Air-water interfacial coherent structures in wind-induced open-channel flows and its effect on gas transfer

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As for the scalar transport phenomena of the issued gases such as CO_2, O_2 and NO_x across an air-water interface, it is essential to clarify the turbulent features in the free-surface region of water flows. Previous studies of wind-induced water-wave flows have been conduced in the coastal engineering and marine sciences. It is especially important to clarify gas exchange mechanism across the air and water. In previous studies, the conditional time-space correlation analysis and the u-v quadrant method have been intensively conducted to investigate coherent structures in sheared flows such as open-channel flows and non-wavy air sheared flows.The present investigation describes an experimental study on turbulent bursting phenomena across air-water interface in wind-induced water waves by using Particle-Image Velocimetry (PIV) in the water layer. The purpose of this study is to clarify coherent structures near the air-water interface in a wind-driven shear layer, and to evaluate an effect on momentum transport due to the busting events from the interfacial water to water-body flow. As the results of experiments, it is shown that the water flow is a combined flow with a wall shear flow and orbital motions, and that the wave-induced fluctuations have major power in the whole velocity fluctuations. In particular, when the bursting events are detected near the air-water interface by human eyes in the time-series of PIV raw images, only the low-frequency fluctuations are enhanced obviously at that time in wavelet-filter analysis. Therefore, it is inferred that the mechanism of water momentum transport is organized by the wave-induced fluctuations from the air-water interface to the water bulk, not by the bursting events with small fluctuations. It is also recognized by PIV measurements that the busting events are closely related to the unsteady wave-induced velocity fluctuations near the interface.