Laser Doppler Velocimetry

Particle Image Velocimetry - PIV

" is usually a planar laser light sheet technique in which the light sheet is pulsed twice, and images of fine particles lying in the light sheet are recorded on a video camera or a photograph. The displacement of the particle images is measured in the plane of the image and used to determine the displacement of the particles in the flow. The most common way of measuring displacement is to divide the image plane into small interrogation spots and cross correlate the images from the two time exposures. The spatial displacement that produces the maximum cross-correlation statistically approximates the average displacement of the particles in the interrogation cell. Velocity associated with each interrogation spot is just the displacement divided by the time between the laser pulses.

If the velocity component perpendicular to the plane is needed, a stereographic system using two lenses can be used. Typically, the PIV measures on a 100 x 100 grid with accuracy between 0.2% and 5% of full scale and spatial resolution ~1mm. But, special design allow for larger and smaller values. Framing rates of most PIV cameras are of order 10Hz, compatible with pulse rates of Nd: Yag lasers, which is too slow for most cinematic recording. Special systems using rapidly pulsed metal vapor lasers and fast cinematic cameras or special high speed video cameras are able to measure up to ~10,000 frames per second. MicroPIV systems have been constructed to measure velocities in cells as small as a few microns. " (http://www.efluids.com/efluids/pages/products/piv.htm)
Particle size measurements

"For taking size measurements of the air-borne spores in flow conditions, the Phase¨CDoppler method was used (Bachalo and Houser, 1984; Sankar and Bachalo, 1991). The angular distribution of scattering from particles which are much larger than the light wavelength can be accurately described by geometrical optics. This method is based on the fact that the phase difference between Doppler signals collected at different scattering angles is a linear function of particle diameter. In practice, the Phase¨CDoppler method requires optics similar to a conventional LDV, except that the two photodetectors are used in the receiver component (Durst and Whitelaw, 1970). When a particle passes through the intersection of the two beams, the scattered light forms an interference fringe pattern with a temporal frequency determined by the Doppler difference frequency. The spatial frequency of the fringe pattern is dependent upon the angle of observation, particle refraction index, beam intersection angle, laser wavelength and particle diameter. The spatial frequency is measured using the two photodetectors placed at a designated distance. The signals received by the two detectors are at the same Doppler frequency, but are phase shifted. For spherical and refracting particles and in given conditions of particle refractive index and for the wavenumber of the laser light, the phase-shift may be expressed as a linear function of particle diameter (Naqwi et al., 1991). -- by Rambert, A., Huber, L. and Gougat, P., 1998, Laboratory study of fungal spore movement using Laser Doppler Velocimetry

LDV online info:

Basic principle and application, plus couple of photos at

A good description at AIAA LDV

Technical Notes on LDV and PIV

Application for explosive

PIV

Basic principle and application

An in depth application on turbulence study

Application on fishes?

Manufacturers/Suppliers

http://www.beckman.com/default.asp

http://www.tsi.com/fluid/products/ldv/ldv.htm

Particle size

Malvern Instrument