Spatio-temporal analysis of flows close to water surfaces

In order to examine the air-water gas exchange, a detailed knowledge is needed about the flow field within and beneath the water-side viscous boundary layer. Therefore a novel measurement technique is developed for the spatio-temporal analysis of flows close to free water surfaces. A fluid volume is illuminated by LEDs. Small spherical particles are added to the fluid, functioning as a tracer. A camera pointing to the water surface from above records the image sequences. The distance of the spheres to the surface is coded by means of a supplemented dye, which absorbs the light of the LEDs. By using LEDs flashing with two different wavelengths, it is possible to use particles variable in size. The velocity vectors are obtained by using an extension of the method of optical flow. The vertical velocity component is computed from the temporal change of brightness. Using 3D parametric motion models the shear stress at surfaces can be estimated directly, without previous calculation of the vector fields. Hardware and algorithmics are tested in several ways: A laminar falling film serves as reference flow. The predicted parabolic profile of this stationary flow can be reproduced very well. Buoyant convective turbulence acts as an example for an instationary inherently 3D flow. The direct estimation of the wall shear rate is applied to sequences recorded in the context of biofluidmechanics, revealing a substantial improvement compared to conventional techniques.