Optimal seeding for long pathlength multi-component laser Doppler anemometers

Abstract

There are optimal combinations of size and number density of seed particles which can maximize the processed data rate of laser Doppler anemometers (LDAs). When optical pathlengths are long, as in large scale water facilities, the choice and density of seeding particles determines the balance between the scattered-light intensity at a measuring volume and its extinction along the rest of the optical path, and so its intensity at a photodetector. These considerations must generally be coupled with knowledge of particle arrival statistics, detector performance, processor characteristics and signal conditioning to optimize data rate. However, in multicomponent LDAs, coincidence requirements in data arrival times enforce a close approximation to a Poisson distribution in the processed data rate as a function of the number density of seed particles. This result leads to simple models for optimal number densities for seeding as a function of optical pathlength and related parameters.