DirichletBC set to boundary normal

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You can project the FacetNormal to be Function, as shown in:

How to plot normal unit vector of faces in a 2D mesh?

The facet normal of an interior interface to either side as discussed here: Integrating over an interior surface You can the project this restriction to a CG1-space and visualize it with Paraview. Code for projecting a normal to a exterior interface has been done at: How to compute curvature of a boundary and looks like. This can easily be modified for an interior interface. mesh = Mesh(UnitDiscMesh.create(MPI.comm_world, 100, 1, 2)) n = FacetNormal(mesh) V = VectorFunctionSpace(mesh, "CG", …

and use it directly in your Dirichlet boundary condition