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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

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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

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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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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
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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 
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Subhasish Mitra
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Q criterion was proposed by Jeong & Hussain (1995), J.Fluid Mech., vol.285, pp.6994.
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February 23, 2016, 10:16 
Different definitions in Fluent and CFDPost

#10 
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C. Meraner
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According to "F.R. Menter, Best Practice: ScaleResolving Simulations in ANSYS CFD, Version 2.00, November 2015" it is: "[...] for historic reasons 0.5 in ANSYS Fluent and 0.25 in ANSYS CFDPost". However, I don't know how it is in ANSYS CFX.


February 24, 2016, 18:15 

#11  
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Subhasish Mitra
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Quote:
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 CTRS88, 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 threedimensional 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.6994.
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