This tutorial is part of viskex.

Plot boundaries mesh tags of a 2D dolfinx mesh

import dolfinx.mesh
import mpi4py.MPI

import viskex

import common_dolfinx as common  # isort: skip

Generate meshes of the unit square by dividing each edge of the square in 6 segments, using either a triangular or quadrangular mesh.

square_tria = dolfinx.mesh.create_unit_square(mpi4py.MPI.COMM_WORLD, 6, 6, dolfinx.mesh.CellType.triangle)
square_quad = dolfinx.mesh.create_unit_square(mpi4py.MPI.COMM_WORLD, 6, 6, dolfinx.mesh.CellType.quadrilateral)

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

• if $x > 0$ and $x \leq \frac{1}{3}$, assign boundary mesh tag equal to $1$;
• if $x \geq \frac{2}{3}$ and $x < 1$, assign boundary mesh tag equal to $2$;
• if $x = 0$, assign boundary mesh tag equal to $3$;
• if $x = 1$, assign boundary mesh tag equal to $4$;
• otherwise, leave the boundary mesh tag unassigned.

square_tria_subdomains = common.mark_subdomains(square_tria)
square_quad_subdomains = common.mark_subdomains(square_quad)
square_tria_boundaries = common.mark_boundaries(square_tria, square_tria_subdomains)
square_quad_boundaries = common.mark_boundaries(square_quad, square_quad_subdomains)

We plot the resulting boundaries.

viskex.dolfinx.plot_mesh_tags(square_tria, square_tria_boundaries, "boundaries")

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(square_quad, square_quad_boundaries, "boundaries")

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 boundaries with tag equal to $4$ (first and second plot), $2$ (third and fourth plot) or both (fifth and sixth plot), which will be displayed with colors. The rest of the mesh facets are still included in the plot, but are colored in gray.

viskex.dolfinx.plot_mesh_tags(
    square_tria, square_tria_boundaries, "boundaries_4", viskex.utils.values_in([4]))

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(
    square_quad, square_quad_boundaries, "boundaries_4", viskex.utils.values_in([4]))

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(
    square_tria, square_tria_boundaries, "boundaries_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(
    square_quad, square_quad_boundaries, "boundaries_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(
    square_tria, square_tria_boundaries, "boundaries_24", viskex.utils.values_in([2, 4]))

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(
    square_quad, square_quad_boundaries, "boundaries_24", viskex.utils.values_in([2, 4]))

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 a scalar field
• repeat this step with firedrake