Turbulence Structure and Turbulent Diffusion Near Gas-Liquid Interfaces

Abstract

In this paper the general features of the turbulence structure and mass and heat transfer at gas-liquid interfaces are reviewed and analysed with particular reference to recent research on large-scale structures in turbulent shear flows, turbulence near boundaries in the absence of mean velocity gradients, numerical simulations of turbulence and of turbulent diffusion.

The discussion is concentrated on the turbulence in liquid below the interface when surface tension and gravitational forces are relatively large enough to prevent significant deformation. It is shown how significant differences in the turbulence below ‘clean’ and ‘dirty’ surfaces are largely confined to surface viscous layers (which are much thicker than viscous ‘sub-layers’ on rigid surfaces). The vertical turbulence below these layers is determined by the spectrum of eddies impinging on the surface and, if expressed in terms of the local energy dissipation rate e, then is found to have a similar form to that in other ‘shear-free’ boundary layers such as in thermal convection boundary layers. Since the impinging turbulence controls the viscous surface layer, it also controls the diffusion layer very close to the surface through which dissolved gases or heat enter or leave the liquid. The turbulent diffusion within the viscous surface layer depends on whether the surface is ‘clean’ or ‘dirty’ and on any significant surface gas motions. Outside the surface viscous layers it is shown how the non-uniform and non-Gaussian turbulence induced by eddies impinging on the free surface diffuses matter or heat down from the surface significantly differently to that in a shear flow, but similar to that in turbulence driven by thermal convection.