Mass transfer across a wavy interface has been investigated by mean of direct numerical simulation. Gas and liquid streams, with physical properties typical of air and water, flow counter-currently. Two cases are presented. In the first, the interface is kept flat in the limit of low Proude and Weber number. The results of this simulation elucidate the effect of turbulence on mass transfer without the complication introduced by waves. In the second case, the interface is free to deform and waves in the capillary and capillary-gravity range can form. The simulation corresponds to a tenmeter wind velocity of ~ 7 m/s. The simulations are for values of the Schmidt number up to 100.
The values of mass transfer velocity obtained from the simulation are in good agreement with laboratory experiments. Waves do not significantly affect mass transfer velocity non dimensionalized by the interfacial frictional velocity. On the liquid side, high mass transfer regions over the interface correlate well with sweeps on the liquid side for all values of the Schmidt number. On the gas side, mass transfer velocity correlates well with sweeps for moderate values of Schmidt number. For high values of Schmidt, however, all turbulence events with significant interface-normal velocity become relevant.
Based on these mechanisms, parametrizations for the mass transfer velocity are proposed, based only on the characteristics of the turbulence structures and without adjustable parameters. They compare well with experiments and indicate that it is the turbulence, rather than the waves characteristics, that controls mass transfer.