Simulation of air-water interfacial mass transfer driven by high-intensity isotropic turbulence

Previous direct numerical simulations (DNS) of mass transfer across the air-water interface have been limited to low-intensity turbulent flow with turbulent Reynolds numbers of R-T <= 500. This paper presents the first DNS of low-diffusivity interfacial mass transfer across a clean surface driven by high-intensity (1440 <= R-T <= 1856) isotropic turbulent flow diffusing from below. The detailed results, presented here for Schmidt numbers Sc = 20 and 500, support the validity of theoretical scaling laws and existing experimental data obtained at high R-T. In the DNS, to properly resolve the turbulent flow and the scalar transport at Sc = 20, up to 524 x 10(6) grid points were needed, while 65.5 x 10(9) grid points were required to resolve the scalar transport at Sc = 500, which is typical for oxygen in water. Compared to the low-R-T simulations, where turbulent mass flux is dominated by large eddies, in the present high-R-T simulation the contribution of small eddies to the turbulent mass flux was confirmed to increase significantly. Consequently, the normalised mass transfer velocity was found to agree with the R-T(-1/4) scaling, as opposed to the R-T(-1/2) scaling that is typical for low-R-T simulations. At constant R-T, the present results show that the mass transfer velocity K-L scales with Sc-1/2, which is identical to the scaling found in the large-eddy regime for R-T <= 500. As previously found for a no-slip interface, also for a shear-free interface the critical R-T separating the large- from the small-eddy regime was confirmed to be approximately R-T = 500.