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An image processing method has been developed, in conjunction with a Fabry-Perot interferometer, for the measurement of time-resolved local velocity of gas flows. The processing technique is used in order to determine, on-line, the Doppler shift in Mie or Rayleigh scattered light form the measurement volume. The method minimizes the adverse effects due to the drifting of laser frequency and interferometer alignment. The scattered light is mixed with unshifted light from the interrogation laser for reference before passing it through the interferometer. The concentric, double-ring pattern images formed at the output of the interferometer are captured by a CCD camera and analyzed for free spectral range and Doppler shift. The original images in polar coordinates are first mapped into a rectangular coordinate system and subsequently analyzed using an auto-correlat ion method in order to determine the Doppler frequency. Measurements were made in the near field of a slightly underexpanded Mach 1.6 supersonic jet. The results obtained by the present image analysis technique compare favorably with those obtained using a direct analysis method.
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Bivolaru et al. (1999) studied this question.
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