Design Considerations for a 3d Fibre Optic Laser Doppler Velocimeter for Turbomachinery Applications

Single headed 3D Laser Doppler Velocimetry (LDV) geometries generally rely upon the use of 3 Doppler difference channels, inclined at differing angles with respect to the mechanical axes of the probe. The transformation between the non-orthogonal measurement co-ordinate system and the Cartesian system can result in large errors in the calculated velocities. A theoretical analysis of the geometrically induced uncertainties in measurements produced by four single headed 3D LDV configurations is presented. These considerations have lead to the development of a single headed fibre optic 3D LDV probe based around the use of two Doppler difference channels to directly measure the transverse velocity channels, and a reference beam channel to measure the on axis velocity component. The f/4 probe head has a working distance of 200 mm, designed to operate within the constraints of the limited optical access available in turbomachinery applications.