Using facet tags to define boundary conditions - GMSH - FEniCSx - dolfinx

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With the first code snippet, you can inspect facet_tags.values and facet_tags.indices,
or write these to file

 with dolfinx.io.XDMFFile(mesh.comm, "ft.xdmf", "w") as xdmf:
     xdmf.write_mesh(mesh)
     xdmf.write_meshtags(facet_tags)

you can inspect the boundaries

ft
ft1280×720 9.33 KB

However, in the second part of your script (the DOLFINx code), you are using

where you should use locate_dofs_topological, as shown in
https://jorgensd.github.io/dolfinx-tutorial/chapter1/fundamentals_code.html?highlight=locate_dofs_topological#defining-the-boundary-conditions
You can for instance do:

locate_dofs_topological(Q, mesh.topology.dim-1, facet_tags.find(100))

if the inflow is at the dofs marked with 100