[Gmsh] Gaps and overlaps when using extrude command

Bart Deschoolmeester bart.deschoolmeester at bbri.be
Fri Aug 14 12:43:32 CEST 2020

```Hi,
I am a beginner.
While working on a larger 3D project I noticed some gaps and overlaps.
I plan to simulate the heat transfer.
I assume gaps and overlaps are to be avoided to get a good simulation. Correct?

I tracked down the origin of the gaps and overlaps and it seems to be the extrude command causing the trouble.
The file below draws a rectangle with a round hole in the middle. In the hole a ring (annulus) is drawn.
If I create the ring with the BooleanDifference command (difference of two circles): all works fine. All triangles line up perfectly around the contact line of ring and hole.
If I create the ring with the Extrude command, things don't line up.

Of course I could opt for the booleandifference command for this problem but:

*   I cannot use this booleandifference solution in my 3D problem
*   I can imagine these holes and overlaps also being produced in other curved contact surfaces.

Is this a bug in GMSH or the be expected in more elaborate meshes?
How can it be resolved?

Thank you!
Bart

KanR = DefineNumber[ 0.1, Name "Parameters/KanR" ];
KanMeshsize = DefineNumber[ 0.05, Name "Parameters/KanMeshsize" ];
Rectangle(1) = {-2.5, -2.5, 0, 5, 5, 0};
Circle(5) = {0, 0, 0, KanR, 0, 2*Pi};
Curve Loop(2) = {5};
Plane Surface(2) = {2};
BooleanDifference{ Surface{1}; Delete; }{ Surface{2}; Delete; }
Point(15) = {0, KanR, 0};
Point(16) = {0, KanR-0.05, 0};
Line(20) = {15, 16};
Extrude {{0, 0, 1}, {0, 0, 0}, 2*Pi} {
Curve{20};
}
Field[1] = Distance;
//+
Field[1].EdgesList = {5};
//+
Field[1].NNodesByEdge = 50;
//+
Field[2] = Threshold;
//+
Field[2].DistMax = 2.5;
//+
Field[2].DistMin = 0;
//+
Field[2].IField = 1;
//+
Field[2].LcMax = 0.25;
//+
Field[2].LcMin = KanMeshsize ;

Background Field = 2;

Mesh.CharacteristicLengthExtendFromBoundary = 0;
Mesh.CharacteristicLengthFromPoints = 0;
Mesh.CharacteristicLengthFromCurvature = 0;

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://onelab.info/pipermail/gmsh/attachments/20200814/9dc745d9/attachment.html>
```