Velocity Measurements in an Unseeded Turbojet Engine Exhaust Using Planar Doppler Velocimetry

Planar Doppler velocimetry (PDV), a non-intrusive measurement technique for obtaining two-dimensional distributions of flow velocity, was applied to a small turbojet engine exhaust containing naturally present aerosol particles. The technique involves using an iodine vapor cell filter to selectively attenuate laser light scattered from the measurement area according to its frequency. A transmission measurement was obtained at each pixel of an image by computing the ratio of the filtered signal to an unfiltered one, yielding the scattered light frequency. A single component of velocity was obtained from the difference between the measured frequency and the laser frequency. Frame-averaged images in the jet exhaust were found to have a noise level of 1.0 m/s RMS, and instantaneous images had a noise level of 9.9 m/s RMS. The results demonstrate an insensitivity of the technique to seeding density.