Labelling of Internal Facets in Mesh Generated with Netgen

[adofarsi]

Describe the current issue
Currently, when generating meshes using Netgen, there is no functionality available to correctly label the internal facets. This lack of labelling can lead to difficulties in identifying and differentiating between various internal facets of the mesh for DG discretisations.

Describe the solution you'd like
Addition of a feature that enables the correct labelling of internal facets in the mesh generated by Netgen.

Additional info
Code for testing below:

import netgen.gui
from netgen.occ import * 
from firedrake import *

outer = Rectangle(1, 1).Face()
outer.edges.name="outer"
outer.edges.Max(X).name = "r"
outer.edges.Min(X).name = "l"
outer.edges.Min(Y).name = "b"
outer.edges.Max(Y).name = "t"

Inner = MoveTo(0.1, 0.1).Rectangle(0.3, 0.5).Face()
Inner.edges.name="interface"
outer = outer - Inner

Inner.faces.name="inner"
Inner.faces.col = (1, 0, 0)
outer.faces.name="outer"

geo = Glue([Inner, outer])
ngmesh = OCCGeometry(geo, dim=2).GenerateMesh(maxh=0.2)
mesh = Mesh(ngmesh)

V = FunctionSpace(mesh, "DG", 2)
u = Function(V)
v = TestFunction(V)
x = SpatialCoordinate(mesh)
f = Function(V).interpolate(sin(x[0]*pi)*sin(2*x[1]*pi))
pen = Constant(1e10)
F = + inner(grad(u), grad(v)) * dx \
    - f * v * dx \
    + pen * (u-2) * v * ds(8) \
    + pen * jump(u) * jump(v) * dS(1) \
    + pen * jump(u) * jump(v) * dS(2) \
    + pen * jump(u) * jump(v) * dS(3) \
    + pen * jump(u) * jump(v) * dS(4) \
    + pen * jump(u) * jump(v) * dS(5) \
    + pen * jump(u) * jump(v) * dS(6) 
bcs = [DirichletBC(V, Constant(2.0), (1,))]
solve(F == 0, u, bcs=[], solver_parameters={"ksp_type": "preonly",
                                            "pc_type": "lu"})
File("testing.pvd").write(u)

[connorjward]

@UZerbinati how easy do you think this will be to do?

[UZerbinati]

@connorjward Is a work in progress in this branch: https://github.com/NGSolve/ngsPETSc/tree/uz/dglabels.
It works for 2D but now needs to be extended to 3D.
The main issue is that it changes the labelling routine we have used so far.
I think it could be useful to create a labelling utility function directly in ngsPETSc that retrieves from a boundary string the corresponding ID. Maybe if a mesh is a netgen mesh we can use this utility function directly in Firdreke DirichletBC?

[connorjward]

@connorjward Is a work in progress in this branch: https://github.com/NGSolve/ngsPETSc/tree/uz/dglabels. It works for 2D but now needs to be extended to 3D. The main issue is that it changes the labelling routine we have used so far. I think it could be useful to create a labelling utility function directly in ngsPETSc that retrieves from a boundary string the corresponding ID. Maybe if a mesh is a netgen mesh we can use this utility function directly in Firdreke DirichletBC?

Maybe. Does DMLabel not support this kind of thing? I think we want to keep netgen code localised to mesh.py where possible.