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Drag coefficients in OpenFOAM

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Old   September 28, 2017, 09:44
Default Drag coefficients in OpenFOAM
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Hello everyone,

currently I am struggling with the calculation of drag for an Ahmed Body.

My first Question is how does OpenFOAM calculate the drag? I assume it integrates the difference in pressure and the shearing stress on the surface and adds them both, is that correct? I am using the library libforces.so:

Code:
  forceCn
    {
        type            forceCoeffs;
        libs ( "libforces.so" );
        writeControl   timeStep;
        writeInterval  1;
        patches         ( "Nose");
        rho             rhoInf;
    pRef        0;
        log             true;
        rhoInf          1.2;
        liftDir         (0 0 1);
        dragDir         (-1 0 0);
        CofR            (0 0 0);
        pitchAxis       (0 1 0);
        magUInf         40;
        lRef            1;
        Aref            0.112;
    }
The drag coefficient at the nose is supposed to be 0.02 http://user.engineering.uiowa.edu/~m...FDEFDahmed.pdf (what seems very low considering the geometry).

My calculations deliver 0.3. Therefore my absolute drag coefficient of the Ahmed Body ist about two times higher than in the paper. The drag coefficients at the back and rear slop are almost the same as in the paper, it is just the drag coefficient at the nose of the body. And why is the drag coefficient at the rear slop and the back positive anyway? Isnt it supposed to be negative, because the pressure difference is negative?


Thanks for any advice, I hope you can help!

Greetings,
Friendly
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