# how to plot Q criterion in fluent

 Register Blogs Members List Search Today's Posts Mark Forums Read

 July 13, 2005, 10:06 how to plot Q criterion in fluent #1 anindya Guest   Posts: n/a Can anyone suggest how I can plot Q criterion in fluent to visualize the coherent structures? Thanks for your help. Anindya

 July 21, 2005, 08:02 Re: how to plot Q criterion in fluent #2 said Guest   Posts: n/a hi anindya, you certainly have the definition of that q criterion which is based on velocity gradient. Fluent do not provide it directly,i mean you have to compose it from computed velocity gradient as a custom field function. for that: define>custom field function and type the name of the criterion "q" for example. you will be then able to visualize the q value on a specified surface on the contour panel... if you want to create an animation of an iso-q surface for example you also have to create an isosurface, hope it helps. best regards Said

 July 22, 2005, 13:33 Re: how to plot Q criterion in fluent #3 anindya Guest   Posts: n/a Thanks a lot for your message. I have already created that custom field function Q. So when I create iso-surfaces, do I create iso surfaces of Q ( say specific values of Q) and then plot Q on those Q iso-surfaces? Or do I plot Q on iso-surfaces of low pressure, vorticity-maginitude, etc? Anindya

 July 24, 2005, 14:06 Re: how to plot Q criterion in fluent #4 Helper Guest   Posts: n/a Fortunately, you don't have to use UDF. Here's a simpler way of visualizing the iso-contours of the second-invariant. You can use the custom field funtion capbility in FLUENT. First you define the second-invariant of deformation tensor with Q = 0.5(W*W - S*S) where W is the vorticity magnitude (you can find it under the "Velocity" menu) and S is the mean rate-of-strain (you can find it under "Derivatives" menu). Once you defined Q, you can generate iso-surfaces of Q for several positive values.

 July 24, 2005, 23:57 Re: how to plot Q criterion in fluent #5 anindya Guest   Posts: n/a Thanks a lot for your help. Anindya

March 20, 2011, 17:51
Q expressed with scalars or tensors...
#6
Member

Join Date: May 2010
Posts: 44
Rep Power: 8
Quote:
 Originally Posted by Helper ;122608 Fortunately, you don't have to use UDF. Here's a simpler way of visualizing the iso-contours of the second-invariant. You can use the custom field funtion capbility in FLUENT. First you define the second-invariant of deformation tensor with Q = 0.5(W*W - S*S) where W is the vorticity magnitude (you can find it under the "Velocity" menu) and S is the mean rate-of-strain (you can find it under "Derivatives" menu). Once you defined Q, you can generate iso-surfaces of Q for several positive values.

Hi Guys,

should not it be the following:
Q=0.25*(W*W - S*S)
???

As the mean rate-of-strain (scalar) is defined as:
S=(2Sij*Sij)^0.5
and similarly the vorticity magnitude (scalar):
W=(2Wij*Wij)^0.5
with Sij being the mean rate of strain tensor and with Wij the mean vorticity tensor (besides, Sij=symmetric, while Wij=antisymmetric part of mean velocity gradient tensor).
Finally the second invariant of velocity grad tensor is defined as:
Q=0.5*(Wij*Wij - Sij*Sij). So I think Q should be: Q=0.25*(W*W - S*S). Is that right?

Besides, this formula of Q is only valid for incompressible flows, more precisely for flows with divergence free velocity field so that divUj=0 <=> Sii=0 !

Thanks!

Last edited by la7low; March 22, 2011 at 21:31.

 May 10, 2011, 05:26 Q expressed with scalars or tensors #7 New Member   Dipanjay Join Date: May 2011 Posts: 1 Rep Power: 0 Hi la7low, I completely agree with you. Cheers..

 October 22, 2012, 08:37 Q criterion Reference #8 New Member   Mohamed Ashar Join Date: Feb 2011 Posts: 12 Rep Power: 7 Hi la7low, Could you please give the reference, from where you have taken this Q criterion. Thanks Last edited by ashar_md2001; October 22, 2012 at 08:52.

February 13, 2016, 02:03
Q criterion
#9
Member

Subhasish Mitra
Join Date: Oct 2009
Location: Australia
Posts: 56
Rep Power: 9
Quote:
 Originally Posted by ashar_md2001 Hi la7low, Could you please give the reference, from where you have taken this Q criterion. Thanks
Q criterion was proposed by Jeong & Hussain (1995), J.Fluid Mech., vol.285, pp.69-94.
__________________
SM

 February 23, 2016, 10:16 Different definitions in Fluent and CFD-Post #10 New Member   C. Meraner Join Date: Dec 2012 Posts: 1 Rep Power: 0 According to "F.R. Menter, Best Practice: Scale-Resolving Simulations in ANSYS CFD, Version 2.00, November 2015" it is: "[...] for historic reasons 0.5 in ANSYS Fluent and 0.25 in ANSYS CFD-Post". However, I don't know how it is in ANSYS CFX.

February 24, 2016, 18:15
#11
Member

Subhasish Mitra
Join Date: Oct 2009
Location: Australia
Posts: 56
Rep Power: 9
Quote:
 Originally Posted by subha_meter Q criterion was proposed by Jeong & Hussain (1995), J.Fluid Mech., vol.285, pp.69-94.
My apology for a correction required in the earlier post. Below are the actual references for the three useful criteria often used for detecting vortex:

1. Q criterion: positive second invariant of velocity gradient tensor

HUNT,J . C. R., WRAY, A.A., & MOIN, P. 1988 Eddies, stream, and convergence zones in turbulent flows. Center for Turbulence Research Report CTR-S88, p. 193.

2. Discriminant (DELTA) criterion: complex eigenvalues of velocity gradient tensor

CHONG, M.S., PERRY, A.E. & CANTWELL, B. J. 1990 A general classification of three-dimensional flow field. Phys. Fluids A 2, 765.

3. Lambda 2 criterion: negative second eigenvalue of the S^2 + W^2 tensor where S = strain rate tensor (symmetric part of velocity gradient tensor) and W = vorticity tensor (antisymmetric part of velocity gradient tensor)

Jeong & Hussain (1995), J.Fluid Mech., vol.285, pp.69-94.
__________________
SM

 August 19, 2016, 15:10 plot in 2-D #12 New Member   amir Join Date: Sep 2014 Posts: 7 Rep Power: 4 How can we plot Q criterion, Discriminant (DELTA) criterion, and Lambda 2 criterion in a 2D plane. what is an important plot to draw?

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is OffTrackbacks are On Pingbacks are On Refbacks are On Forum Rules

 Similar Threads Thread Thread Starter Forum Replies Last Post ivanbuz FLUENT 12 November 16, 2016 14:38 ibnkureshi FLUENT 5 June 9, 2011 13:43 khimkhim FLUENT 0 March 16, 2010 00:49 Renold FLUENT 0 December 8, 2009 13:00 Luo FLUENT 24 April 11, 2000 06:07

All times are GMT -4. The time now is 16:50.