I want to calculate gradient of volume fraction. And I can't store the values temporary using solve set expert. So I have stored the C_VOF(c,t) in C_UDSI(c,t,0). What boundary conditions should I give to this scalar? I have given velocity inlet BC at inlet and outflow BC at outlet. I know the profile of volume fraction at inlet but at outlet I know neither the value nor the flux for this UDS.

Thank you