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Q-criterion in 2D flows

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Old   March 9, 2011, 14:25
Default Q-criterion in 2D flows
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Joshua Counsil
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Good day!

I have been modelling 2D URANS flows over airfoils. In a recent report, I had several plots of vorticity magnitude. A reviewer complained that the vorticity magnitude was not sufficient for vorticity detection. The reviewer suggested we plot the Q-criterion (second invariant of the velocity tensor) or Okubo-Weiss criterion (Weiss, J., The Dynamics of Enstrophy Transfer in 2‐Dimensional Hydrodynamics, Physica D: Nonlinear Phenomena, 48, pp. 273294 (1991)) for better visualization.

After talking to people within my department, I think the reviewer's suggestion may be incorrect. I've never seen anybody use the Q-criterion in 2D and/or URANS flows. I argue that the Q-criterion (and Okubo-Weiss criterion) has no significance for 2D flows, which are what I'm studying. However, I can't describe why.

Am I right in my assumption? If so, why? Can you direct me to an example that has used either criterion in 2D and/or URANS flows?

Thanks for all your help.
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Old   June 14, 2013, 03:52
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Ehsan
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hi Josh
Knowing its a sowewhat old thread,
But if you read and remember did you find probably your answer?
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Old   June 15, 2013, 05:19
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I agree that vorticity magnitude is not automatically a detection of vortical structure (e.g., think of the case of Poiseuille flow), however I see no necessity in trying to use the Q criterion for detecting vortices in a 2D URANS-based solutions....
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Old   June 15, 2013, 05:55
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Hi
than how to detect vortices in 2D case?
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Old   June 15, 2013, 06:12
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Quote:
Originally Posted by immortality View Post
Hi
than how to detect vortices in 2D case?
- Definition of a vortex tube, as described by the book of Chorin and Marsden. You just have a 2D closed curve since the tube is infinitely extended in the orthogonal direction of the flow plane.
- A 2D recirculation region is detected by closed isolines of the stream-function
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Old   June 15, 2013, 07:13
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I used Q command and don't know what other setting to do in paraView for seeing vortices.
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