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May 23, 2012, 20:41 

#21 
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Sony
Join Date: May 2010
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Thanks,
I will try it and get back to you. Cheers, Sony 

May 24, 2012, 01:51 

#22 
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Arif
Join Date: Mar 2012
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Hi,
I am looking for a freelancer to simulate supersonic flow through CD nozzle by fluent 6.2.16 Please contact upal_arif@yahoo.com Quote:


August 3, 2012, 15:20 
Small error in the code

#23 
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Guillaume Beardsell
Join Date: Aug 2012
Posts: 1
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Hello,
If I can save some time to the people who downloaded the lambda2.c, there is a small error in the definitionof p, (which has a huge impact on the output, of course)... Instead of p=b/(3.0*a); One should read p= b/(3.0*a); Hope this can be helpful to someone ! 

August 3, 2012, 15:38 

#24 
Senior Member

I knew there was something wrong with it (can't say i didn't write it clear ) but i lost my reference on the cubic equation and put it aside. Thank you very much for your attention, it will help me.


May 6, 2013, 05:48 

#25 
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Tom
Join Date: May 2013
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Hi all,
I've just implemented both criteria, Lambda2 and the Qcriterion. However the two methods yield almost the same result, which lead me to the questions:
My data set under consideration is a measurement and contains noise. Both methods are sensitive of the gradient computation of the vector field. Considering noise in data, can we make a statement about which methods works best for measured data sets? Thank you for your feedback 

May 6, 2013, 06:50 

#26 
Senior Member

The following reference should answer some of your questions:
http://www.aero.polimi.it/~quadrio/papers/1999ejmb.pdf 

May 6, 2013, 07:00 

#27 
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Tom
Join Date: May 2013
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Hi,
thank you, in fact most of my questions can be answered with your reference. bests, brunntho 

June 19, 2013, 21:06 
lambda2 refernce

#28 
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MonaK.
Join Date: Jun 2013
Location: Ein Sof
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Hey,
Can anybody point out where in Lesieur's book(Turbulence in Fluids) can I find the definition of lamda2 ? esp. one that Filippo used in his script? Thanks! ~e 

June 20, 2013, 11:03 

#29 
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Tom
Join Date: May 2013
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maybe following paper will also be helpful for you.
http://journals.cambridge.org/action...ine&aid=353418 

June 21, 2013, 02:08 
lambda2 reference

#30 
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MonaK.
Join Date: Jun 2013
Location: Ein Sof
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Hey all CFDists,
Firstly, I need correct my last post: the script that "Paolo" (not Filippo) wrote. All Italian names sound the same. Secondly, if Paolo or whoever went through the script of lambda2.c would you please tell me how you calculate the P11, P22, P33, P12, P13, P23? I can not find any reference for it. Thirdly, thank you, brunntho, I started writing my code based on the paper of Jeong & Hussain. I agree with you. Although it's a pretty helpful article,I couldn't find any reference for calculating the value of Pij Cheers! ~e 

June 21, 2013, 05:26 

#31 
Senior Member

Dear Eddieme,
first of all, the reference book is: Lesieur, Metais, Comte  "Large Eddy Simulation of Turbulence"  Cambridge. There you can find that P_ij is actually S_ij S_jk + W_ij W_jk (sum of squared symmetric and antisymmetric parts of velocity gradient), which i calculated by hand. Then i used some reference to compute the roots of the cubic equation. I guess you are right not trusting my computation of P_ij in lambda2.c as i already made a mistake in the cubic equation. But i don't think you will find a reference for it, you need to do it by hand. If you actually find a mistake, however, i would be glad to know. 

December 22, 2013, 18:47 

#32 
New Member

Great contribution, Thanks sbaffini for this precious piece of code, it's really valuable for me.


April 2, 2014, 04:18 

#33 
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Mostafa
Join Date: Apr 2013
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Quote:
Dear All, I am new in the field and I work with compressible flows. Does anyone knows if the lambda 2 criterion in the mentioned format will work for my problems (compressible flow)? because in compressible flow this formula does not recover the pressure Hessian matrix. Best, Mostafa 

April 2, 2014, 13:01 

#34 
Senior Member

Dear Mostafa,
the short answer is no. In the link i posted above: http://www.aero.polimi.it/~quadrio/papers/1999ejmb.pdf there is also a derivation for the compressible pressure Hessian, which is obviously different from the incompressible one. I don't remember if a solution for compressible flows is also effectively presented. The first link i got from Google on the matter is instead: http://www.wseas.us/elibrary/confer.../MAFLUH03.pdf which seems to discuss some compressible aspects (still, i didn't read it). 

April 3, 2014, 03:26 

#35 
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Mostafa
Join Date: Apr 2013
Posts: 3
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Thank you Paolo.
I think I should go to Delta criteria. I found one paper from Farrell and Martin (JOT 2009) where they showed the Delta criteria. Their results are nice, but they use a lot of filtering! Best, Mostafa 

August 25, 2014, 11:56 
Lambda 2

#36 
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Azadeh Saeedi
Join Date: Mar 2014
Location: Canada
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hello every one,
I actually have a question. I calculated the eigenvalues for the lambda 2, but the range includes the positive numbers as well as negative ones. So my question is did I do my calculation wrongly, and I only should have negative numbers? or I only should choose the negative numbers from the range ? Thanks Last edited by Azy; August 25, 2014 at 15:06. 

August 25, 2014, 13:05 

#37 
Senior Member

You should just pick up a reasonable negative number to use as value for the lambda2 isosurface. By reasonable i mean close to 0 but different enough to produce a nice picture. Obviously, by proper nondimensionalization of lambda2, it may be possible to pickup more physically sound values... but your eyes are probably gonna work better than this.


August 25, 2014, 15:12 

#38 
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Azadeh Saeedi
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Thanks Paolo, Ok so it is alright to have positive numbers, I should just pick up negative ones to plot the iso surfaces. And is it better for my numbers to be as close as possible to zero? I dont know why I think in reverse :/ :/


August 25, 2014, 16:04 

#39 
Senior Member

In theory, you need a strictly negative value for lambda2 in order to select zones where the pressure has a local minimum. This is all you need to know if you want to use lambda2 as a visualization tool (this might not be true if you need lambda2 for other means).
According to this, i really suggest a trial and error approach in order to find the best value (which might be even dependent on the specific visualization/isosurface routines). 

August 25, 2014, 17:13 

#40 
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Azadeh Saeedi
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yes I just want it for visualization. Thanks a lot Paolo


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