[Gmsh] Issue with periodicities

[Alain Berod]

Dear all,

First of all, I would like to thanks the Gmsh developers and team for
providing such a robust and versatile meshing tool.

I encountered a strange behaviour related to periodicities. When I run the
attached file *not_working.geo* with an alpha parameter (line 8 of the
script) different to pi/4 (try pi/8 if you want to see the error), the code
correctly finds the corresponding periodic surfaces *2-5 and 8-1* and
performs the 2D meshing without any error.
Nevertheless, the 3D meshing outputs the following errors:




*Info    : Found two exactly self-intersecting facets.Info    :   1st:
[297, 2694, 3786] #2Info    :   2nd: [297, 2694, 9] #2*

*Error   : Invalid boundary mesh (overlapping facets) on surface 2 surface
2Error   : No elements in volume 35*

Taking a look more carefully at the 2D surface mesh, I found that 2 out of
the 4 periodic faces have overlapping triangles (see attached image
*not_working.png*). This seems to be related to the orientation of the
ellipse being different on each periodic surface, but this just an
hypothesis.
I tried with the Mesh.Algorithm options 1,5 and 6 without any success.

This bug does not appear when using a pi/4 alpha parameter, which might be
due to the fact that the previously badly oriented edges are now circles.

I managed to find a workaround that works for any alpha parameter (see
*working.geo*) using ThruSections functions (see line 45 of *working.geo*),
if that can help you debugging this issue.

My Gmsh building options are the following (printed by Gmsh):












*Version       : 4.5.3-git-5d81a7c49License       : GNU General Public
LicenseBuild OS      : Linux64Build date    : 20200213Build host    :
PC-Alain-BERODBuild options : 64Bit ALGLIB Ann Bamg Blas[generic] Blossom
DIntegration Dlopen DomHex Fltk Gmm Hxt Hxt3D Jpeg Kbipack Lapack[generic]
LinuxJoystick MathEx Med Mesh Metis Mmg3d Mpeg Netgen ONELAB
ONELABMetamodel OpenCASCADE OpenCASCADE-CAF OpenGL OptHom Parser Plugins
Png Post QuadTri Solver TetGen/BR Voro++ ZlibFLTK version  : 1.3.4OCC
version   : 7.3.0MED version   : 4.0.0Packaged by   : alainWeb site      :
http://gmsh.info <http://gmsh.info>Issue tracker :
https://gitlab.onelab.info/gmsh/gmsh/issues
<https://gitlab.onelab.info/gmsh/gmsh/issues>*

Do not hesitate to contact me directly if something is not clear.

Kind regards,

Alain BEROD
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