if (particle diameter > D_filter_size)
{
boundary condition is DPM_BC_REFLECT;
}
if (particle diameter < D_filter_size)
{
boundary condition DPM_BC_INTERIOR;
}
For interior DPM, it's just like the "porous jump" boundary, interior for DPM, and particles can pass through the boundary without anything happening ("filter" surface in my figure).