FREE-SURFACE TURBULENCE AND AIR−WATER GAS TRANSFER

Concurrent measurements of the two-dimensional free-surface velocity field and bulk gas-transfer velocity for the case of zero-mean-shear mechanically generated turbulence are reported. Laboratory experiments in an oscillating grid-stirred tank have been performed that reveal evidence of the role of surface divergence on air−water gas transfer for varying free-surface rheological and dynamical conditions. Temporal measurements of waterside dissovled oxygen concentration were used to infer the gas-transfer rate, and simultaneous DPIV (digital particle image velocimetry) and PTV (particle tracking velocimetry) measurements provided the velocity field for a small free-surface patch. Additionally, the impact of surfactants on the surface flow and gas transport has been explored. Ensemble-averaged free-surface dynamical quantities such as vorticity and turbulent velocity fluctuations are seen to scale with a bulk flow parameterization for a range of surface conditions. However, such quantities are unable to provide a unique relationship for the transfer velocity under all surface conditions. Rather, the gas-transfer velocity is found to scale with the surface divergence as k ~ √α. Estimates of the transfer velocity based on a hydrodynamic model incorporating the present surface divergence data are in agreement with the measured values, confirming that the divergence is a critical parameter involved in the interfacial transport process.