I understand the reasoning behind the relaxation of terms, but why should I do that?nThe diffusion term in the UDS equation is not a problem when it is implemented directly in the UDS equation. When I set D=1e-15 so that I can test if my source term (the same diffusion term) is okay, then the solver blows up. Does the solver follow a different approach in each term? nThe same for the VOF equation. Volume fraction equation in VOF + diffusion term = my equation of UDS. I am solving always the same equation, but I am doing different test to see where it is the problem.nFurthermore, there is a situation (VOF + source term for some specific values of the diffusivity) where the solver does not blow up. Just gives meaningless results.n