how to plot Q criterion in fluent
Can anyone suggest how I can plot Q criterion in fluent to visualize the coherent structures?
Thanks for your help. Anindya 
Re: how to plot Q criterion in fluent
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 
Re: how to plot Q criterion in fluent
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 
Re: how to plot Q criterion in fluent
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. 
Re: how to plot Q criterion in fluent
Thanks a lot for your help.
Anindya 
Q expressed with scalars or tensors...
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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! 
Q expressed with scalars or tensors

Q criterion Reference
Hi la7low,
Could you please give the reference, from where you have taken this Q criterion. Thanks 
Q criterion
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Different definitions in Fluent and CFDPost
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.

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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. 
plot in 2D
How can we plot Q criterion, Discriminant (DELTA) criterion, and Lambda 2 criterion in a 2D plane. what is an important plot to draw?

This reference may be helpful for defining different vortex detection criteria in 2d
http://aip.scitation.org/doi/10.1063/1.4927647 
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Ps: If I am not wrong, I remember is due to some historical reason to use 0.5 and 0.25 in Fluent 
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