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

[Christophe Geuzaine]

CEM ALBUKREK wrote:
> 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...
> 

We'll have a look.



> Thank you.
> 
> Cem
> 
> 
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-- 
Prof. Christophe Geuzaine
University of Liege, Electrical Engineering and Computer Science
http://www.montefiore.ulg.ac.be/~geuzaine