The Periodic Condition node can be used to define periodicity for the mass transport between two sets of boundaries. The node prescribes continuity in the concentration and the mass flux between the “source” and the “destination” side respectively. Note that these names are arbitrary and does not influence the direction in which mass is transported. It is dictated by mass transfer equations in the adjacent domains.

For more complex geometries, it might be necessary to add the Destination Selection subnode, which is available from the context menu (right-click the parent node) as well as from the Physics toolbar, Attributes menu. With this subnode, the boundaries that constitute the source and destination surfaces can be manually specified.