Dear researchers,
The following questions are the issues we had when using PIV. We would truly appreciate it if you could help us solve them.
1- Why are velocity vectors highly disorderly shown and recorded with reversed directions or mostly upward directions, and very low values during data collection by rtcontrol software?
2- Although the obstacle on flow path cuts the laser emission coordinates, why are velocity vectors coordinates illustrated continuously rendering the flow crossing path not distinguishable?
3- Why is velocity vector illustrated in the empty space above the flow (with no water), too?
4- What is the best settings for initial pass, overlap, and final window size options?
Best regards,
Maryam
1. I don't quite understand the description. Can you show a picture of that?
2. It could be light diffraction or interpolation in the PIV analysis.
3. Again light diffraction could be why. Also, ambient light source (visible or invisible) could be captured by cameras with excellent dynamic range. Micro particles, such as dust, can serve as tracer. Combining above, you are likely to measure velocity above water surface under such condition. However, this kind of velocity should be filtered out or masked.
4. There is a must-read book for PIV measurement: www.springer.com/us/book/9783319688510
Hope this helps!
Hi Maryam,
You've mentioned you are using "rtcontrol" in your setup. Is it a third party software? Most of the PIV systems currently used in US and Europe are built by either LaVision or Dantec and therefore use DaVis and DynamicStudio since they are their propitiatory software packages. Since I'm not familiar with rtcontrol, I'll try to answer your questions in the general sense.
1. What you've described does not appear to be a software artifact. It could be due to a variety of reason including, but not limited to, improper seeding, low laser intensity, scatter and laser reflections from surfaces. Your processing settings could also contribute to what you are seeing. I suggest playing with your seed density and laser light intensity to see if the processed vector fields improve. You've also mentioned "very low values" which I assume refers to the magnitude of some of the vectors in your field. I wouldn't be surprised to see them in any velocity field but if your post-processing is robust, it should throw those vectors away. I finally, I'd suggest double-checking your calibration constants to make sure they are low (would be best if they are below 0.5.)
2. I couldn't understand your question..
3. This one's easy, there are always spurious vectors in any vector field due to a variety of reasons. While I don't completely agree Wei-Liang on ambient light, I do believe that reflections and light scattering can cause outlier vectors to show up in your vector field. As I said before, your post-processing algorithm should eliminate such vectors.
4. As a rule of thumb, you should reduce your window size in the follow-up passes after the initial one. While there's no one size fits all solution and I'm not familiar with your setup, I'd say you can start with a 32x32 pixel (or 64x64) window with 50% overlap and then continuously reduce your window size. I've never used more than 50% overlap while processing my low-speed data but different setups and flow might call for a different overlap.
Wei-Liang Chuang suggested a great reference and I'd definitely recommend it.
Good luck with your experiments!
I should make a more specific statement on the ambient light that I was thinking of. It comes from any objects in the lab with their surfaces being able to reflect lights. In this case light source is most likely from the light scattering of laser beam or laser light sheet. The energy of such ambient light is undoubtedly much lower than that of light scattering. If there are objects with excellent reflective surface in the lab, chances are that such energy level may still be high enough to illuminate dark or even low-light area in the filed of view and sensed by the camera sensor with extremely high sensitivity. However, this situation is much less likely to happen because it causes safety issues and should always be avoided.
For open source PIV analysis tool, you can consider PIVlab, a MATLAB-based software package. I have been using DaVis for many years, and PIVlab for about a year. For now I don't see PIVlab is any different from DaVis in terms of velocity determination.
Dear Maryam,
1-As mentioned by other researchers it is better to use free PIV softwares like PIVlab or OenPIV. PIVlab is very user friendly and robust. Although LaVision might be the best but I am aware that it is not easy to get it in your country.
The problem you mentioned could be related to different reasons: algorithm, laser and camera misalignment, thick laser sheet,etc.
2-I think what you mean from this question is: why while the obstackle presents in the flow, you can steel see some vecotrs passing through the obstakle, right?
You should make sure that you are focosing on the right plane (sdjusting the depth of focus is very important) and capturing a completly 2D flow. You could also apply MASK (removing obstacle from flow path) on raw images before processing them with PIV software (it greatly helps).
3-As Ata mentioned, PIV algorithm could be a main reason
4-I have worked with 75% overlap when the fluid velocity was high or I needed to have higher resolution especially if you want to investigte a FSI (fluid structure interaction) problem. However it is better to compare the results. In the case of 50% overlap the spurios vectors could be less but as I mentioned you have less data point for the same meausrement plane.
Hope it works!
Good luck
Dear friends (Wei, Ata, and Adel),
Thank you very much for your useful reply and good explanations. Your responses and resources were thoroughly useful for me. My late response is due to my intention to apply your comments to my experiments and see the result. Fortunately, the result was completely satisfactory and my data have been normalized. I only need to learn how to use pivlab in order to analyze my data. I would appreciate it if you could refer me to a source on how to use it.
Thanks and regards,
Maryam
Hi Maryam, may I ask what version of rtControl software you used? I have a problem with the installation, I used version 1.6 which very old version and special for Windows XP and 7. Do you have a new version? Thank you
Dear Riza Siregar
I used version 1.6 that was suitable for Windows 7. I am sorry because I did not use the new version yet.
I hope you can fix it.
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