Inconsistent normal orientation of Fenics mesh

[evantwout]

The normal orientation of the surface meshes extracted from FEniCSx has changed from version 0.8.0 to 0.9.0. The normals can be inspected with Gmsh following this example code.

from mpi4py import MPI
import dolfinx
from dolfinx.fem import functionspace
from dolfinx.mesh import create_unit_cube
import bempp.api
from bempp.api.external import fenicsx

mesh = create_unit_cube(MPI.COMM_WORLD, 10, 10, 10)
fenics_space = functionspace(mesh, ("CG", 1))
trace_space, trace_matrix = fenicsx.fenics_to_bempp_trace_data(fenics_space)

bempp.api.export("cube"+str(dolfinx.__version__)+".msh", grid=trace_space.grid)

For Dolfin 0.8.0, all normals of the surface mesh are pointing outwards. However, for Dolfin 0.9.0, half of the triangles have outward pointing normals, and the other half have inward pointing normals. Hence, the FEM-BEM tutorial provides a wrong result.