This tutorial is part of viskex.

Plot boundaries mesh tags of a 1D dolfinx mesh

import dolfinx.mesh
import mpi4py.MPI

import viskex

import common_dolfinx as common  # isort: skip

Generate a mesh of the unit interval, divided into six cells.

interval = dolfinx.mesh.create_unit_interval(mpi4py.MPI.COMM_WORLD, 6)

Mark boundaries according to the $x$ 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.

interval_subdomains = common.mark_subdomains(interval)
interval_boundaries = common.mark_boundaries(interval, interval_subdomains)

We plot the resulting boundaries.

viskex.dolfinx.plot_mesh_tags(interval, interval_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 plot), $2$ (second plot) or both (third 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(
    interval, interval_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(
    interval, interval_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(
    interval, interval_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