[Gmsh] Combine single facets to surface group OR multiple STL files (closed surface) merged to get 3D mesh

[Meiner Auchmeiner]

Hello gmsh group,

first of all I would like to thank Mr. Geuzaine and Mr. Remacle, and
all the other contributors, for this great software, thank you very
much!

I hope that somebody here can give me a hint towards the solution of
my problem. I'm trying to import a STL geometry (with one closed
surface) into gmsh and mesh it as a volumetric body with 3D elements.
The resulting mesh should ideally also consist of multiple surface
definitions on that body, in order to apply boundary conditions in the
framework of the Finite Element Method (FEM).
Firstly, I was able to import the STL file
(rabbit_mod_opt_500faces.stl) into gmsh (rabbit_mod_opt_500faces.geo)
and did get a decent 3D mesh (see screen1.jpg). But unfortunately I
wasn't able to select single facets - or find any other ways - to
define physical surface groups on that body (see screen2.jpg).
In a next approach I split-up the initial shell surface in advance
(thanks to meshlab) into two parts (see
rabbit_mod_opt_500faces_subset1.stl and
rabbit_mod_opt_500faces_subset2.stl) and merged them into gmsh (see
rabbit_mod_opt_500faces_subsets.geo). Now I'm able to add physical
surface groups but unfortunately I wasn't able to get a correct 3d
mesh (see screen3.jpg). I also played around with the orientation of
the input faces of the two STL files but didn't succeed (but I'm not
sure if I did it right).

My questions are now:
* Is it possible to pick single facets to combine them to multiple
physical surface groups?
* Is it possible to get a 3d mesh out of multiple surface shells (STL
files) merged together? If it is, what intermediate step would be
necessary to do so?
* I read in a previous mail about an experimental feature called
compound surfaces - is it possible that I could use this to solve my
problem?


I would greatly appreciate any assistance.
Thanks,
Uwe

P.S.: The geometry I used for testing purpose comes from:
http://thevirtualheart.org/anatomyindex.html
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