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[Thomas Ward <question109629@xxxxxxxxxxxxxxxxxxxxx>]

New question #109629 on DOLFIN:
https://answers.launchpad.net/dolfin/+question/109629

I am struggling with a very simple problem

unit cube
div(grad(u))=0
du/dn=-1.4 on x=0
du/dn=1.4 on x=1
du/dn+1.4*1.4/9.81*u=0 on z=1
du/dn=0 on y=0,y=1,z=0

below is my attempt with pydolfin, but it gives u=zero everywhere
any help will be much appreciated, thanks, Tom

from dolfin import *
mesh = UnitCube(5, 5, 5)
V = FunctionSpace(mesh, "CG", 1)

subdomains = MeshFunction("uint",mesh, mesh.topology().dim())
left, right = compile_subdomains(["(fabs(x[0]) < DOLFIN_EPS) && on_boundary", "(fabs(x[0] - 1.0) < DOLFIN_EPS) && on_boundary"])
btm, top = compile_subdomains(["(fabs(x[2]) < DOLFIN_EPS) && on_boundary", "(fabs(x[2] - 1.0) < DOLFIN_EPS) && on_boundary"])

top.mark(subdomains, 1)
right.mark(subdomains, 2)
left.mark(subdomains, 3)

v = TestFunction(V);
u = TrialFunction(V);
	
g = Expression("1.4")			
alpha=Expression("1.4*1.4/9.81")

a = inner(grad(v), grad(u))*dx +(alpha*v*u)*ds(1) 
L =  g*v*ds(2)-g*v*ds(3)

problem = VariationalProblem(a, L)
u = problem.solve()
plot(u, interactive=True)

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