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sakti July 28, 1998 09:04

laser doppler
Is there anybody has used laser doppler anemometry or others to measure the velocity in the flow ??

would you tell me how to use the apparatus (principally) and what is the output?

thank you for the attention.


John C. Chien July 28, 1998 18:31

Re: laser doppler
Laser doppler anemometer has been in use since early 70's ( maybe even in late 60's) to measure flow velocity indirectly ( the velocity of the seeded particles ). In real applications, the seeding a problem. When a particle enters the prob volume created by two laser beams at different frequencies, there will be a doppler shift in frequency in the reflected laser light. The frequency shift is directly related to the particle's velocity. When the receiving end of the sensor is placed ahead of the prob volume, it is called forward scattering system. When the sensor is placed on the source side of the laser, it is called the back scattering system. So, you need a system to count the frequency of wave generated by the particle entering the prob volume. For turbulent flows, you collect large smaples and run the data reduction program to give you the average velocity and other turbulence quantities. It takes time,because you have to wait for a particle to enter the small prob volume. Commercial systems have been available since mid-70's. It is also a common tools used in large government laboratories and universities. There are books, reports, and conferences on this subject. This is what I learned from my colleague back in early 70's when he was measuring supersonic jets with shocks.( particles are much larger and havier than gas molecules, so it's tricky to measure gas velocity in this case ). Beyond this point, try to get some information from a commercial LDA system Lab.

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