A direct interface implies that the two boundaries composing the interface should be intersected and, if they are non-conformal, some new discretization will be created. An explicit connection is created between cells on each side of the interface so that mass and/or heat transfer will occur across the intersected area.
When a direct interface is created from two boundaries, two new boundaries associated with that interface are automatically created as well. The faces of these new boundaries are composed of the intersection of the faces of the original boundaries. The original boundaries from which the interface is created are not destroyed; however, the surface area of each will be reduced by the amount it overlaps with the other. Direct interfaces can be created between boundaries with meshes that are either conformal or non-conformal and can belong to the same region or different regions depending on the type selected.
The three allowable direct type topologies are: