The process of designing an appropriate drag force is given as an example here. We will outline the process of designing the Ergun drag force,

F = 150.0*e*nu*rho/(pow(e*D, 2.0)) + 1.75*rho()*Ur/(e*D)

where we need to work out both porosity e and the fluid velocity Ur = U_MD-U_CFD. A far more detailed version is given in the next section. The minimal changes are given here, add the following to the CPL_force.h header file,

class CPLForceErgun : public CPLForceGranular {


    using CPLForceGranular::CPLForceGranular;

    //Force specific things
    double drag_coefficient(double r[], double D, std::vector<double> Ui_v);


and the following code to CPL_force.cpp which changes the drag coefficient

double CPLForceErgun::drag_coefficient(double r[], double D, std::vector<double> Ui_v) {
    double eps = CPLForceGranular::get_eps(r);
    if (eps == 0.0) {
        return 0.0;
    } else {
        return 150.0*eps*(mu/rho)*rho/(pow(eps*D, 2.0)) + 1.75*rho/(eps*D);

That's it, provided the force can be defined in terms of position, D and relative velocity Ur with all fields from pre-force eSum and vSums.