(281d) Open-Channel Flow Turbulence and Air-Water Gas Transfer

Transfers of mass across air-water interfaces have gained renewed attention due to efforts to track pollutants and nutrient chemicals in environmental systems such as rivers, lakes and oceans. Despite many decades of investigation of this subject, predictions of the transfer rate still carry uncertainty factors of two or greater, mostly because the fluid motions very near the air-water interface have eluded measurement and characterization. This talk presents results from new particle image velocimetry (PIV) experimental techniques that measure and characterize the water motions within one millimeter of the air-water interface. Laboratory open-channel flows of varying bulk velocity and depth were used for generating the turbulence studied here. The air-water transfer rate is measured by stripping the oxygen from the upstream water and measuring the rate of uptake of the gas at downstream locations. Bulk turbulence characteristics are also measured with PIV. Several theories for predicting the interfacial transfer rate are tested with the experimental data. The surface divergence theory and surface renewal theories are seen to agree with measurements in open-channel flows, but both require poorly understood corrective coefficients. A better understood theory of interfacial transfer is formulated and tested, with the goal of eliminating corrective coefficients. Surfactants are seen to have a large effect on interfacial motion despite significant efforts to clean the interface. Future directions for research of air-water gas transfer are proposed.