8th European Postgraduate Fluid Dynamics Conference

6th – 9th July 2016
Warsaw, Poland

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Individual water drop-droplet collisions observed in a laminar flow with holography

Anna Gorska1, Jacob Fugal2, Subir Mitra3, Szymon Malinowski4, Stephan Borrmann2
1Institute of Geophysics, Faculty of Physics, University of Warsaw, Warsaw, Poland; Particle Chemistry Department, Max Planck Institute for Chemistry, Mainz, Germany
2Particle Chemistry Department, Max Planck Institute for Chemistry, Mainz, Germany; Institute of Atmospheric Physics, Johannes Gutenberg University, Mainz, Germany
3Institute of Atmospheric Physics, Johannes Gutenberg University, Mainz, Germany
4Institute of Geophysics, Faculty of Physics, University of Warsaw, Warsaw, Poland
We investigate collisions of water drop and droplets in a laminar flow in the Mainz vertical wind tunnel by means of in-line holography. We levitate a small rain drop (ca. 550-770 \(\mu \)m) in a vertical stream of cloud droplets (20-70 \(\mu \)m) moving with the terminal velocity of the drop (ca. 3 m/s). We record holographic images with a high speed camera. Processing them allow us to determine sizes and 3D positions of both drop and droplets. Connecting droplets into tracks (up to 6 point tracks) and choosing only tracks in the surrounding of the drop enable to find tracks leading to collisions. We found 106 collisions in a data series of about 50 s. Furthermore, thanks to holography, we are able to characterize the flow, how laminar and uniform it is and determine droplets size distributions and number concentrations and relative and terminal velocities of both. Making this experiment in turbulent conditions will likely be of significance for cloud and climate numerical
Keywords: Experimental techniques, Geophysical flows
Figure 1: Position of droplets around the drop in drop-centered and -normalized coordinates in the plane perpendicular to the flow. The scatter of collision positions outside the drop cross-section are due to measurement