Simulation and measurement of the water-sided viscous shear stress without waves

The local viscous sheer stress in the watersided boundary layer of the air-water interface was measured at the Aeolotron, an annular wind-wave facility located at the Institute of Environmental Physics in Heidelberg. The measurements were conducted at low wind speeds up to u_10 ~ 6.1 m/s, under non-stationary conditions directly after turning the wind on, while periodically switching the wind on and off, as well as under stationary conditons with the wind- and water flow fields being in equilibrium. The wave formation was suppressed by adding 3.3 mg/L of the surfactant Triton X-100 to the water. Active thermography was used for heating thin lines on the water surface, and monitoring their broadening. This method combines the advantages of an air-sided and contactless setup and independence on the prevailing wind profile. Using an approach based on simulation developed in this thesis, the viscous shear stress could be obtained with a maximal temporal resolution of one second. Thereby the temporal development of the line widths were simulated numerically, varying three parameters. These were the initial line width and the spatially constant components of the shear stress with respect to depth and the direction parallel to the lines. The results of the simulations were interpolated and matched with the measured line widths. The hereby obtained values for the viscous shear stress were for the stationary measurements compared to a direct measurement based on particle streak velocimetry. The discrepancies were smaller than 20 %