(108c) Gas Transfer and Surface Divergence at an Air-Water Interface in Channel Flow

Prediction of air-water gas transfer rates using the full advection-diffusion equation is rare due to the difficultly in obtaining the necessary velocity and concentration fields. Consequently most predictions of air-water gas transfer have relied on simplifications of the advection-diffusion equation, such as penetration models mediated by a surface renewal frequency (Dankwerts 1951, Komori 1990), stagnation flow or roll cell models (Chan & Scriven 1970, Fortesque & Pearson 1967, Banerjee 1968), or rms surface divergence models (McCready 1986, Tamburrino & Gulliver 2002, Law & Khoo 2002). Of these simplifications the surface divergence models have recently shown the most promise (Banerjee 2005, McKenna & McGillis 2004, Turney 2005). However the surface divergence model requires knowldege of patterns and intensities of fluid divergence at the interface. In turbulent flow this becomes the chief difficulty. Here we explore the relationship between surface divergence motions, bulk turbulence, and air-water gas transfer in turbulent open channel flow. We present PIV measurements of fluid velocity in open channel flow, from regions of fluid in the bulk of the flow and also from within a few millimeters of the air-water interface. From this we test a "blocking" theory of homogenous turbulence impacted on a fritionless plane (Hunt & Graham 1978) and a related theory to predict surface divergence intensities and air-water gas transfer (Banerjee 1990). The end goal of our analysis is the development of simplified predictive equations for air-water gas transfer.