface waves has a magnitude much larger than the viscous transport. The interfacial transport process is dominated by the pressure forces.