dolfin team mailing list archive

["Garth N. Wells" <g.n.wells@xxxxxxxxxx>]

Jake Ostien wrote:
> Hi,
>
> I'd like to use a subset of the solution vector to calculate other 
> quantities.  For example, if I have solved for the displacement field on 
> a 2D mesh, how can a get at the displacement in the x direction on a 
> subset of the mesh.  I was think along the lines of populating a 
> std::vector<real> which contains u[0] for a subset (possibly a 
> SubDomain) of the mesh.  What I can't figure out is how to access the 
> actual values of u[0] inside of DOLFIN.  

This is not supported directly. The best way to extract just the 
component to be used in other calculations is to project u[0] onto 
another Function, so the FFC form file should look something like

  element_u  = VectorElement("triangle", 1)
  element_ux = FiniteElement("triangle", 1)

  v  = TestFunction(element_ux)
  ux = TrialFunction(element_ux)
  u  = Function(element_u)

  a = v*ux*dx
  L = v*u[0]*dx

If you want the Function to act only in a subdomain, you could introduce 
a user-defined function using a DG0 basis and set it to one in the 
domain of interest, and zero elsewhere. Then multiply your problem by 
this function.

Garth


Of course I can write the
> solution to a file, but I am trying to avoid that post-processing step.  
> I have found lots of useful hooks for looking at the reference mesh, but 
> not for the solution (and I guess for Functions in general).
>
> Thanks for any help.
>
> -Jake
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