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Reference values

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Old   August 16, 2023, 08:19
Default Reference values
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Saeed Jamshidi
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Hello everyone,
Would you please tell know what is the lRef and Aref in force function. I,m going to calculate drag and lift coefficients of circular cylinder in openfoam. Assume that the diameter of cylinder is 0.04 and we have considered the thickness in z direction equal to 0.02.

Best,
Saeed


Code:
functions
{
    forces
    {
        type            forceCoeffs;
        functionObjectLibs ( "libforces.so" );
        outputControl   timeStep;
        outputInterval  1;
        patches         ( cylinder );
        pName           p;
        UName           U;
        rhoName         rhoInf;
        log             true;
        rhoInf          1;
        liftDir         (0 1 0);
        dragDir         (1 0 0);
        CofR            (-0.5 0 0);
        pitchAxis       (0 1 0);
        magUInf         1;
        lRef            1;
        Aref            1;
    }
}
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Old   August 17, 2023, 05:15
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Yann
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Hello Saeed,

These are the reference length and area to compute the coefficients.
https://doc.openfoam.com/2306/tools/...s/forceCoeffs/

The choice of these reference dimensions is arbitrary and depends on the application.

For a circular cylinder I guess the diameter should do the job as the reference length, and frontal area as reference area. But if you intend to compare your results with reference data, the best way would be to check what reference dimensions has been used, and use the same dimensions for your case.

Cheers,
Yann
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Old   August 17, 2023, 05:37
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Saeed Jamshidi
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Dear Yann, thank you for your response 🙏.
As you said, Aref is frontal area which means diameter of the cylinder times to the height of the cylinder. I'm worry about the value of the height of the cylinder, because it's arbitrary and on the basis of it we will have different Aref. Shall we consider it to 1 m?
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Old   August 17, 2023, 05:44
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Yann
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No, keep using the actual height of your cylinder. So you can compare coefficients between cylinders of different sizes.
What would be wrong would be to change the dimension you are using, or to not use the same characteristic dimension than the data you want to compare to. (like using something else than the cylinder height)

But with a circular cylinder there are not really a lot of options to choose from to define characteristic dimensions
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