[Gmsh] Unexpected behaviour with Geometry.AutoCoherence=0 and duplicatedsurfaces.

Christophe Geuzaine cgeuzaine at ulg.ac.be
Wed May 5 11:49:38 CEST 2010


On 30/04/10 12:46, Mark Starnes wrote:
> Hi everyone,
>
> For various reasons, the meshes I produce use Geometry.AutoCoherence=0
> to prevent the collapsing of coincident objects.  I perform collapsing
> at a later point when needed.
>
> When copying a surface, however, extra nodes seem to appear on the
> duplicated surface and when meshed, Gmsh shows a Seg. Fault.  I tried
> this on windows and linux, and am using version 2.5.0-svn.
>
> The following script shows the issue.  The behaviour can be turned on or
> off by commenting the Geometry.AutoCoherence=0 command:
>
> -----------------
> Geometry.AutoCoherence = 0;
>
> Point(1) = {0,0,0}; // centre
> Point(2) = {1,0,0};
> Point(3) = {0,1,0};
> Point(4) = {-1,0,0};
> Point(5) = {0,-1,0};
>
> Circle(1) = {2, 1, 3};
> Circle(2) = {3, 1, 4};
> Circle(3) = {4, 1, 5};
> Circle(4) = {5, 1, 2};
>
> Line Loop(5) = {2, 3, 4, 1};
> Plane Surface(6) = {5};
>
> test[] = Translate{0,0,-0.1} {Duplicata{Surface{6};}};
> -----------------

Hi Mark - Indeed: the old internal CAD engine translates surfaces by 
translating its boundaries separately first; so without the 
"RemoveDuplicate" phase, the resulting geometry is not correct (the 
meshing algorithm should not crash anyway, but... well ;-)

In any case, just translating the geometry will *not* give you identical 
meshes. Currently you would have to use a hack based on extrusion for 
this: see

http://www.geuz.org/pipermail/gmsh/2009/004416.html

In the very near future we will also add true "mesh copying" facilities 
to generate periodic grids.

Best,

Christophe


>
> I noticed this, as I have coincident faces which need identical
> meshes for coupling to occur on a node to node basis between different
> domains.
>
> If anybody has a suggestion for how to match meshes on coincident but
> numerically separate faces, I'd love to hear it!  Currently, all I can
> think of is the Duplicata approach above.
>
> As a side, I have just been awarded my Doctorate of Aeronautics from
> Imperial College, London:  I used Gmsh throughout the project (and
> referenced it as requested!), and have posted a few times, receiving
> much help from the community.  So, thanks for this great software and
> thanks for the support over the years!
>
> Best regards,
>
> Mark.
>
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-- 
Prof. Christophe Geuzaine
University of Liege, Electrical Engineering and Computer Science
http://www.montefiore.ulg.ac.be/~geuzaine