d4_p2D.py

# script taken from the FEniCS project tutorial at http://fenicsproject.org

from fenics import *
import numpy

# Create mesh and define function space
mesh = UnitSquareMesh(30, 30)
V = FunctionSpace(mesh, 'Lagrange', 1)

# Define boundary conditions
u0 = Expression('1 + x[0]*x[0] + 2*x[1]*x[1]')
p  = Expression('x[0] + x[1]')

c  = conditional( lt(u0, 1.75), 0, u0)
File('c.pvd') << project(c,V)

def u0_boundary(x, on_boundary):
    return on_boundary

bc = DirichletBC(V, u0, u0_boundary)

# Define variational problem
u = TrialFunction(V)
v = TestFunction(V)
f = Constant(-6.0)
a = p * inner(nabla_grad(u), nabla_grad(v)) * dx
L = f*v*dx

# Compute solution
u = Function(V)
solve(a == L, u, bc)

#plot(u)

print """
Solution of the Poisson problem -Laplace(u) = f,
with u = u0 on the boundary and a
%s
""" % str(mesh)

# Dump solution to the screen
u_nodal_values = u.vector()
u_array = u_nodal_values.array()
coor = mesh.coordinates()
if coor.shape[0] == u_array.shape[0]:  # degree 1 element
    for i in range(len(u_array)):
        print 'u(%8g,%8g) = %g' % (coor[i][0], coor[i][1], u_array[i])
else:
    print """\
Cannot print vertex coordinates and corresponding values
because the element is not a first-order Lagrange element.
"""

# Note: u_nodal_values.array() returns a copy
print id(u_nodal_values.array()), id(u_array)

# Verification
u_e = interpolate(u0, V)
u_e_array = u_e.vector().array()
print 'Max error:', numpy.abs(u_e_array - u_array).max()

# Compare numerical and exact solution
center = (0.5, 0.5)
print 'numerical    u at the center point:', u(center)
print 'exact        u at the center point:', u0(center)
print 'interpolated u at the center point:', u_e(center)