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[Marie Rognes <question109900@xxxxxxxxxxxxxxxxxxxxx>]

Question #109900 on DOLFIN changed:
https://answers.launchpad.net/dolfin/+question/109900

    Status: Open => Answered

Marie Rognes proposed the following answer:
Achim Schroll wrote:
> New question #109900 on DOLFIN:
> https://answers.launchpad.net/dolfin/+question/109900
>
> for a plain Poisson eqn with pure Neumann b.c., how to specify the scaling condition u*dx = 0 ?
>   


If you have a recent dolfin, you can introduce a constant c acting as a 
Lagrange multiplier corresponding to the constraint. See example below.

   
from dolfin import *

mesh = UnitSquare(32, 32)
V = FunctionSpace(mesh, "CG", 1)
Q = FunctionSpace(mesh, "R", 0)
M = V * Q

(u, c) = TrialFunctions(M)
(v, d) = TestFunctions(M)

f = Expression("x[0]*x[1]*sin(pi*x[0])")

a = dot(grad(v), grad(u))*dx + d*u*dx + c*v*dx
L = v*f*dx

pde = VariationalProblem(a, L)
u_h = pde.solve()

plot(u_h[0])
interactive()


--
Marie

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