[Gmsh] Zero volume tets

[Christophe Geuzaine]

> On 12 May 2020, at 11:49, Karin&NiKo <niko.karin at gmail.com> wrote:
> 
> Dear Gmsh Gurus,
> 
> I am facing some troubles with a brep CAD I am trying to mesh with Gmsh. 
> If I try the meshing default settings, the mesh is being built and Gmsh is proud to announce that no illegal tet stand in the mesh. *But* if I simply run "gmsh -check", Gmsh admits that zero volume tets stay in the mesh. So it is unusable for computation.

There maybe too many messages, but the optimizer indeed reports that there are 6 tets below the quality threshold in volume 2 with the default algo (we should issue a warning if the quality is indeed approaching zero):

Info    : Optimizing volume 2
Info    : Optimization starts (volume = 3.52969e+07) with worst = 0 / average = 0.757222:
Info    : 0.00 < quality < 0.10 :       212 elements
Info    : 0.10 < quality < 0.20 :       362 elements
Info    : 0.20 < quality < 0.30 :       166 elements
Info    : 0.30 < quality < 0.40 :       222 elements
Info    : 0.40 < quality < 0.50 :       287 elements
Info    : 0.50 < quality < 0.60 :       765 elements
Info    : 0.60 < quality < 0.70 :      3802 elements
Info    : 0.70 < quality < 0.80 :      7720 elements
Info    : 0.80 < quality < 0.90 :      4721 elements
Info    : 0.90 < quality < 1.00 :      4214 elements
Info    : 321 edge swaps, 0 node relocations (volume = 3.52969e+07): worst = 0 / average = 0.765757 (Wall 0.022079s, CPU 0.022s)
Info    : 338 edge swaps, 0 node relocations (volume = 3.52969e+07): worst = 0 / average = 0.765884 (Wall 0.0357411s, CPU 0.035583s)
Info    : 339 edge swaps, 0 node relocations (volume = 3.52969e+07): worst = 0 / average = 0.765909 (Wall 0.0503662s, CPU 0.050068s)
Info    : 6 ill-shaped tets are still in the mesh


> I have succeeded in producing a correct mesh by using the Frontal 3D algorithm (Mesh.Algorithm3D = 4;). 
> Perhaps a check of the presence of zero volume elements could be an interesting enhancement. 

You can also try

Mesh.Algorithm3D = 10;

which is (here a bit) faster.

Christophe


> You'll find all necessary files in the attached archive.
> 
> Regards,
> Nicolas
> <foo.tar.gz>_______________________________________________
> gmsh mailing list
> gmsh at onelab.info
> http://onelab.info/mailman/listinfo/gmsh

— 
Prof. Christophe Geuzaine
University of Liege, Electrical Engineering and Computer Science 
http://www.montefiore.ulg.ac.be/~geuzaine