Velocity measurement of compressible air flows utilizing a highspeed video camera for separate image capture

Kurzfassung

Particle image velocimetry (PIV) is applied with growing success to an increasing wide variety of flows. The most important step towards adapting this technique to different applications was the development and use of the image shifting technique as described by Adrian (1986), Lourenco (1988) and Grant et al. (1990). This technique eliminates directional ambiguity, and also allows a free selection of the pulse separation in spite of possible restrictions in the utilized evaluation methods. Furthermore, if the two necessary exposures are recorded on the same film or video chip, thistechnique helps to avoid an overlap of particle images in regions of lower flow velocities. Free choise of the pulse separation means thatthis measuring method offers excellent metrological and spatial resolutions, as well as a higher tolerance towards out-of-plane flow components and large velocity gradients. This makes it highly adaptable to a wide variety of conditions. The same advantaves can be achieved by recording the different exposures on separate images. Howover, the use of a video camera, which is dependent on the video rate, gives rise to new time restrictions as reported by Willert and Gharib (1991) and Merzkirch et al. (1994). The use of a high-speed video camera has been suggested by Cho (1989) as a possibility for circumventing those restrictions. This article explains which devices and procedures can best be used to perform this kind of measurement in high-speed applications. The attainable quality is demonstrated by showing a result of a current measurement.