Photographical vs. Video PIV "Experimental experience - theoretical aspects"

Abstract

The performance of a new digital evaluation method for photographic particle image velocimetry (PIV) recordings is described. Rather thanevaluating the recordings with optical systems, which require contact copies, high resolution scan of the actual negatives are digitally processed using a modified cross-correlation method. The primary advantages are a increased spatial resolution in the obtained velocity vector maps (due to smaller interrogation areas), increased evaluation speed, and a simple calibration procedure. The accuracy of the technique matches, and in some cases exceeds, the performance of the presently available optical PIV evaluation method under a variety of conditions. Important parameters are the particle image diameter, with an optimum at 2 to 3 pixels (typ. 10-30 microm.on film), the interrogation window size, and the number of pulses used in the generation of the PIV recording. The performance and limitations of the method are illustrated with several examples of applications of PIV in DLR wind tunnels: a cylinder wake, a wake behind a 2-D car model and a transonic flow behind a flat plate.