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Mechanisms are reviewed here for the distortion of turbulent flows near thin density interfaces and their effects on transfer processes across them. Firstly the results of rapid distortion calculations show how the in homogeneous eddy structure depends on whether the turbulence is generated above or below the interface, or in both regions. The flow is unstratified and the buoyancy forces are stable and strong enough relative to the inertial forces that the interface is continuous. It is shown that as the surface blocks the vertical turbulent eddy motions, horizontal straining motions are induced which affect the surface viscous layers and can then induce motions some distance from the interface on the opposite side from where the turbulence is generated. Secondly the paper reviews the physical mechanisms controlling how wind flows over monochromatic and groups of surface waves. The results of triple deck theory for turbulent shear flows, i. e. combining sheltering and unsteady critical layer mechanisms, explain why groups are the most efficient mechanism for waves to extract energy from the wind and therefore enhance transfer properties between atmosphere and water bodies. The third section of the paper reviews the mechanisms for the generation of turbulence coherent roll structures in the ocean surface layer, resulting from surface shear turbulence (normal stress variations), wave-mean shear vortex stretching and rotation, i.e. Langmuir cells, and unstable buoyancy forces (i.e. cooling at the surface) plus mean shear also via vortex rotation. Since these mechanisms are generally additive - exceptional situations exist - they are effective in transporting fluxes downwards into the ocean surface layers.

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Gas Transfer at Water Surfaces 2010


Kyoto University Press