Investigation of the water-sided shear layer at a wind-driven wavy surface by active thermography
A local, active thermographic measurement method was advanced and used for measuring the mean viscous shear stress in the water-sided boundary layer of a wind driven air-water interface in the presence of water waves, at low wind speeds up to u 10 = (4.8 ± 0.3) m/s. Higher wind speeds of up to u 10 = (10.7 ± 0.7) m/s were considered to explore the boundaries of the application of the method. The measurements were conducted at the annular wind-wave facility Aeolotron in Heidelberg, Germany. The measurement technique utilizes a thin line which is heated onto the surface perpendicular to the wind direction. The broadening of the line is enhanced by Taylor dispersion due to shear flow in the boundary layer. The temporal development of the line width is monitored by an infrared camera. The broadening is compared to numerically simulated line widths, enabling the determination of the viscous shear stress. This initial study with a rough surface showed promising results for both the evaluation of longer time intervals, and temporally resolved measurements on the order of few seconds. Stationary conditions with the air and water compartments in dynamical equilibrium, corresponding to a quasi- infinite fetch, and non-stationary conditions after turning the wind on were considered. The stationary results were compared to results for the viscous shear stress obtained by water-sided particle streak velocimetry, with results deviating by less than 15%. The non-stationary measurements indicated an overshoot in the viscous shear stress shortly after turning the wind on.
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