Q-criterion in 2D flows

Josh · March 9, 2011, 14:25

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. 273–294 (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.

immortality · June 14, 2013, 04:52

hi Josh
Knowing its a sowewhat old thread,
But if you read and remember did you find probably your answer?

FMDenaro · June 15, 2013, 06:19

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....

immortality · June 15, 2013, 06:55

Hi
than how to detect vortices in 2D case?

FMDenaro · June 15, 2013, 07:12

Quote:

Originally Posted by immortality

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

immortality · June 15, 2013, 08:13

I used Q command and don't know what other setting to do in paraView for seeing vortices.

March 9, 2011, 14:25	Q-criterion in 2D flows	#1
Josh Senior Member Joshua Counsil Join Date: Jul 2009 Location: Halifax, Nova Scotia, Canada Posts: 366 Rep Power: 18	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. 273–294 (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.

June 14, 2013, 04:52		#2
immortality Senior Member Ehsan Join Date: Oct 2012 Location: Iran Posts: 2,208 Rep Power: 27	hi Josh Knowing its a sowewhat old thread, But if you read and remember did you find probably your answer? __________________ Injustice Anywhere is a Threat for Justice Everywhere.Martin Luther King. To Be or Not To Be,Thats the Question! The Only Stupid Question Is the One that Goes Unasked.

June 15, 2013, 06:55		#4
immortality Senior Member Ehsan Join Date: Oct 2012 Location: Iran Posts: 2,208 Rep Power: 27	Hi than how to detect vortices in 2D case? __________________ Injustice Anywhere is a Threat for Justice Everywhere.Martin Luther King. To Be or Not To Be,Thats the Question! The Only Stupid Question Is the One that Goes Unasked.

June 15, 2013, 08:13		#6
immortality Senior Member Ehsan Join Date: Oct 2012 Location: Iran Posts: 2,208 Rep Power: 27	I used Q command and don't know what other setting to do in paraView for seeing vortices. __________________ Injustice Anywhere is a Threat for Justice Everywhere.Martin Luther King. To Be or Not To Be,Thats the Question! The Only Stupid Question Is the One that Goes Unasked.

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June 15, 2013, 06:19		#3
FMDenaro Senior Member Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,882 Rep Power: 73	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....