An LDA system yielding long-term averages and velocity distribution (channel flow and erosion experiments)

Abstract

Describes a frequency tracker for laser Doppler anemometry (LDA). The circuit makes use of the phase-locked loop principle. Special features include a data validation circuit and a twin counter for forming long-term averages of velocity data. A measured profile of average velocity in a turbulent open-flow channel is presented. By suitable programming, a commercial microprocessor is made to function as multichannel analyser for LDA velocity data. Accumulated data are processed for moments of the velocity distribution such as the mean, the mean square deviation and, using LDA optics at +or-45 degrees to the mean flow direction, the Reynolds shear stress. In a differencing mode the analyser gives the distribution function of acceleration which indicates the time scale of velocity fluctuations.