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[Johan Hake <question102549@xxxxxxxxxxxxxxxxxxxxx>]

Question #102549 on DOLFIN changed:
https://answers.launchpad.net/dolfin/+question/102549

    Status: Open => Answered

Johan Hake proposed the following answer:
On Friday 26 February 2010 12:34:59 James Lai wrote:
> New question #102549 on DOLFIN:
> https://answers.launchpad.net/dolfin/+question/102549
> 
> Hi,
> 
> After obtaining my solution using dolfin through python
> 
> problem = VariationalProblem(a,L,bcs)
> U = problem.solve()
> 
> is there any way to integrate the solution over a given boundary edge?
> 
> Lets say I set up subdomains, then if I want to integrate n dot u over
>  subdomain 1, I tried
> 
> ds_in = Measure('exterior_facet',1,None)
> n_in = Constant((-1,0))
> val = Integral(dot(n_in,u),ds_in)
> 
> It seems like when i do this, val is a not a number, but a Integral type. 
>  Is there any way to evaluate this integral and get a number out of it ?

What you get there is an integral form, like a and L above, but scalar. You 
need to assemble it to get the value. A slightly smaller example using 
predefined Measures and Facet normals should also work:

  val = assemble(dot(FacetNormal(mesh),u)*ds(1), \
           exterior_facet_domains=domain, mesh=mesh)

Here domain is a either an instance of SubDomain or a MeshFunction defining 
the dumain for your exterior integral form. Note that you probably need to 
pass the mesh as you are assembling a functional.

Johan

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