facet.exterior() always False?

Question

I am trying to specify a DirichletBC using boolean expression for the boundary facet midpoints, but DirichletBC only allows for discarding facets using the check_midpoint.

I tried to create a FacetFunction and marks facets using their midpoint, but I cannot see an equivalent to the on_boundary functionality.

If I run

from dolfin import *
mesh = RectangleMesh(0,0,1,1,5,5)
for facet in facets(mesh):
 #print [facet.midpoint().x(),facet.midpoint().y()]
 print facet.exterior()

I get False for all of the facets.

Answer 1

Hi, you need to initialize the connectivity between facets and cells first.

from dolfin import *
mesh = RectangleMesh(0,0,1,1,5,5)
tdim = mesh.topology().dim()
mesh.init(tdim-1, tdim)
for facet in facets(mesh):
    print facet.exterior()