Define boundary conditions for gmsh meshes

[tr3920]

Dear all,

I am a complete beginner with MFEM. I would like to import a Gmsh mesh into MFEM, as I need to create the mesh from a .step file.

Could you please explain a basic method for creating boundary elements and assigning values to them based on the physical entities defined in the Gmsh file?

Thank you very much in advance for your help.

[tzanio]

Hi @tr3920, is this helpful: https://mfem.org/tutorial/meshvis/#importing-meshes-from-gmsh-and-cubit?

[tr3920]

Thank you that's very helpful!

[tr3920]

Hello all,

Sorry for the delay — I'm still running into some issues while adapting the tutorial to a custom geometry.

I successfully generated a working mesh using a .geo file (see screenshot below), and everything runs fine with MFEM in that case:

However, when I try generating the same geometry using the Gmsh C++ API, I get a segmentation fault. The issue seems to be that pmesh.bdr_attributes.Max() returns 0, so when I try to set boundary conditions, the program segfaults.

Here's the C++ code I use to generate the mesh:

#include <set>
#include <iostream>
#include <gmsh.h>

int main(int argc, char **argv)
{
gmsh::initialize(argc, argv);
gmsh::option::setNumber("Mesh.MshFileVersion", 2.2);
gmsh::model::add("simple_mesh");

double L = 1.0;
double Rc = 0.05 * L;

int box1tag = gmsh::model::occ::addBox(0, 0, 0, L, L, L);
int box2tag = gmsh::model::occ::addBox(0.25 * L, 0.25 * L, 0.25 * L, 0.5 * L, 0.5 * L, Rc);
int box3tag = gmsh::model::occ::addBox(0.25 * L, 0.25 * L, 0.75 * L, 0.5 * L, 0.5 * L, Rc);

// Subtract electrodes from the main box
std::vector<std::pair<int, int>> electrodes = {{3, box2tag}, {3, box3tag}};
std::vector<std::pair<int, int>> ov;
std::vector<std::vector<std::pair<int, int>>> ovv;
gmsh::model::occ::cut({{3, box1tag}}, electrodes, ov, ovv, 10);
gmsh::model::occ::synchronize();

// Add physical groups
gmsh::model::addPhysicalGroup(3, {box1tag}, 50); //outer box

gmsh::model::addPhysicalGroup(2, {7, 8, 9, 10, 11, 12}, 1); // electrode 1
gmsh::model::addPhysicalGroup(2, {13, 14, 15, 16, 17, 18}, 2); // electrode 2

gmsh::model::mesh::generate(3);
gmsh::write("simple_mesh.msh");

if (!std::setstd::string(argv, argv + argc).count("-nopopup"))
gmsh::fltk::run();

gmsh::finalize();
return 0;
}