Calculation issues of velocity

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Aidana Boribayeva

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Mar 27, 2025, 4:12:58 AM3/27/25
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Dear PIVlab Developers,

I am experiencing an issue with velocity calculations in PIVlab and would appreciate your guidance. My frames are recorded at 250 FPS (set as 4 ms) with a dense flow of particles. I have applied masking and calibration settings as shown in the images, and all other settings are configured according to the tutorials. However, I am encountering the following issues:

  1. The calculated velocity magnitude is approximately twice the expected value, which is also evident from the polyline analysis.

  2. I tried using "Subtract mean U and mean V" with both 0 and the suggested values, but in both cases, the mean U and mean V appear to be very large.  In some zones, the velocity vectors change direction opposite to the actual flow direction, which is from top to bottom.

  3. Should I apply velocity limits? When I tried, it resulted in shortened data with NaN values. Subtract flow.pngSubtract flow.pngColormap.png

  4. How can I normalize the velocity magnitude with the colormap?

I would appreciate any suggestions on improving my PIVlab settings.

Calibration.png

Best regards,

Aidana Boribayeva
PhD Candidate, Nazarbayev University, Astana, Kazakhstan



thielickeoptolution

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Mar 27, 2025, 4:20:22 AM3/27/25
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Hi, raw data is required for any help.

Aidana Boribayeva

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Mar 27, 2025, 4:27:13 AM3/27/25
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Thank you very much, I appreciate your work!
The distance I set as 10 mm as below
Sph_15_trial1_250_FPS_S0001000001.bmpScale 1.png

thielickeoptolution

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Mar 27, 2025, 4:28:21 AM3/27/25
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How can you do PIV with only one image?

Aidana Boribayeva

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Mar 27, 2025, 4:42:12 AM3/27/25
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I am very sorry there was issue with uploading zip file. Please,, downdload from this link Images for PIVlab

William

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Mar 27, 2025, 5:59:29 AM3/27/25
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Hi, I think your data looks very good! You will get accurate results with this.
Concerning your questions:
4: Normalization is not possible, why do you need this?
3: Your data is very good, no need to set strict velocity limits
2:  Why do you want to do this? It changes the data and is only needed if you want to change the frame of reference.
1: There are exactly three possibilities that PIVlab gives values that differ from your expectations:
  • Your camera does not report the correct fps
  • Your length reference is wrong
  • Your expectations are wrong
I would recommend to use the horizontal distance between the white tape as a reference. Larger distances on the screen are better for a precise calibration.

Aidana Boribayeva

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Mar 27, 2025, 7:34:52 AM3/27/25
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Thank you very much! I will try to work on with your suggestions and I hope to come back with positive outcomes.
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