About specified flux

John

Hi, everyone. I have a case in which there are mass diffusion form the wall. My method is to use a UDS to model the mass diffusion. However I found there are two options in the UDS bondary condition of wall: one is "specified value", another is "specified flux" . The help document said that "(i.e., the negative of the term in parenthesis on the left hand side of Equation 9.3-2 dot [as in the dot product of] n [as in the vector, n], where n is the normal into the domain). ". My understanding is n.(Zeta*grad(phi)). Is it right? In addition, what is the physical meaning of this "specified flux"?