| The mechanisms governing turbulent heat and mass transfer across fluid interfaces are difficult to unravel because measurements and simulations are difficult to perform due to interface deformation. We have developed direct numerical simulation techniques, that allow investigations of transport processes between two turbulent streams separated by a wavy interface.; The results indicate qualitative similarity between turbulence structures, in the vicinity of a continuous interface, to those in wall turbulence—if the shear rates correspond. There are detailed differences, which depend on the interfacial tension, fluid densities and viscosities of the gas and liquid streams. At atmospheric conditions, and restricting consideration to the wave scales important for scalar transfer (capillary waves), the differences are primarily on the liquid side. The liquid-side tangential turbulent fluctuations peak right at the interface, whereas on the gas side they peak a small distance away—like in wall turbulence.; The patterns of shear stresses and pressure axe primarily controlled by the quasi-streamwise vortical structures on the gas-side, with their associated sweeps and ejections. Quasi-streamwise vortices also arise near the interface on the liquid side, but the associated sweeps and ejections do not correlate with the interfacial shear stress pattern.; Heat and mass transfer mechanisms are primarily controlled by the sweeps, with ejections playing an important role for high Schmidt numbers on the gas side. An understanding of the dominant mechanisms allows simple parametrizations of the scalar transfer velocity on each side, which compare well with experiments and DNS. Capillary waves appear to have little effect on these parametrizations, provided the correct frictional velocity scale is used in the nondimensionalization. More of a wave effect is seen for scalar transfer from a solid wavy wall arising from flow separation and reattachment around the crests in a wave train. Such separation does not appear to occur for capillary waves at a mobile interface and this explains the small effect seen. While these results pertain primarily to scalar transfer between separated streams there are indications that they carry over to more complex flow regimes if the length scales of the interfacial structures are of the order of, or larger, than capillary waves. |
