Best practices of define the boundary.

[xfdywy]

Hello, everyone. If the boundary is complex, could anyone tell me what is the best practice of defining the boundary? For example, considering the case I need to distinguish the left, right, top, and bottom boundary. Can I use a python function to define the different boundary just like in fenics:

# Boundaries
def right(x, on_boundary): return x[0] > (1.0 - DOLFIN_EPS)
def left(x, on_boundary): return x[0] < DOLFIN_EPS
def top_bottom(x, on_boundary):
    return x[1] > 1.0 - DOLFIN_EPS or x[1] < DOLFIN_EPS

# No-slip boundary condition for velocity
noslip = Constant((0.0, 0.0, 0.0))
bc0 = DirichletBC(W.sub(0), noslip, top_bottom)

# Inflow boundary condition for velocity
inflow = Expression(("-sin(x[1]*pi)", "0.0", "0.0"), degree=2)
bc1 = DirichletBC(W.sub(0), inflow, right)

Or in this way to modified the facet markers directly like in fenics:

  bndry = MeshFunction("size_t", mesh, mesh.topology().dim()-1)
   for f in facets(mesh):
       mp = f.midpoint()
       if near(mp[0], 0.0):  # inflow
           bndry[f] = 1
       elif near(mp[0], L):  # outflow
           bndry[f] = 2
       elif near(mp[1], 0.0) or near(mp[1], W):  # walls
           bndry[f] = 3
       elif mp.distance(center) <= radius:  # cylinder
           bndry[f] = 5

Or in this way like in fenicsx:

def walls(x):
    return np.logical_or(np.isclose(x[1],0), np.isclose(x[1],1))
wall_dofs = locate_dofs_geometrical(V, walls)
u_noslip = np.array((0,) * mesh.geometry.dim, dtype=PETSc.ScalarType)
bc_noslip = dirichletbc(u_noslip, wall_dofs, V)

Thanks~

[ksagiyam]

RelabeledMesh (https://www.firedrakeproject.org/firedrake.html#firedrake.mesh.RelabeledMesh) lets you make a new mesh with new labels; see https://github.com/firedrakeproject/firedrake/blob/master/tests/regression/test_mark_entities.py for examples.

[enricofacca]

Hello,
I am reading this discussion because I am trying to label some faces using an indicator function but only on the boundary in order to define some Dirichlet boundary condition.
It is there way to do it using RelabeledMesh or other existing Firedrake tools without manipulating directly the dm_topolgy object?
Thank you.
Enrico

[connorjward]

This discussion is very old. Can you please open a new one to ask this?

[enricofacca]

yes, sure.