[reza2031]

I am using forceCoeffs function object to generate a cylinder's drag and lift coefficients during the simulation.
The issue is that the values are different in the serial and parallel runs. Even when the number of CPUs changes, the results change.
I am using pisoFoam.
Any idea?

In controlDict:

functions
{
forces
{
    type            forceCoeffs;
    libs            ("libforces.so");
    writeControl    timeStep;
    writeInterval   10;

    patches         ("cylinder");
    rho             rhoInf;      // Indicates incompressible
    log             true;
    rhoInf          1;           // Redundant for incompressible
    liftDir         (0 1 0);
    dragDir         (1 0 0);
    CofR            (0 0 0);  // Axle midpoint on ground
    pitchAxis       (0 0 1);
    magUInf         0.535;
    lRef            0.04;        // Wheelbase length
    Aref            0.16;        // Estimated
}
}

[reza2031]

Since no one replied, I am replying to my post!
After days of research on this matter, I concluded that because of the intercommunication between cups in a parallel run, the point at which vortex shedding starts is different than in a serial run.
The shedding frequency and the Strouhal number are the same.
If you integrate variables over time, nothing changes in both runs.