[DarioP]

I struggled on this, with bits of (outdated) information scattered all over the place, so here is a brief tutorial, any comment, correction, improvement is welcome!

In OpenFOAM v9 let's consider tutorials/compressible/rhoPimpleFoam/RAS/mixerVessel2D.
There we have the following geometry where the rotation of central region is modeled according to MRF approach (but AMI would be the same for our sake).



The question is: what would be the momentum required to sustain the rotation of the stirrer at the specified velocity?

To get that we need to add the following lines in system/controlDict

functions
{
    forces
    {
         type forces;
         libs ("libforces.so");
         patches (rotor);
         CofR (0 0 0);
         executeControl adjustableRunTime;
         executeInterval 0.01;
         writeControl adjustableRunTime;
         writeInterval 0.01;
    }
};

The results get written in the postProcessing folder while the solver runs.