DGV’s Accuracy Dependence Upon Laser Beam Intensity Profile

Abstract

A Doppler Global Velocimeter (DGV) system was designed for use in high speed rotating equipment at the Turbomachinery Laboratory. Due to the rapidly varying periodic nature of flows inside turbines, compressors, and pumps, it is desirable to use a pulsed laser as the light source. An ND-YAG laser was selected for use based upon the 9 ns pulse duration and the ability for the laser to operate with a 15 MHz light bandwidth which is tunable to the absorption line filter used in the DGV system. However, when applied to the system it was discovered the DGV system did not work properly. The output of a line CCD array used to monitor the laser frequency was closely scrutinized. The light intensity across the laser beam was not Gaussian in nature but contained a very large amount of “noise”. Since the DGV system measures light intensity variations to infer Doppler shifts and hence velocity distributions, the rapidly spatially varying light intensity across the laser beam was suspected as the cause of the system’s inaccuracy. An analysis to quantify how the laser beam light intensity profile noise affects a DGV system accuracy is performed and possible remedies are suggested.