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April 11, 2011, 12:42 
Reynolds Stress Tensor

#1 
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Sergio
Join Date: Apr 2011
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Hello Foamers
I am quite new to CFD and OpenFoam, and I was wondering if someone can help me with this: How can be calculated / displayed the Reynolds Stress Tensor, in order to, say, calculate the stresses at a given point of the domain. Please also note that I am using a LES turbulence model Thanks in advance for your help 

April 11, 2011, 13:29 

#2 
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David Aljure
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Location: CTTC Universidad Politécnica de Catalunya. Spain
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In the controlDict, the last part you can choose to add functions:
Code:
functions { fieldAverage1 { type fieldAverage; functionObjectLibs ( "libfieldFunctionObjects.so" ); enabled true; outputControl timeStep; outputInterval 100; fields ( U { mean on; prime2Mean on; base time; } p { mean on; prime2Mean off; base time; } ); } } This function will give a symmetric tensor field with 6 components for the times you choose. 

April 12, 2011, 10:42 

#3 
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Sergio
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Hi David, thanks for your answer. I will try it.
On the other hand, checking commands, I found stressComponents and typed it. The thing, is that it created at each time step a sigmaxx, sigma xy.... sigmazz files. However when I opened any of those files the units I found are [0 2 2 0 0 0 0], while I was expecting [1 1 2 0 0 0 0]. Do you, or any other foamer have something to say about this? Thanks 

April 12, 2011, 12:55 

#4 
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David Aljure
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In its source code, stressComponents is defined as:
laminarTransport.nu()*2*dev(symm(fvc::grad(U))) so I guess its the deviatoric stress tensor divided by the density, hence the units [0 2 2 ...] These stress components are different than the Reynolds stresses: Re Stresses: velocity fluctuation. Dev. Stresses: deformation. 

April 12, 2011, 14:01 

#5 
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Sergio
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Gracias David
The tech support folks from OpenFoam, have said the following about this issue stressComponents was written for incompressible laminar flow. For LES you need to accumulate the Reynolds stress by averaging during the run.It looks as if it is time for this newbie to dig into the programming aspects of OpenFoam (auch!!!) 

April 12, 2011, 14:20 

#6 
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Sergio
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It finally makes some sense:
Tau= mu*dV/dy stressComponents uses nu [m2 s1] instead of mu [kg m1 s1], and that is the reason why the stress tensor yields [m2 s2] However I still need to know how to calculate the stress tensor components 

April 13, 2011, 03:23 

#7 
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David Aljure
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If you need to calculate the Re stress tensor use the info in my first post, at the end of control dict add the fieldaveraging function, UPrime2mean gives the Re stress tensor.
I'm also a newbie at foam and programing, but I have digged several times into the source codes. Have fun 

April 18, 2014, 07:30 

#8 
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Tony
Join Date: Nov 2013
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Hi David,
I saw you post about the Reynolds stress tensor. I also found that UPrime2mean yielded six columns which should be stress tensor. I was just wondering how I can get the RMS value from the UPrime2mean file. Hope you can give me some hints. Thank you very much. Kind regards, Tony 

January 4, 2017, 11:06 

#9 
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DimitriF
Join Date: Dec 2016
Location: London
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Hey David,
This is quite an old post but maybe I'm lucky and get a reply. I want to plot the Reynolds stress versus the normal wall direction at a specific point. To do so, I used the post process utilities of OpenFOAM (simpleFoam postProcess func R latestTime) but I cannot sample this tensor along a line. (see my post here Sample of Reynolds Stress Normal to Wall) I thought I can use your option which is to write the UPrime2Mean using your proposed function above. However, comparing the Reynold stress that I got from the post process utility and UPrime2Mean, there is quite a difference. Any idea why? Cheers, Dimitri 

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