When reading a mesh file into FLUENT an error is reported:
Error: Cannot change [a boundary zone name] to interior because there is only one adjacent cell thread.
This error occurs when an edge, in 2D, or a face, in 3D, is assigned the boundary type interior in Gambit when that edge/face only has a fluid region on one side. If your mesh is intended to have fluid on both sides of this edge/face, then open Gambit and check for a duplicate edge/face. To avoid this problem ensure that adjacent faces share a common edge if fluid is intended to cross the shared edge.