Choosing the right circulation value

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hawk...@mail.usf.edu

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Feb 4, 2021, 12:52:11 PM2/4/21
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Hi! 

I am relatively new to using PIVlab for analysis so I apologize if this question seems rather simple. 

I am trying to calculate the vorticity of paired vortices that shed off the fin of a fish I am studying. I have followed the instructions to find tangential velocity that are available on the blog, but I am not sure what value I actually need to extract. If I extract all the values on the line, I have 30 tangential velocities but that does not tell me the "maximal" circulation. Do I just have to record the specific coordinates and the circulation at the vortex core and use that in the pivlab_GUI code?  

William Thielicke

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Feb 4, 2021, 1:41:27 PM2/4/21
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Hi, the integral of the tangential velocity (on a path around a vortex) will give you the circulation. Pivlab gives you the integral already when you e.g. select to extract tangential velocity of a circle series.
If you need further help, please share a picture of what you are doing.

Manojkumar Gupta

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Oct 5, 2025, 2:14:33 AM (7 days ago) Oct 5
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Could you please clarify the difference in circulation calculation between the following two methods in PIVlab:

  1. Line integral of tangential velocity around a series of circles (extraction along polyline — Series of Circles, SS1).

  2. Area integral of vorticity over a circular region (area extraction of vorticity and integration to obtain circulation — SS2).

    Please see the attached screenshots for reference.

Thank you.

SS2_area integral of vorticity_cirulation.png
SS1_tangential vel_circulation calculation.png

William

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Oct 5, 2025, 3:42:58 AM (7 days ago) Oct 5
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Hi, these are just the two independent available independent options to calculate circulation from a velocity map. For your case they match relatively well, so it seems theory is correct ... again! :)
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