[Gmsh] Problem with mesh over extruded surface where other surfaces are defined

[Christophe Geuzaine]

> On 6 Feb 2019, at 23:14, Isaac Haïk Dunn <anjeeisaac at gmail.com> wrote:
> 
> Hello,
> My goal is to create a parallelepiped with two rectangles on each of the top and bottom surfaces. That way I could import the mesh and solve an equation on that volume and apply boundary conditions on each of the 4 rectangular regions over the solid.
> 
> I've almost completed the task by extruding a rectangular shape and by defining the rectangles over the top and bottom surfaces. However when I mesh 3D, the bottom surface and the volume are correctly meshed, but not the top surface. The mesh does not accommodate to the two rectangular shapes, hence my final mesh over that surface has two meshes overlapping on each rectangle, which is a problem.
> I applied the exact same logic than for the bottom surface, so I do not really understand why the result differs, nor how I could fix it. I would greatly appreciate some guidance!
> 

Even for the bottom part, you have defined overlapping surfaces. Only surface 47 is correctly defined with holes...

You can "fragment" everything, i.e. add

   BooleanFragments{ Surface{:}; Delete; }{ Volume{:}; Delete; }

add the end of your file to fix the mess, but 

1) it would clearly be better to define everything correctly in the first place
2) you won't be able to use structured meshing commands in the extrusion (i.e. you have to remove "Layers ..." in Extrude)

Christophe

> Here's the code:
> 
> SetFactory("OpenCASCADE");
> Mesh.RandomFactor=1.0e-6;
> lc=0.35;
> a = 2;
> b = 1;
> c = 0.06;
> 
> // bottom motif
> Point(1) = {-a/2, -b/2, -c/2, lc};
> Point(2) = {-a/2, b/2, -c/2, lc};
> Point(3) = {a/2, b/2, -c/2, lc};
> Point(4) = {a/2, -b/2, -c/2, lc};
> Line(1) = {1,2};
> Line(2) = {2,3};
> Line(3) = {3,4};
> Line(4) = {4,1};
> Line Loop(5) = {1,2,3,4};
> Plane Surface(6) = {5};
> 
> // auxiliary points over top and bottom surface
> // rectangle 1
> Point(7) = {-a/10 + a/4.4,-b/10, c/2};
> Point(8) = {-a/10 + a/4.4,b/10,c/2};
> Point(9) = {a/10 + a/4.4, b/10,c/2};
> Point(10) = {a/10 + a/4.4,-b/10,c/2};
> Line(11) = {7,8};
> Line(12) = {8,9};
> Line(13) = {9,10};
> Line(14) = {10,7};
> Line Loop(15) = {11,12,13,14};
> Plane Surface(16) = {15};
> 
> // rectangle 2
> Point(17) = {-a/16+a/4,-b/16, -c/2};
> Point(18) = {-a/16+a/4,b/16,-c/2};
> Point(19) = {a/16+a/4, b/16,-c/2};
> Point(20) = {a/16+a/4,-b/16,-c/2};
> Line(21) = {17,18};
> Line(22) = {18,19};
> Line(23) = {19,20};
> Line(24) = {20,17};
> Line Loop(25) = {21,22,23,24};
> Plane Surface(26) = {25};
>  
> // rectangle 3
> Point(27) = {-(-a/16+a/4),-b/16, c/2};
> Point(28) = {-(-a/16+a/4),b/16,c/2};
> Point(29) = {-(a/16+a/4), b/16,c/2};
> Point(30) = {-(a/16+a/4),-b/16,c/2};
> Line(31) = {27,28};
> Line(32) = {28,29};
> Line(33) = {29,30};
> Line(34) = {30,27};
> Line Loop(35) = {31,32,33,34};
> Plane Surface(36) = {35};
> 
> // rectangle 4
> Point(37) = {-a/22+a/14,-b/22, -c/2};
> Point(38) = {-a/22+a/14,b/22,-c/2};
> Point(39) = {a/22+a/14, b/22,-c/2};
> Point(40) = {a/22+a/14,-b/22,-c/2};
> Line(41) = {37,38};
> Line(42) = {38,39};
> Line(43) = {39,40};
> Line(44) = {40,37};
> Line Loop(45) = {41,42,43,44};
> Plane Surface(46) = {45};
> 
> 
> // bottom mesh
> Line Loop(46) = {2, 3, 4, 1};
> Line Loop(47) = {42, 43, 44, 41};
> Line Loop(48) = {22, 23, 24, 21};
> Plane Surface(47) = {46, 47, 48};
> Physical Surface(98) = {26};
> Physical Surface(99) = {46};
> Physical Surface(100) = {47};
> 
> surfaceVector[] = Extrude {0, 0, c} {
>      Surface{6};
>     Layers{15};
>      Recombine;
>     };
> 
> // volume and top mesh
> Physical Volume("internal") = surfaceVector[1];
> Physical Surface(201) = {16};
> Physical Surface(202) = {36};
> Line Loop(54) = {49,51,52,47};
> Plane Surface(203) ={54, 15, 35};
> Physical Surface(203)={203};
> 
> 
> In Gmsh, to see that the bottom surface mesh is correct, go to Visibility and select Surfaces 98, 99 and 100. To see that the top surface mesh is wrong, select the surfaces 201, 202 and 203.
> 
> Thanks!
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— 
Prof. Christophe Geuzaine
University of Liege, Electrical Engineering and Computer Science 
http://www.montefiore.ulg.ac.be/~geuzaine