Gmsh Mesh

Geometry

We create the geometry directly from the box primitive provided by the OpenCascade geometry engine in Gmsh.

Meshing using Python Script

We use the Python script generate_gmsh.py to create the geometry and generate the mesh. The script makes use of the gmsh_utils.GenerateMesh class (discussed in gmsh_utils), which provides the command line arguments and boilerplate methods. In our generate_gmsh.py Python script, we create a class App that implements the functionality missing in gmsh_utils.GenerateMesh. We must implement the create_geometry(), mark(), and generate_mesh() methods that are abstract in the GenerateMesh base class. We use the Gmsh MeshSize options to define a uniform discretization size of 3 km.

Listing 39 Run the generate_gmsh.py Python script to generate the mesh.

# Generate a mesh with hexahedral cells and save it to `mesh_hex.msh` (default filename).
$ ./generate_gmsh.py --write

# Save as above but start the Gmsh graphical interface after saving the mesh.
$ ./generate_gmsh.py --write --gui

# Create only the geometry and start the Gmsh graphical interface.
$ ./generate_gmsh.py --geometry --gui

# Show available command line arguments.
$ ./generate_gmsh.py --help

By default the Python script will generate a finite-element mesh with hexahedral cells and save it to the file mesh_hex.msh. You can view the mesh using Gmsh either by using the --gui command line argument when you generate the mesh or running Gmsh from the command line and opening the file.

Listing 40 View the Gmsh mesh file mesh_hex.msh using Gmsh.

gmsh -open mesh_hex.msh

Fig. 36 Finite-element mesh with hexahedral cells generated by Gmsh.