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Turbulence and Shear Flow Phenomena -1 First International Symposium

ISBN:
1-56700-135-1 (Print)

FREE-SURFACE TURBULENCE AND AIR−WATER GAS TRANSFER

Sean P. McKenna
Department of Applied Ocean Physics and Engineering Woods Hole Oceanographic Institution Woods Hole, MA 02543, USA; Department of Ocean Engineering Massachusetts Institute of Technology Cambridge, MA 02139, USA

Wade R. McGillis
Department of Applied Ocean Physics and Engineering Woods Hole Oceanographic Institution Woods Hole, MA 02543, USA

Erik J. Bock
Department of Applied Ocean Physics and Engineering Woods Hole Oceanographic Institution Woods Hole, MA 02543, USA

Abstract

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.