Two-frequency planar Doppler velocimetry (2ν-PDV)

Abstract

A planar Doppler velocimetry (PDV) system has been designed which is able to generate two beams from a single source separated in frequency by 690 MHz. This allows a common-path imaging head to be constructed, using a single imaging camera instead of the usual camera pair. Both illumination beams can be derived from a single laser and a set of acousto-optic modulators used to affect the frequency shifts. One illumination frequency lies on an absorption line of gaseous iodine, and the other in a region of zero absorption. The beams sequentially illuminate a plane within a seeded flow and Doppler-shifted scattered light passes through an iodine vapor cell onto the camera. The reference beam that lies in a zero absorption region is unaffected by passage through the cell, and provides a reference image. The signal beam, the frequency of which coincides with an absorption line, encodes the velocity information as a variation in transmission dependent upon the Doppler shift. Images of the flow under both illumination frequencies are formed on the same camera, ensuring registration of the reference and signal images. This removes a major problem of a two-camera imaging head, and cost efficiency is also improved by the simplification of the system. The dual illumination technique has been shown to operate successfully with a spinning disc as a test object and is currently achieving a velocity resolution of about +/−2 ms−1, limited by the quality of the light sheet generated from the multimode fiber. Automatic superposition of the signal and reference images is achieved, and polarization errors caused by the beam splitter in the conventional system are eliminated. Measurements have also been made on an axisymmetric air jet, seeded with a commercial smoke generator, which has maximum velocities of ∼100 ms−1. A comparison with data obtained simultaneously, using a conventional two camera PDV arrangement has been made and the difference between the measurements found to be within a few m/s.