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Old   April 11, 2011, 12:42
Default Reynolds Stress Tensor
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Sergio
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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
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Old   April 11, 2011, 13:29
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David Aljure
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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;
            }
        );
    }
}
I'm pretty sure UPrime2Mean is the Reynolds stress tensor, (if its not, somebody please correct me)

This function will give a symmetric tensor field with 6 components for the times you choose.
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Old   April 12, 2011, 10:42
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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
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Old   April 12, 2011, 12:55
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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.
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Old   April 12, 2011, 14:01
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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.

Take a look at stressComponents.C and you will see how the stress is calculated;
it is a kinematic rather than dynamic stress.
It looks as if it is time for this newbie to dig into the programming aspects of OpenFoam (auch!!!)
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Old   April 12, 2011, 14:20
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It finally makes some sense:

Tau= mu*dV/dy

stressComponents uses nu [m2 s-1] instead of mu [kg m-1 s-1], and that is the reason why the stress tensor yields [m2 s-2]

However I still need to know how to calculate the stress tensor components
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Old   April 13, 2011, 03:23
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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
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Old   April 18, 2014, 07:30
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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
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