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Mohammad Al-Shannag July 1, 2000 07:04

PIV Images
Hello every body,

I am using Particle Image velocimetry, PIV, technique to obtain the velocity vectors in some planes within the flow domain, which has curved cylindrical geometry. The problem I face is the distortion of the images due to the presence of curved Plexiglas wall. I will be so appreciated if someone will tell me how can I get rid off this problem or how can I correct the distortion of the taken images.

Thank you in advance,

Your sincerely, Mohammad

John C. Chien July 1, 2000 08:44

Re: PIV Images
(1). Can you create a window on the wall? (2). The flow near the window will not be useful, but the flow away from the window shoud be all right. (3). The window can be enclosed. It can also have different shape and size, depend on your measuring system requirement.

Mohammad Al-Shannag July 1, 2000 09:22

Re: PIV Images
Dear John,

Thank you for the fast response. The problem is that the wall is curved, 500 mm in diameter and CCD Video camera has focus with 5 cm in diameter. This means that it is difficult to joint it without entering a part of it in the flow domain. Another thing there will be flow in the hole space between the camera lens and its front boundary, 6 cm depth which will affect the flow. Also the lens of the camera will be wetted and affect the image.

Thank you

Your sincerely, Mohammad

John C. Chien July 1, 2000 12:30

Re: PIV Images
(1). I know that there are physical constraints on the system. (2). What I am saying is the curved wall effect can be reduced by replacing a portion of the curved wall by a hole. This is apparently not acceptable, because there will be nothing between the hole and the camera. (3). Now normally what people do is to put a small piece of flat window (glass window) there to replace the original curved wall. In this way, the distortion will be gone. But since the inner wall of the glass window is flat, and if you are interested in the flow in the near wall region, you are not going to get the original flow field back there. (everything is relative in size). (4). You can keep that piece of flat window glass small, and use additional lenses between the window and the camera lense. This will reduce the flat window effect to a minimum. (5). Now you have the camera and lense, then optical system, then the small flat glass window , then the flow field inside the 500mm pipe. In this way, the camera will stay outside and dry. (6). You can repeat the same process at different locations of the curved wall, and move the lense system and camera around.

Mohammad Al-Shannag July 2, 2000 06:21

distortion correction
Hi John,

I would like to thank you very much for the detailed explanation you provided me. Is there a posibility of making the images with their distortion and then to correct it in some way?. I prefered this but if it is not efficient I am going to do what you told me, "glass window".


John C. Chien July 2, 2000 17:49

Re: distortion correction
(1). I guess, anything is possible,especially in the image processing area. (2). But unfortunately, I am no expert in this area. So, I would say Yes, it is possible to correct the distortion, but I can't help you. (3). You can start looking into image processing area first. And it is likely that people who are using this method had similar problem before. They could give you direct suggestions. (4). Some people in the IC engine research lab have been using this method. So I think, it is a good idea to contact them in person through e-mail. So, look through Internet first, see whether you can locate these researchers first. good luck.

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