This tutorial is part of viskex.

Plot subdomains mesh tags of a 3D dolfinx mesh

import dolfinx.mesh
import mpi4py.MPI

import viskex

import common_dolfinx as common  # isort: skip

Generate meshes of the unit cube by dividing each edge of the cube in 6 segments, using either a tetrahedral or hexahedral mesh.

cube_tetra = dolfinx.mesh.create_unit_cube(mpi4py.MPI.COMM_WORLD, 6, 6, 6, dolfinx.mesh.CellType.tetrahedron)
cube_hexa = dolfinx.mesh.create_unit_cube(mpi4py.MPI.COMM_WORLD, 6, 6, 6, dolfinx.mesh.CellType.hexahedron)

Mark subdomains according to the $(x, y, z)$ position of the vertices of the mesh:

• if $x \leq \frac{1}{3}$, assign subdomain mesh tag equal to $1$;
• if $x \geq \frac{2}{3}$, assign subdomain mesh tag equal to $2$;
• otherwise, leave the subdomain mesh tag unassigned.

cube_tetra_subdomains = common.mark_subdomains(cube_tetra)
cube_hexa_subdomains = common.mark_subdomains(cube_hexa)

We plot the resulting subdomains.

viskex.dolfinx.plot_mesh_tags(cube_tetra, cube_tetra_subdomains, "subdomains")

error: XDG_RUNTIME_DIR is invalid or not set in the environment.
MESA: error: ZINK: failed to choose pdev
glx: failed to create drisw screen

viskex.dolfinx.plot_mesh_tags(cube_hexa, cube_hexa_subdomains, "subdomains")

error: XDG_RUNTIME_DIR is invalid or not set in the environment.
MESA: error: ZINK: failed to choose pdev
glx: failed to create drisw screen

We can also plot all subdomains with tag equal to $2$, which will be displayed with colors. The rest of the mesh cells are still included in the plot, but are colored in gray.

viskex.dolfinx.plot_mesh_tags(
    cube_tetra, cube_tetra_subdomains, "subdomains_2", viskex.utils.values_in([2]))

error: XDG_RUNTIME_DIR is invalid or not set in the environment.
MESA: error: ZINK: failed to choose pdev
glx: failed to create drisw screen

viskex.dolfinx.plot_mesh_tags(
    cube_hexa, cube_hexa_subdomains, "subdomains_2", viskex.utils.values_in([2]))

error: XDG_RUNTIME_DIR is invalid or not set in the environment.
MESA: error: ZINK: failed to choose pdev
glx: failed to create drisw screen

Alternatively, we can clip the plot so that it only shows the subset of the mesh with subdomain tag equal to $2$.

viskex.dolfinx.plot_mesh_tags(
    cube_tetra, cube_tetra_subdomains, "subdomains_2",
    lambda grid: grid.threshold(2))  # type: ignore[no-untyped-call]

error: XDG_RUNTIME_DIR is invalid or not set in the environment.
MESA: error: ZINK: failed to choose pdev
glx: failed to create drisw screen

viskex.dolfinx.plot_mesh_tags(
    cube_hexa, cube_hexa_subdomains, "subdomains_2",
    lambda grid: grid.threshold(2))  # type: ignore[no-untyped-call]

error: XDG_RUNTIME_DIR is invalid or not set in the environment.
MESA: error: ZINK: failed to choose pdev
glx: failed to create drisw screen

What's next?

• next step: plot boundaries
• repeat this step with firedrake