Function forces is included in file controlDict. Definition of forces functions are created in file forces.fun. This functions can evaluate aerodynamic forces acting on selected model patches:

forcesBlade
{
    type                    forces;
    functionObjectLibs      ( "libforces.so" );
    outputControl           timeStep;
    outputInterval          1;
    region                  rotor;
    patches                 ( blade_rot_wall );
    pname                   pMean;
    Uname                   UMean;
    rhoName                 rhoInf;
    rhoInf                  1; // blueSolver uses rho = 1
    log                     true;
    CofR                    (0 0 0);
}
forcesWall
{
    type                    forces;
    functionObjectLibs      ( "libforces.so" );
    outputControl           timeStep;
    outputInterval          1;
    region                  rotor;
    patches                 ( hub_rot_wall_wall hub_rot_wheel_wall hub_rot_tip_wall );
    pname                   pMean;
    Uname                   UMean;
    rhoName                 rhoInf;
    rhoInf                  1; // blueSolver uses rho = 1
    log                     true;
    CofR                    (0 0 0);
}

During the computation the results are written in directory forces/0/forces.dat.