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LES: Qcriterion- visualize/recognize turbulence

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Old   August 28, 2013, 10:11
Default LES: Qcriterion- visualize/recognize turbulence
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Dear all,

I am using the LES model for my stenosis geometry since I found this strange behaviour for the velocity magnitude (and also for the subgrid turbulent viscosity ratio) (see pics) a few diamters behind the stenosis and also read some papers dealing with the apperance of vortices behind a stenosis. But when I plot the Q criterion it is just zero for the whole domain. Does this mean there are no turbulences? What is a criterion for the apperance of a "real" turbulence and how can I visualize this?

It would be really nice if somebody could give me ahint!
Thanks amillion in advance!
Lilly
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File Type: jpg SubgridTurbViscRatio_Stenosis.jpg (23.6 KB, 104 views)
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Old   August 28, 2013, 10:28
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When you say that the value of the Q-criterion is zero, do you mean that it is REALLY zero or do you mean that its magnitude throughout the domain is very low compared to the maximum that fluent uses for scaling the countour range?

If it is the first option, did you make sure that the q-criterion is among the result file quantities?
If it is option 2, rescale the contour plot to an appropriate range.
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Old   August 29, 2013, 03:16
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Thank you so much, flotus1!

It's really zero. And I did the post processing with fluent itself (so I guess the Q criterion should be among the result quantities, or is this wrong?)

Is there another way to visualize turbulence? Or might there be no turbulence if Q is zero, even the the vel magnitude and the viscosity looks so? Or am I doing something wrong at plotting the Q criterion?

Thank you for any idea!
Lilly
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Old   August 29, 2013, 03:52
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The normalized Q-criterion is not among the standard quantities that fluent writes to the result file. You could try to add it in the "run calculation" tab under "data file quantities.
But since it is a variable derived from the velocity field, it should be available no matter if it is written to the result file or not, at least when you do the post-processing in fluent directly.
This is odd...

Some other identifiers for turbulent flow regions may be the vorticity, helicity, \lambda_2-criterion and of course the fluctuations of velocity and pressure.
The two contour plots you show in your first post clearly indicate that there are regions of turbulent flow.
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Old   August 29, 2013, 08:53
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Thank you again, flotus1!
I was looking at the results of another sim that I am running at the moment and the Q criterion lies between 9e+6 and 9e-6 there (is it normal that it is negative by the way?), so I have no idea what happend to results of the first simulation...
Thank you for the description of the other identifiers of turbulence! So a plot of them with non-zero values would indicate turbulence?

I made a iso surface plot (Iso Q criterion=100) and colored it by vorticity magnitude. Would this wavy structure at the left of the pic would be an inidicator of turbulence there?
Thank you again for any idea!
Lilly
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Old   August 29, 2013, 10:24
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Quote:
lies between 9e+6 and 9e-6 there (is it normal that it is negative by the way?)
No negative values here

Quote:
Thank you for the description of the other identifiers of turbulence! So a plot of them with non-zero values would indicate turbulence?
As always, it depends. The vorticity for example has non-zero values in any shear flow, no matter if turbulent or not.
A similar problem seems to be present in the picture you attatched.
You will have to play around with the values of the isosurfaces until the plot looks the way you want it to.

The most obvious measure for identifying regions of turbulent flow are the fluctuations of velocity and pressure.
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Old   August 30, 2013, 03:46
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Quote:
Quote:
lies between 9e+6 and 9e-6 there (is it normal that it is negative by the way?)
No negative values here
Thank you again, flotus1!
I should definitely think before writing : of course the range is 9e+6 to -9e+6, so there are really negative values for the Q criterion (?)

I also realized why the Q criterion was zero: A really strange thing, I have to visualize the "Vorticity Magnitude" before visualizing the "Q criterion". Then the "Q criterion" is not zero anymore, I have no idea why this happens...
So as a measure of turbulence you would show some plots of the the velocity magnitude at the same time of the period for different periods? If they are not congruent, there are fluctuations in within the velocity field and turbulence is present(?)
So a simple eddy at a single time as shown in my first pic is not an evidence of turbulent, yet?

Thank you again for any idea! It is really helpful to talk to somebody how is familiar with turbulence modelling, thank you so much!
Lilly
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Old   August 30, 2013, 03:56
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Hi,
i just wanted to add little bit of information regarding Q criterion.. please excuse me if you had already discussed about this..

Q criterion defines the difference between the vorticity magnitude and the strain rate. Therefore sometimes it is better to visualise turbulence throuigh Q criterion instead of vorticity or pressure plots.
So basically, if the rotation dominates the strain/shear then the Q criterion is positive and you have active vortices in the flow.

you can also manually define the Q criterion inside Fluent through custom defenitions, if i remember correctly.
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Old   September 4, 2013, 05:01
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Thank you for your information, Ananthakrishnan!

So would a negative Q criterion mean that there is no turbulence or would a Qcriterion value of zero mean there is no turbulence present?

Thank you for any idea!
Lilly
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Old   September 4, 2013, 13:30
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The Q scalar is defined as:

Q=-0.5*(dudx^2+dvdy^2+dwdz^2)-dudy*dvdx-dudz*dwdx-dvdz*dwdy

Hence, any parallel flow (e.g., laminar flows in ducts) will have Q = 0.

In general, Q = (d^2p/dx^2 + d^2p/dy^2 + d^2p/dz^2)/(2*rho) and will be zero whenever the pressure has just a linear spatial variation, is positive for local pressure minima and negative for local pressure maxima. As you can see:

1) If Q is exactly zero, the flow is necessarily laminar
2) If Q is not zero the flow is NOT necessarily turbulent. Laminar examples with Q not 0 are all the flows with a 2D rotating pattern (just du/dy*dv/dx < 0 and constant, e.g., u=-k*y, v=k*x with k = 0.5 times the z component of the vorticity)
3) Turbulent flows will exhibit local maxima and minima for Q whose value are strictly dependent on the Re number of the flow

What matters for turbulent flow visualizations are the positive Q iso-surfaces. The specific value to pick is problem/visual appeal dependent.
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