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

demesy guillaume.demesy at fresnel.fr
Wed Feb 24 15:09:36 CET 2010

Hi Christophe,

thanks for your kind reply. 
Scaling the whole system by *1e9 is not bothering, I only had to scale 2
physical constants in getdp...

I also think that two surface extrusions would be more appropriate here.
Unfortunately I couldn't mesh the attached minimal (not-)working example:
- I'm using one of your tricks
(http://www.geuz.org/pipermail/gmsh/2009/004416.html) to generate 2 faces
of a parallelepiped with identical mesh and I delete the generated entries
(volume and surfaces). so far, so good.
- Then I try to extrude in the other direction, but gmsh seem to mesh the
faces // to the extrusion direction before the desired one, which seem to
be the source of the problem (please see attached image).

There might be a workaround here. If you can think of someting, I'm still
very interested.

Thanks again,


On Wed, 24 Feb 2010 13:21:33 +0100, Christophe Geuzaine
<cgeuzaine at ulg.ac.be> wrote:
> 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?
> Best,
> Christophe
>> 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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