Abstract
A planar Doppler velocimetry system to measure flow velocity fields is described. The technique uses a Mach–Zehnder interferometric filter to convert Doppler frequency shifts into intensity variations. The free spectral range of the filter can be selected by adjusting the optical path difference of the interferometer. This allows the velocity measurement range, sensitivity and resolution to be varied. Any laser wavelength may be used. An experimental arrangement is described that incorporates a phase-locking system designed to stabilize the interferometric filter. Two techniques to process the interference fringe images are presented. The first uses the shift of the fringe pattern to determine the Doppler shift along profiles. The second provides a full-field measurement by normalizing the received light intensity at each pixel in the image. Results are presented here for measurements of a rotating disc with maximum velocities of ∼ ± 70 m s−1 at the edge. Standard deviations in the measured velocities were found to be about 2.9 and 2 m s−1 for the two methods. Measurements on a seeded air jet with a nozzle diameter of 20 mm and an exit velocity of ∼85 m s−1 are also presented.
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