Outflow
The Outflow boundary condition subnode is available from the context menu (right-click the Species parent node) or from the Physics toolbar, Attributes menu.
The Outflow subnode is used on boundaries where the species mass is to be transported out of the model domain.
If an imaginary slice is drawn through the cross section of a geometry, then there is no net flux across the slice due to diffusion, since diffusion corresponds to random motion of the atoms and molecules. There can, however, be a net flux due to convection, migration or thermophoresis, but outlet boundary conditions for temperature and electric field also impose zero gradient. An appropriate “open” boundary condition is to set the normal component of the diffusive flux to zero, which implies that diffusive transport of flux across the imaginary slice is zero. The boundary condition becomes:
which assumes that all mass passing through the boundary is due to convection.