[Gmsh] Issue with periodicities

[Christophe Geuzaine]

Dear Alain,

> On 14 Feb 2020, at 09:31, Alain Berod <alain.berod at gmail.com> wrote:
> 
> 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] #2
> Info    :   2nd: [297, 2694, 9] #2
> Error   : Invalid boundary mesh (overlapping facets) on surface 2 surface 2
> Error   : 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.

Indeed: we have trouble when OpenCASCADE changes the orientation. A partial solution has been committed in 4.5.3, but there's still room for improvement.

Christophe




> 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-5d81a7c49
> License       : GNU General Public License
> Build OS      : Linux64
> Build date    : 20200213
> Build host    : PC-Alain-BEROD
> Build 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++ Zlib
> FLTK version  : 1.3.4
> OCC version   : 7.3.0
> MED version   : 4.0.0
> Packaged by   : alain
> Web site      : http://gmsh.info
> Issue tracker : https://gitlab.onelab.info/gmsh/gmsh/issues
> 
> Do not hesitate to contact me directly if something is not clear.
> 
> Kind regards,
> 
> Alain BEROD
> <not_working.geo><working.geo><not_working.png>_______________________________________________
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— 
Prof. Christophe Geuzaine
University of Liege, Electrical Engineering and Computer Science 
http://www.montefiore.ulg.ac.be/~geuzaine