CFD Online Discussion Forums - internal face in blockMesh

Hi Foamers,

I have tried to run two concentric cylinders but when I define patches I have an error message.
Error messages:
Trying to specify a boundary face 4(4 5 17 16) on the face on cell 1 which is either an internal face or already belongs to some other patch. This is face 0 of patch 5 named evap.
From function polyMesh, polyMesh
(
const IOobject& io,
const pointField& points,
const cellShapeList& cellsAsShapes,
const faceListList& boundaryFaces,
const wordList& boundaryPatchTypes,
const wordList& boundaryPatchNames,
const word& defaultBoundaryPatchType
)
in file C:\tmp\OpenFOAM-1.5\src\OpenFOAM\meshes\polyMesh\polyMeshFromShape Mesh.C at line 483.
FOAM aborting

could anybody help me to know how we can define an internal face in blockMesh?