Instantaneous, two camera, three dimensional planar Doppler velocimetry using imaging fiber bundles

This paper describes a planar Doppler velocimetry (PDV) technique that is capable of measuring the three, instantaneous components of velocity in two dimensions using a single pair of signal and reference cameras. PDV can be used to measure the instantaneous 3-D velocity of a fluid by using an absorption line filter (ALF) to determine the Doppler shifted frequency of a narrow line pulsed laser (Nd:YAG) that has been scattered off particles seeded into the flow. In the technique presented here the three views required to obtain three dimensional velocity information are ported from the collection optics to a single imaging plane using flexible fiber imaging bundles. These are made up of a coherent array of single fibers and are combined at one end as the input plane to the measurement head. A fourth leg of the imaging bundle is used to image the individual laser pulses and allow correction for pulse-to-pulse frequency variations. The results reported in the paper are from the development phase of the system and are of the velocity field of a rotating wheel.