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Analyzing the footprints of turbulence producing mechanisms at the free water surface
At the free wind-sheared water surface, various turbulence producing mechanisms contribute significantly to enhanced transfer rates of heat and gas across the air-water boundary. As a result, surface convergence and divergence form and can be visualized in infrared images of the water surface. Within this work the footprints of turbulent processes in the surface temperature pattern are analyzed. Image processing techniques, such as motion estimation and classification, are adapted and applied to the infrared images of the water surface. Dense flow fields are estimated without the suppression of surface divergence and up- and down-welling sites are identified. Data from a range of laboratory facilities is evaluated with a focus on small spatial scales, low to moderate wind stress and the impact of surfactants on the surface dynamics. In this wind regime shear induced turbulence, small Langmuir circulations and microscale breaking waves are identified as the dominant processes that drive heat exchange. The turbulent cell size is suggested as a characteristic feature of shear and Langmuir turbulence and is related against friction velocity and wave field. Within this work, a novel method is suggested to assess the impact of individual processes on the overall heat transfer rate and tested on the measured laboratory data also with respect to the influence of increasing wind stress and surfactant coverage. The following key points are found: Shear induced turbulence is a major contributor to enhanced transfer rates at low wind stress. Its relevance increases if the surface is covered by surfactants as the onset of waves is delayed. Langmuir circulations play a major role at intermediate wind stress and cause a significant increase of transfer rates. Surfactants on the water surface delay the evolution of Langmuir circulation and intensity considerably. Microscale breaking is a dominant contributor at moderate wind stress and is responsible for approximately 50% and more of the transfer rate.
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530 Physik
530 Physics