Problem import 3d mesh from gmsh

General Discussion
1

Hi,
I’m trying to import a 3d mesh from gmsh to Freefem++ in order to simulate an object. The problem is that Freefem++ give me this :

$ FreeFem++ real_3d.eps
– FreeFem++ v4.5 (Tue Feb 11 17:02:48 CET 2020 - git v4.5-18-gcf799e13)
Load: lg_fem lg_mesh lg_mesh3 eigenvalue
1 : load “gmsh”
2 :
3 : mesh3 Th2 = gmshload3(“real_3d.msh”);
4 :
5 : plot(Th2,wait=1);
6 : sizestack + 1024 =1248 ( 224 )

Element of type 3 is not considered in Freefem++

Here are my files : real_3d.geo :
nb=10;

Point(1) = {0, 0, 0};
Point(2) = {0, 0.5, 0};
Point(3) = {1, 0.5, 0};
Point(4) = {4, 0.5, 0};
Point(5) = {5, 0.5, 0};
Point(6) = {5, 0, 0};

Point(7) = {1, 0.5, 0};
Point(8) = {4, 0.5, 0};
Point(9) = {5, 1.5, 0};
Point(10) = {3.5, 1.5, 0};
Point(11) = {1.5, 1.5, 0};
Point(12) = {0,1.5,0};

Point(13) = {0, 1.5, 0};
Point(14) = {1.5,1.5,0};
Point(15) = {3.5, 1.5, 0};
Point(16) = {5, 1.5, 0};
Point(17) = {5, 2, 0};
Point(18) = {0, 2, 0};

Line(30) = {1,2};
Line(31) = {2,3};
Line(32) = {3,4};
Line(33) = {4,5};
Line(34) = {5,6};
Line(35) = {6,1};

Line(36) = {2,3};
Line(37)= {3,7};
Line(38) = {7,8};
Line(39) = {8,4};
Line(40) = {4,5};
Line(41) = {5,9};
Line(42) = {9,10};
Line(43) = {10,11};
Line(44) = {11,12};
Line(45) = {12,2};

Line(46) = {13,14};
Line(47) = {14,11};
Line(48) = {11,10};
Line(49) = {10,15};
Line(50) = {15,16};
Line(51) = {16,17};
Line(52) = {17,18};
Line(53) = {18,13};

Line Loop(60) = {30,31,32,33,34,35};
Line Loop(61) = {36,37,38,39,40,41,42,43,44,45};
Line Loop(62) = {46,47,48,49,50,51,52,53};

Plane Surface(63) = {60};
Plane Surface(64) = {61};
Plane Surface(65) = {62};

surfaceVector[] = Extrude {0, 0, 1} {
Surface{63};
Layers{2};
Recombine;
};
surfaceVector[] = Extrude {0, 0, 1} {
Surface{64};
Layers{2};
Recombine;
};
surfaceVector[] = Extrude {0, 0, 1} {
Surface{65};
Layers{2};
Recombine;
};

AND real_3d.eps :

load “gmsh”

mesh3 Th2 = gmshload3(“real_3d.msh”);

plot(Th2,wait=1);

2

FreeFEM doesn’t support quadrilateral or hexahedral elements, only simplicial elements (triangles and tetrahedra). Gmsh can generate purely triangular and tetrahedral meshes and indeed will do so by default. Here, just omit the ‘Recombine’ commands which recombine tetrahedra into hexahedra.

Also, when generating a mesh with Gmsh, I’d highly recommend defining a Physical Volume for the domain (or multiple for multiple subdomains, called regions in FreeFEM) and a Physical Surface for the boundary (or multiple for mixed boundary value problems: inlet, outlet, wall, &c.). See http://gmsh.info/dev/doc/texinfo/gmsh.html#Elementary-entities-vs-physical-groups. When defining these physical groups, Gmsh saves only those to the MSH file rather than everything which is often superfluous; also, the physical group tags are accessible from within FreeFEM if required for multiple subdomains or boundary-patches.

1 Like
3

OK
Thanks for the help