[Gmsh] too slow coherence function

Christophe Geuzaine cgeuzaine at ulg.ac.be
Sun Aug 21 16:50:55 CEST 2016

> On 19 Aug 2016, at 21:34, andrew <armitatz at gmail.com> wrote:
> ​Hi,
> I try to create a 3d mesh from an extruded 2d mesh. The mesh is a simple box but with cuts in it of zero width. The geometry is created ok and the meshing even when I have 2000-3000 cuts is done in less than a few minutes. However when I convert the mesh for use with openfoam I was getting errors. By adding the command 'Coherence mesh;' after the creation of the mesh, the mesh is converted correctly for use with openfoam by using the ultility 'gmshToFoam'.

Indeed, it's a limitation of our extrusion algorithm with embedded curves ("Line In Surface"): it currently creates duplicate vertices. The real fix is thus for us to enhance extrusion of embedded curves, but it's not trivial due to the way extruded meshes are generated.

> The problem is in the use of the 'Coherence Mesh'. It takes almost two hours to execute while the meshing takes a few minutes. Is there a way to improve the Coherence command or to make the same geometry without the duplicate nodes that need so much time to be removed?
> I attached a file that needs 2.5 minutes on a quad core for meshing and more than ten minutes to remove 1600 duplicates.
> I have to create thousands of these meshes so a speedup would be welcomed.

That seems a bit slow: on my laptop meshing and duplicate removal take about the same time (2 minutes each). Which version of Gmsh do you use?

> kind regards
> Andrew tsiantis
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Prof. Christophe Geuzaine
University of Liege, Electrical Engineering and Computer Science 

Tetrahedron V, July 4-5 2016: http://tetrahedron.montefiore.ulg.ac.be
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