Planar Doppler velocimetry measurements of flows using imaging fibre bundles

The development of a planar Doppler velocimetry is described. The technique 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. The velocity of the fluid is determined using the Doppler formula and is dependent on the laser direction and the viewing direction. Hence, only one velocity component of the flow is measured. This component can be measured in two spatial dimensions using an array detector such as a CCD camera. To capture the three components, three such measurement heads have been used viewing from different angles. In the technique presented here the three views are ported from the collection optics to a single imaging plane using flexible fibre imaging bundles. These are made up of a coherent array of single fibres and are combined at one end as the input plane to the measurement head. The paper discusses the issues involved in developing a full three-dimensional velocity measurement system.