[Gmsh] Unsolved problem! mesh cannot adapt from fine to coarse in a simple 2D geo

ryan kim realnumeric at gmail.com
Thu Apr 23 23:43:17 CEST 2015


Hi German and gmsh team,

Thanks for the discussion.
This manual point adding can play the trick for this problem but it is "*not
a systematic"* way to apply. For a general irregular geometry, to
automatically add this point or even a number of points in the empty area
is a problem! I expect gmsh has the feature to make this fine to coarse
transition. However, I dig deep into gmsh literature and did not find *this
feature to have fine mesh ONLY around the small geometry*.

I hope gmsh team can confirm this with me, or advise someway to achieve
this other than the trick to insert point in empty area manually.

Thanks,
Kim

2015-04-23 4:12 GMT-07:00 Theler German Guillermo <gtheler at cites-gss.com>:

>  These three lines at the end of your geo file will do the trick:
>
> Plane Surface(22) = {1, 2, 3, 4, 5};
> Point(23) = {0, 0, 0, lc};
> Point {23} In Surface {22};
>
>
>   --
> *Germán Theler :: **CTO Eng & IT*
>
> *CITES* – Centro de Innovación Tecnológica Empresarial y Social S.A.
> Dirección General Sancor Seguros
> Grupo Sancor Seguros
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> gtheler at cites-gss.com <ccipolatti at cites-gss.com>
> *www.cites-gss.com <http://www.cites-gss.com>* -
> www.gruposancorseguros.com
>
>
>
>   On Wed, 2015-04-22 at 01:38 -0700, ryan kim wrote:
>
> I have used gmsh in my application for a while. However, one kind of
> simple and standard geometry still makes bad mesh result. The geometry is
> just a rectangular plane with four small holes at each corner like this:
> -------------------------------------
> |                                    |
> |                                    |
> |     __                    __    |
> |     |  |                    |   |   |
> |     ---                     ----   |
> |                                    |
> |                                    |
> |     __                     __   |
> |     |   |                    |   |  |
> |     ----                     ----  |
> |                                    |
> |                                    |
> |____________________ |
>
>
>  Surprisingly, gmsh generated fine mesh not only around the four holes,
> but also in the area between the holes. Therefore, the entire mesh size is
> huge. The ideal mesh should only have fine mesh around the holes at corner
> since coarse mesh is enough for the center area.
>
>
>  I read the history of the mailing list and find some discussion about
> this problem. However, no good solution has been provided. Someone
> suggested using
> Mesh.CharacteristicLengthExtendFromBoundary= 0;
>
>  However, it indeed generated coarse mesh at the center but the triangle
> quality is bad with very sharp angle.
>
>  Using fields: "Attractor", and "Threshold" as in tutorial t10 does not
> work as well.
>
>
>
>  Here is the script of the simple test case as described above. I also
> attached the bad mesh result as I mentioned. Hope someone can discuss this
> classic problem with me.
>
> lc = 10;
> lc1 = 1;
> Point(1) = {-100, -100, 0, lc};
> Point(2) = {100, -100,  0, lc} ;
> Point(3) = {100, 100, 0, lc} ;
> Point(4) = {-100,  100, 0, lc} ;
> Line(1) = {1,2} ;
> Line(2) = {3,2} ;
> Line(3) = {3,4} ;
> Line(4) = {4,1} ;
> Line Loop(1) = {4,1,-2,3};
>
> Point(5) = {49, 49, 0, lc1};
> Point(6) = {49, 51,  0, lc1} ;
> Point(7) = {51, 51, 0, lc1} ;
> Point(8) = {51, 49, 0, lc1} ;
> Line(6) = {5,6} ;
> Line(7) = {7,6} ;
> Line(8) = {7,8} ;
> Line(9) = {8,5} ;
> Line Loop(2) = {9,6,-7,8};
>
> Point(11) = {49, -49, 0, lc1};
> Point(12) = {49, -51, 0, lc1};
> Point(13) = {51, -51, 0, lc1};
> Point(14) = {51, -49, 0, lc1};
> Line(10) = {11,12} ;
> Line(11) = {13,12} ;
> Line(12) = {13,14} ;
> Line(13) = {14,11} ;
> Line Loop(3) = {13,10,-11,12};
>
> Point(15) = {-49, 49, 0, lc1};
> Point(16) = {-49, 51,  0, lc1} ;
> Point(17) = {-51, 51, 0, lc1} ;
> Point(18) = {-51, 49, 0, lc1} ;
> Line(14) = {15,16} ;
> Line(15) = {17,16} ;
> Line(16) = {17,18} ;
> Line(17) = {18,15} ;
> Line Loop(4) = {17,14,-15,16};
>
> Point(19) = {-49, -49, 0, lc1};
> Point(20) = {-49, -51,  0, lc1} ;
> Point(21) = {-51, -51, 0, lc1} ;
> Point(22) = {-51, -49, 0, lc1} ;
> Line(18) = {19,20} ;
> Line(19) = {21,20} ;
> Line(20) = {21,22} ;
> Line(21) = {22,19} ;
> Line Loop(5) = {21,18,-19,20};
>
> Plane Surface(1) = {1, 2, 3, 4, 5} ;
> //Mesh.CharacteristicLengthExtendFromBoundary= 0;
>
>
>
>  Thanks,
>
>  Kim
>
>
>
>
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