UFL Measures on Sub-domains not Evaluated

[dokken]

To calculate the «length» of a domain, you would need an interface marker of the facets of the subdomain, or you can create a submesh for each subdomain and then evaluate ds as done for the full mesh.

Note that your example is not reproducible as you haven’t provided a mesh.

If you want to make an interface marker after meshing see

@dokken

cells = np.full(num_cells_local, 4, dtype=np.int32)
cells[bottom_cells] = 3
ct = dolfinx.mesh.meshtags(
    mesh, tdim, np.arange(num_cells_local, dtype=np.int32), cells
)
all_b_facets = dolfinx.mesh.compute_incident_entities(
    mesh.topology, ct.find(3), tdim, fdim
)
all_t_facets = dolfinx.mesh.compute_incident_entities(
    mesh.topology, ct.find(4), tdim, fdim
)
interface = np.intersect1d(all_b_facets, all_t_facets)
values[interface] = 5

mt = dolfinx.mesh.meshtags(mesh, mesh.topology.dim - 1, facets, values)

in Discontinuity at interface using mixed domains - #2 by dokken