dolfin team mailing list archive

[Johan Hake <johan.hake@xxxxxxxxx>]

On Tuesday September 28 2010 05:34:06 Achim Schroll wrote:
> New question #127166 on DOLFIN:
> https://answers.launchpad.net/dolfin/+question/127166
> 
> Hi!
> 
> Thanks for the new tutorial (aug 2010)!
> 
> Ultimately I need to specify a flux condition on a (small) part of the
> boundary as explained in Section 6.3. But, is it true that "it is not
> possible to perform integrals over different parts of the ... boundary
> using ... the VariationalProblem instance"?

This should be possible. Just pass a FacetFunction which marks the different 
Boundary domains, as the exterior_facet_domains. 

Can you point me to the tutorial. I think I do not have the latest copy.

> Background: I am solving the nonlinear DEMOSTRAT model and want to model
> sediment inflow through a river bed. Just recently, before the summer, I
> updated my code to implement the fully nonlinear system and to use DOLFINs
> features like automatic form building and nonlinearity. I don't want to go
> back to manual linearization etc...
> 
> Another detail:
> Any idea what could be wrong when using the code on page 24 to evaluate the
> total flux: n = FacetNormal(mesh)
> flux = -dot(grad(u),n)*ds
> total_flux = assemble(flux)
> A runtime error appears: "Unable to extract mesh from form (no mesh found)"
> when executing the last line.

This because the dolfin::Form which is used to pass the ufc::form to the 
assembler only chech the FunctionSpaces of the Test and TrialSpaces for a 
mesh. So you need to pass this manually by:

  total_flux = assemble(flux, mesh=mesh)

I think it would be convinient to be able to assemble a scalar from a 
Function. The mesh is contained in the FunctionSpace so it should be possible. 
I then need to figure out if a GenericFunction is a Function or an Expression. 
Maybe we can add a has_function_space() method to the GenericFunction 
interface?

Johan


> I am runing DOLFIN 0.9.7 under Ubuntu 9.10
> 
> Best, Achim