Measurement of acoustic and streaming velocities in fluids using laser Doppler anemometry

Abstract

The optical technique of laser Doppler anemometry (LDA) can be used to measure both acoustic and streaming velocities in a fluid. In a typical LDA system, two coherent laser beams intersect within the fluid to form a series of interference fringes. Whenever a small tracer particle suspended in the fluid passes through these fringes, light is scattered with an intensity that fluctuates at a frequency proportional to the particle velocity. This intensity is measured using a photodetector, and the resultant signal is processed to yield the fluid velocity. An overview of several signal processing methods will be given, and the inherent strengths and weaknesses of the LDA technique will be discussed. The authors have used LDA with burst spectrum analysis and recursive Fourier averaging to measure standing-wave acoustic velocities in the range of 0.02–15 m/s, and to measure Rayleigh streaming velocities in the range of 0.1–20 cm/s in the presence of standing-wave acoustic velocities that were 2–3 orders of magnitude larger. [Work supported by ONR.]