Q-criterion in 2D flows

Josh · March 9, 2011, 13:25

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, 03:52

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

FMDenaro · June 15, 2013, 05:19

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, 05:55

Hi
than how to detect vortices in 2D case?

FMDenaro · June 15, 2013, 06:12

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, 07:13

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

March 9, 2011, 13:25	Q-criterion in 2D flows	#1
Josh Senior Member Joshua Counsil Join Date: Jul 2009 Location: Halifax, Nova Scotia, Canada Posts: 366 Rep Power: 17	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, 03:52		#2
immortality Senior Member Ehsan Join Date: Oct 2012 Location: Iran Posts: 2,208 Rep Power: 26	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, 05:55		#4
immortality Senior Member Ehsan Join Date: Oct 2012 Location: Iran Posts: 2,208 Rep Power: 26	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, 07:13		#6
immortality Senior Member Ehsan Join Date: Oct 2012 Location: Iran Posts: 2,208 Rep Power: 26	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.

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
SIMPLE pressure correction in compressible flows	Maarten de Jong	Main CFD Forum	7	April 30, 2012 02:23
y+ in high Pr flows	vishyaroon	Main CFD Forum	0	April 15, 2010 14:24
Laminar Solid-Liquid (Two-phase flows) flows in CFX	freemankofi	ANSYS	0	April 8, 2010 16:59
Free surface flows and multiphase flows	Vidar	Main CFD Forum	0	September 18, 2008 05:05
calculating recirculating turbulent flows with k-e	Abhijeet Vaidya	Main CFD Forum	10	November 18, 2002 08:21

June 15, 2013, 05:19		#3
FMDenaro Senior Member Filippo Maria Denaro Join Date: Jul 2010 Posts: 6,768 Rep Power: 71	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....