%0 Generic
%A Kunz, Jakob
%C Heidelberg
%D 2017
%F heidok:22903
%R 10.11588/heidok.00022903
%T Active Thermography as a Tool for the Estimation of Air-Water Transfer Velocities
%U https://archiv.ub.uni-heidelberg.de/volltextserver/22903/
%X Active thermography offers a tool that can be used to measure air-water transfer velocities at spatial and temporal scales unprecedented by any other measurement technique available. Key to successful measurements is the precise control of a locally applied heat flux density at the water surface. A new way of beam shaping with diffractive optical elements is introduced that significantly increases the spatial homogeneity of the heat  flux density. A new multifrequency excitation scheme is developed and successfully implemented that reduces measurement times by a factor of 4 to 12 depending on wind  speed. Systematic studies about the dependence of heat transfer on surfactant concentration  and fetch length are carried out in the annular wind-wave facility Aeolotron in Heidelberg. The presence of the artificial surfactant Triton X-100 reduces heat transfer  for an intermediate wind speed range, but not for extremely low or for high wind speeds. It is found that the heat transfer velocity is strongly dependent on fetch length  for low wind speeds. Experiments in the Aeolotron with actual sea water from the North Sea containing natural surfactants show similar results as the measurements with the artificial surfactant Triton X-100. A measurement series in the linear wind-wave facility Pytheas in Marseille, France, shows that wave breaking enhances heat transfer in an intermediate wind speed regime. Simultaneous measurements of heat and gas transfer velocities are in agreement with each other for the sea water conditions investigated  in the Aeolotron, when scaled to the transfer velocity of a gas with the same Schmidt number. However, heat transfer velocities are measured to be twice as large as gas  transfer velocities in another experimental series with unknown biological activity in the Aeolotron.