Planar Doppler velocimetry using optical fibers

Abstract

Planar Doppler Velocimetry (PDV) measures the Doppler frequency shift of light scattered by particles entrained in a moving stream of air or gas. The technique uses a laser light sheet and one or more viewing directions to obtain up to three components of velocity within the sheet. Continuous wave (cw) lasers can be used to measure time-averaged velocity and pulsed lasers for instantaneous and time-averaged velocity. Optical fibres can be used to provide flexible deployment of the light sheet and can be combined with a beam scanner to generate a “top-hat” beam profile which provides more uniform illumination across the light sheet and can help improve signal to noise ratio. Coherent fibre bundle arrays can be used to provide flexible imaging of the light sheet. These bundles can be used with both cw and pulsed lasers and provide significant simplification of the optical instrumentation and by coupling to different imaging optics they enable a wide range of flow measurement applications. For example, standard SLR lenses can be used for external flow applications such as windtunnels, whilst the use of borescopes attached to the end of the fibre bundles enables internal flow measurement applications such as gas turbine compressors. This paper will introduce the technique of Planar Doppler Velocimetry (PDV), describe the technique used for Doppler frequency shift to intensity transduction and methods to obtain three components of velocity instantaneously and time-averaged from a laser light sheet. Optical fibres for laser beam delivery and coherent imaging fibre bundles for imaging of the laser light sheet will be presented and demonstrated on seeded air flows.