Flow field and momentum balance in the Heidelberg Aeolotron, a large annular wind/wave facility

The Heidelberg Aeolotron is an annular wind/wave facility with 10m diameter. One of the main advantages in contrast to a linear facility is the nearly infinite fetch. The conditions on the water surface, and so the gas transfer are more similar to conditions at the open ocean. The wind field in the Aeolotron was investigated with a pitot tube mounted on a translation stage. The radial wind speed was measured for a complete cross section at ten positions. To measure near the water surface surfactants were added to dampen the wave field. Because of the inhomogeneous wind field and secondary currents due to the annular geometry the friction velocity u* cannot be determined by the logarithmic wind profile. But as an advantage of the annular geometry the momentum balance in water can be used to get u*. Using the increase of the water velocity after turning on the wind, the friction at the walls and u* can be determined. An acoustic velocity sensor was used during a large number of experiments to quantify the friction at the walls. We found a dependence on the mean water velocity and on wave motion, which we parametrized. Together with the velocity in equilibrium, the wind induced friction velocity was determined for a large range of wind speeds up to 10m/s. Additional experiments with two concentrations of the surfactant Triton X-100 show the influence of different wave fields on the friction velocity. The annular geometry generates secondary currents in water, too. This could be quantified at the Aeolotron by measuring the 3D water velocity in a cross section. The cross wind velocity components account for up to 10% of the mean flow at medium wind speeds.