Development of Planar Doppler Velocimetry for Aircraft Exhausts

A diagnostic method is developed for measuring 2D planar distributions of velocity in an engine exhaust flow. Planar Doppler velocimetry (PDV) measures the Doppler shift of laser light scattered from soot particles naturally present in the flow, and may also be used for measuring non-combustion flows outside the exhaust using naturally occurring environmental aerosols. An optical image processing filter that employs iodine vapor absorption transforms frequency variations in the scattered light to variations in image intensity. The technique is demonstrated here to provide 2D distributions of one-component instantaneous velocity in a cross-section of the exhaust flow from a small turbojet engine. The Doppler shifts observed were in good agreement with those expected based on pitot tube measurements of the same flow. Velocities of up to 233 m/s were measured, with uncertainties ranging from 8.2 to 16.7 m/s, limited primarily by image registration when normalizing by the reference image.