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Abstract

In this thesis the dynamics of the air sided boundary layer above wind induced water waves was investigated by velocity measurements. Within the scope of this study a measurement technique based on particle tracking was developed, which enables to measure the flow directly above the water surface down to the viscous boundary layer with high accuracy. At the big linear wind-wave facility in Marseille, France, experiments with different wind speeds (u10 = 3−15.5m/s) and different wave conditions (pure wind waves and mechanically generated waves) were performed. In addition the water elevation was measured using laser induced fluorescence, which allows the use of wave following coordinates and phase dependent averaging of the velocity fields. Turbulent, wave coherent and viscous stress contribution to the total drag were determined. The pressure induced momentum transport was indirectly estimated by measurering the total momentum flux and closing the momentum balance. For the first time measurements of the complete high resolved stress partitioning from inside the viscous boundary layer (<1mm) to approximately 25cm above the mean water level were accomplished for high wind speeds and large waves. It was found that directly at the water surface the viscous shear decreases from 70% of the total stress at 3m/s to only 10% at 15.5m/s. Additionally, the viscous shear at the wave crest is larger than the mean and strongly depends on the wave steepness.