Thanks for reply, I want to simulate rain drops on porous media using dpm which converts into vof when it comes into contact with the porous surface, so that VOF on porous media can be checked. But, I could only use DPMtoVOF model on wall surface, and only when wall film to vof transition is also initialized. VOF was not formed on porous surface(air-porous medium interface). Should I use UDFs?