Citation:
Tom O. H. Charrett, Helen D. Ford, David S. Nobes and Ralph P. Tatam. Dual Illumination Planar Doppler Velocimetry using a Single Camera. Proceedings of the SPIE Optical Diagnostics for Fluids, Solids, and Combustion II. 3-4 August 2003, San Diego, CA, USA. Volume 5191, 113. Eds. Patrick V. Farrell, Fu-Pen Chiang, Carolyn R. Mercer, Gongxin Shen
Abstract:
A Planar Doppler Velocimetry (PDV) illumination system has been designed which
is able to generate two beams, separated in frequency by about 600 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, using acousto-optic modulators to effect the frequency
shifts. One illumination frequency lies on an absorption line of gaseous iodine,
and the other just off the absorption line. The beams sequentially illuminate a
plane within a seeded flow and Doppler-shifted scattered light passes through an
iodine vapour cell onto the camera. The beam that lies at an optical frequency
away from the absorption line is not affected by passage through the cell, and
provides a reference image. The other 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. The benefits of combining the dual
illumination system with a three-component, fibre-linked imaging head developed
at Cranfield will be discussed.
