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July 13, 2005, 10:06 
how to plot Q criterion in fluent

#1 
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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 
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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 isoq 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 
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Thanks a lot for your message. I have already created that custom field function Q.
So when I create isosurfaces, do I create iso surfaces of Q ( say specific values of Q) and then plot Q on those Q isosurfaces? Or do I plot Q on isosurfaces of low pressure, vorticitymaginitude, etc? Anindya 

July 24, 2005, 14:06 
Re: how to plot Q criterion in fluent

#4 
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Fortunately, you don't have to use UDF. Here's a simpler way of visualizing the isocontours of the secondinvariant.
You can use the custom field funtion capbility in FLUENT. First you define the secondinvariant 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 rateofstrain (you can find it under "Derivatives" menu). Once you defined Q, you can generate isosurfaces of Q for several positive values. 

July 24, 2005, 23:57 
Re: how to plot Q criterion in fluent

#5 
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Thanks a lot for your help.
Anindya 

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

#6  
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Join Date: May 2010
Posts: 44
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Quote:
Hi Guys, should not it be the following: Q=0.25*(W*W  S*S) ??? As the mean rateofstrain (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. 

October 22, 2012, 08:37 
Q criterion Reference

#8 
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Mohamed Ashar
Join Date: Feb 2011
Posts: 12
Rep Power: 6 
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. 

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