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how to plot Q criterion in fluent

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Old   July 13, 2005, 10:06
Default how to plot Q criterion in fluent
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anindya
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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
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Old   July 21, 2005, 08:02
Default Re: how to plot Q criterion in fluent
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said
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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 iso-q surface for example you also have to create an isosurface,

hope it helps. best regards

Said
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Old   July 22, 2005, 13:33
Default Re: how to plot Q criterion in fluent
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anindya
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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
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Old   July 24, 2005, 14:06
Default Re: how to plot Q criterion in fluent
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Helper
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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.
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Old   July 24, 2005, 23:57
Default Re: how to plot Q criterion in fluent
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anindya
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Thanks a lot for your help.

Anindya
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Old   March 20, 2011, 17:51
Default Q expressed with scalars or tensors...
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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.
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Old   May 10, 2011, 05:26
Default Q expressed with scalars or tensors
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Dipanjay
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Hi la7low, I completely agree with you.

Cheers..
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Old   October 22, 2012, 08:37
Default Q criterion Reference
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Mohamed Ashar
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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.
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