[Gmsh] generate unstructured mesh between indentic structured surface mesh of a non extrudable geometry

Christophe Geuzaine cgeuzaine at ulg.ac.be
Wed Feb 24 13:21:33 CET 2010

On 18/02/10 01:05, gdemesy at physics.utoronto.ca wrote:
> Dear Gmsh Users and Developers,
> I would need to impose pseudo-periodic boundary conditions on two sets
> of parallel surfaces in the attached geometry (respectively parallel to
> zOx and zOy).
> Originally, I was planning to use identical unstructured mesh on these
> surfaces, but as far as I understood, my only options consist in using
> (i) an extrusion of surfaces or (ii) the Transfinite Algo which both
> lead to structured surface mesh.
> I have considered these two last options:
> (i) I did not manage to extrude the mesh in both directions. Thus, after
> extruding one surface in one direction (say Ox) and deleting all the
> middle entries (as mentioned in
> http://www.geuz.org/pipermail/gmsh/2009/004416.html), I still have to
> extrude it in the Oy direction, which leads me to define some points two
> times (?).
> (ii) I tried to use Transfinite Surfaces, like in the attached geo file.
> I used MeshAdapt for the 2D part which generates proper 2D meshes.
> Everything seems OK until the 3D meshing part:
> - Delaunay overwrites my identical surface mesh as mentioned in some
> past mail.
> - Frontal leads to unexpected multiple "Error : Edge a - b multiple
> times in surface mesh"
> Is there any workaround?

Hello Guillaume,

Unfortunately we cannot correct the error in the Frontal algorithm: it 
fails due to some internal floating point precision issue. A workaround 
is to define nm=1 in your geometry, and apply a global scaling factor 
(using Mesh.ScalingFactor=1.e-9) when saving the mesh...

I also think until we have actual "mesh copying" operations (work in 
progress... almost done but not quite!), Extrusion would be a better way 
forward. Why didn't extrusion work in your case?



> Wouldn't you use Extrude instead?
> Eventually, I would like to end up with a fine mesh in the so-called
> groove physical volume (n°6000), and ideally a mesh that goes
> progressively coarser when reaching the top (resp. bottom) of the
> physical region 5000=PML_top (resp. 1000=PML_bot). However, it seems
> that Transfinite doesn't enjoy this kind of setting, e.g.
> "paramaille_hol>paramaille". Would you have any solution in mind,
> compatible with my early issue ?
> Thank you very much for your time.
> Best,
> Guillaume Demésy,
> Postdoc at University of Toronto.
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Prof. Christophe Geuzaine
University of Liege, Electrical Engineering and Computer Science