[Gmsh] Extruding a NAS closed surface consisting of 2-3 open shells

[CEM ALBUKREK]

Dear Christophe,

I have a quick question on extruding closed NAS surfaces, with multiple part
ids (PIDS). What I mean by this kind of mesh is that a given closed NAS mesh
consists of unclosed shell partitions. I have been able to build nice volume
meshes around such surface mesh data successfully using GMSH. So somehow
GMSH must be interpreting the vertices at the seams of the PIDS correctly
when volume meshing. In fact, there is no duplicate vertex at the seam in
the NAS file definition.

Unfortunately, I do not see consistent behaviour with surface extrusion from
the same mesh, receiving "could not find extruded vertex" error at the seams
of PIDS. I think the problem is that GMSH interprets each PID (shell)
independently of its neighbors when applying extrusion. Hence the vertex
normals can not be computed correctly at the seams. Am I correct with this?
Do you see a way around, or could you point me to the section of the code
which might require some improvement for this? I think vertex normal
calculation at shared edges needs the tweak...

Attached is a sample file pair (.geo and .nas) for you to observe the
problem. At the header of the .geo file there is a switch to turn off the
extrusion (boundary layer mesh) to verify the volume meshing works well.
Please let me know how I can tackle the issue. Note that I have tested the
same case for which the NAS file has one single PID - one closed shell - and
extrusion works...

Thank you.

Cem
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