Laser Doppler Velocimetry (LDV) is a fluid mechanics experimental measurement technique whereby a pair of intersecting laser beams create an interference fringe pattern that, as flow tracer particles pass through the beam crossing, scatters light at a frequency proportional to the velocity of the particle.
The benefits of LDV include its non-invasive nature, ability to measure very high velocities, suitability for a large dynamic range of velocity measurements, and data-rates into the 100s of kHz. The final output of an LDV system is a time-history of individual velocity measurements corresponding to every instant that a particle passed through the measurement volume.
One application of LDV is the measurement of multiphase flows. In these applications, the velocities of particles representing multiple phases are measured simultaneously. In a standard LDV setup, the velocities of particles representing the different phases would be measured in the same way, that is, a frequency measurement would be taken for every particle passing through the measurement volume. One inherent advantage of LDV is that since it is an optical technique and relies on the measurement of a scattered light signal, the intensity of the signal can also be measured. Using this extra piece of information about each particle measurement, the data can be segmented based on the signal intensity, thus allowing the velocity measurements to be grouped separately based on which phase is being measured.
The figure shows a histogram of the measured intensities from a sand/water velocity measurement. The water was seeded with ~10-micron glass spheres and the sand particles were on the order of several hundreds of microns in diameter. Due to the size difference and the properties of Mie Scattering, upon which LDV depends, the intensity of scattered signal from the sand particles was much brighter than the signal generated by the 10-micron tracers. By subranging the data based on intensity, it is possible to analyze the velocity information of the water and sand separately through a simultaneous measurement.