Experiments on gas transfer at the air–water interface induced by oscillating grid turbulence

The gas transfer process across the air–water interface in a turbulent flow environment, with the turbulence generated in the water phase far away from the surface, was experimentally investigated for varying turbulent Reynolds numbersRe_Tranging between 260 and 780. The experiments were performed in a grid-stirred tank using a combined particle image velocimetry – laser induced fluorescence (PIV-LIF) technique, which enables synoptic measurements of two-dimensional velocity and dissolved gas concentration fields. The visualization of the velocity and concentration fields provided direct insight into the gas transfer mechanisms. The high data resolution allowed detailed quantification of the gas concentration distribution (i.e. mean and turbulent fluctuation characteristics) within the thin aqueous boundary layer as well as revealing the near-surface hydrodynamics. The normalized concentration profiles show that asRe_Tincreases, the rate of concentration decay into the bulk becomes slower. Independent benchmark data for the transfer velocityK_Lwere obtained and their normalized values (K_LSc^0.5/u_HT) depend onRe_Twith exponent −0.25. The spectra of the covariance termc′w′ indicate that the contribution ofc′w′ is larger in the lower-frequency region for cases with smallRe_T, whereas for the other cases with higherRe_T, the contribution ofc′w′ appears to be larger in the higher-frequency region (small eddies). These interrelated facts suggest that the gas transfer process is controlled by a spectrum of different eddy sizes and the gas transfer at different turbulence levels can be associated with certain dominant eddy sizes. The normalized mean turbulent flux $\overline{c^\prime w^\prime}$ profiles increase from around 0 at the interface to about 1 within a depth of approximately 2δ_e, where δ_eis the thickness of the gas boundary layer. The measured turbulent flux (c′w′) is of the same order as the total flux (j), which shows that the contribution ofc′w′ to the total flux is significant.