Image processing for interferometric Mie and Rayleigh scattering velocity measurements

An image-processing method that is used in conjunction with a Fabry-Perot interferometer has been developed for time-resolved local velocity measurements in gas flows. The processing method is used to determine the Doppler shift in Mie or Rayleigh scattered light from 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 it is passed through the interferometer. The concentric, double-ring pattern images (from both the signal and the reference light) formed at the output of the interferometer are captured by a charge-coupled device camera and analyzed for free spectral range and Doppler shift simultaneously using an autocorrelation technique. Measurements have been made in the near field of a Mach 1.6 supersonic jet to determine the structure of the velocity field. Velocity data obtained using the present image analysis technique has been compared with those obtained using a direct analysis method in the same jet.