On Sidesets, what do forward, reverse, and both mean?
Surfaces have a “natural” direction, the direction in which the normals point. For surfaces bounding a single volume, normally this points out of the volume. For surfaces which are part of two volumes, this surface direction is oriented “forward” with respect to one volume and “reverse” wrt the other volume (assuming a volume’s natural direction is outward). Putting a surface in a sideset gets translated in the mesh output as a 3d element and a side number of that element. The reverse, forward, or both qualifiers on a surface in a sideset designates which geometric volume from which that 3d element gets chosen. Note that in the case of merged/non-manifold surfaces, getting the directionality wrong will have the effect of reversing the sign of the Neumann boundary condition in the target analysis.
