Laser velocimetry

Laser-Doppler velocimetry (LDV) is the method of measuring fluid velocities by detecting the Doppler frequency shift of laser light that has been scattered by small particles moving with the fluid. The technique was originally discussed in a pioneering paper by Cummins et al. [1], in which they measured the Brownian motion of an aqueous suspension of micron-sized particles by observing the spectrum of the scattered light. In these measurements the quantity of interest was the broadening of the laser light spectrum due to the random particle motion. However, they also observed a net shift in the frequency of the light, an effect that they attributed to small convection currents that generated mean velocities in their water cell. Hence, almost inadvertently, they performed the first measurenments of fluid velocity by laser-Doppler velocimetry. Shorlty thereafter, Yeh and Cummins [2] carried out an experiment intended expressly to demonstrate the measurement of fluid velocites.