Consider a simple example of a small cube placed over a big cube. If these two volumes are in proper contact, they would have an unique surface in common. In the Fig. 1a shown below, it appears that the two volumes are in proper contact, but they are not. The easiest way to verify whether these two volumes are in contact or not is, assign different materials to these volumes with the help of the command “Assign” of the “Physics\Material Table” dialog box. See that the color of the each material is distantly different from the other. Observe the color of the edges of the common surface ABCD in Fig. 1b. It is blue which is the same color of the other edges of the small cube. Had the surface ABCD been a proper common surface to both the volumes, the color of its edges would have been light gray which is neither the color of the small cube nor the big cube. Since the color of the edges of the surface ABCD is blue, the geometry shown in Fig. 1 is not valid for the field analysis.
Fig. 1a Fig. 1b
The geometry shown in Fig. 2 and Fig. 3 are valid because the color of the edges of the contacting surface ABCD is light gray. You may choose any one of these two valid geometries for the field analysis.
Fig. 2 Fig. 3