181 Three dimensional flow dynamics beneath the air-water interface D. Engelmann“, C. Garbeb, M. Stöhrb and B. Jähnea,b a In terd iscip lin a ry C en t e r fo r S cien tific C om puting, IW R , U n iversity o f H eid elb erg , Im N eu en h eim e r F eld 368, D -6 9 1 2 0 H eid elb erg , G erm a n y e-m a il: D irk.E n g elm a n n @ iw r.u n i-h eid elb erg .d e b In stitu te fo r E n viro n m en ta l P h ysics, U n iv e r s ity o f H eidelberg, Im N eu en h eim e r F eld 366, D -6 9 1 2 0 H eid elb erg , G erm a n y Abstract The physical relation between shape of the surface, flow dynamics beneath the sur face and related exchange processes such as gas transfer are not well understood. In order to gain insight into the dynamics of three dimensional flow fields close the air/sea shear layer we present a stereo particle tracking velocimetry method. The well established particle tracking velocimetry for two dimensional image se quences is combined with a stereo correlation algorithm. The stereo correlation is applied to the trajectories extracted from each image sequence of the stereo camera setup. Particle features can be extracted not only from one single image but from the whole sequence. These are then used in the correlation process, thus making the algorithm more efficient. A precise sub pixel calibration allows quantitative study of the properties of the flow field. Only two cameras are needed and therefore this method simplifies the experimental setup compared to multiple camera setups. Stereo particle tracking was applied for flow field investigations in a wind-wave flume and in a gas liquid reactor. 1 Introduction The gas transfer rate between the atmosphere and the sea increases when wind induced water waves are generated [11]. Small scale waves of a few cm wavelength such as capillary waves are especially important. The coupling mechanisms of the wind induced shear stress on the air- water interface and the physical properties of the 3D-flow field beneath the interface are of interest. Our method is based on classical two dimensional particle tracking ve locimetry (PTV, see [2],[9]) and on a stereo correlation method (Fig. 1). A sub