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Old   June 22, 1999, 08:32
Default Verification using LDA
Bo B. B. Jensen
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A question a little bit out of the ordinary:

When using LDA (Laser Doppler Anemometri) does anyone have any hints to some of my "problems":

Problems: 1) I have curvatured surfaces (valve houseing and pipes) wherein I want to measure the velocity profiles. I have read somewhere that problems arises when the laser beam passes through the curvatured surface, and that these problems can be minimized by putting a square box around with water around the pipe. Is that correct? and if yes, where can I find some information on this subject.

2) Thickness of the plexiglass. Is there a limit to the thickness of the plaxiglass through which the laser beam is going.?

3) Other hints are very welcome?


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Old   June 22, 1999, 08:55
Default Re: Verification using LDA
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1) The problem is refraction (and reflection near the surfaces). Any LDA reference book.

2) Generally, the thicker, the more the beams wander off. The point at which no more reliable readings can be taken is a function of a great many things. It is quite common to make slot shaped "windows" of thin material through which the beams pass to take readings whilst maintaining a thick wall elsewhere to stop the rig distorting/falling apart.

3) Talk to an LDA experimentalist.
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Old   June 22, 1999, 11:14
Default Re: Verification using LDA
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andy is correct. talk to an LDA researcher. the one I've been in contact with has said there is definite problem with the lasers going through the housing or material of a pipe or wind tunnel. they took so many measurements that they had to be done with a wind tunnel door partially open which meant that the flow velocity had to be very low, but since LDA requires (I believe) a carrier particle such as an oil, the flow still has to be fast enough to keep the particle aloft so in avoiding the problems of correcting for refraction, they introduced a flow distortion by opening the tunnel door. perhaps a door with a bunch of removable slit covers would allow for minimal flow distortion and still allow for the many flow measurements that are desirable. LDA can certainly be a very helpful tool in conjunction with CFD.
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Old   June 30, 1999, 22:15
Default Re: Verification using LDA
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1) If you are on the center plane of your pipe, you will probably be alright. It depends on the radius of curvature of the pipe. If the pipe is rather large and the angle between the laser beams is not, then you should be able to make the measurements. If you traverse the probe volume in the direction of the beams (E x B) then you should be OK. If you traverse in the radial direction perpendicular to this you will have problems because the two beams will intersect the curved surface at different angles.

2) The thickness of the plexiglass will depend on the quality of the material (in terms of particles present that can scatter light). If you could use glass for the section you are probing you will be better off.

3) The uncertainty in LDV measurements is greater than many people realize. The best method is to calibrate the instrument in-house, and then to make your measurements WITHOUT CHANGING ANY OF THE OPERATING PARAMETERS. It is almost impossible to quantify the uncertainty in phase doppler measurements if you operate the instrument under conditions other than those in which it was calibrated.

Hope this helps. Feel free to e-mail me if you have any other questions.

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