dolfin team mailing list archive

[Anders Logg <logg@xxxxxxxxx>]

On Wed, Feb 13, 2008 at 08:49:09PM +0100, Shilpa Khatri wrote:
> We (Dag and I) are doing this because we would like to have Dolfin solve the
> Stokes/Navier-Stokes equations as one part of a timestep in our code where we
> are moving interfaces that are defined as a random set of points in the
> domain. 
> 
> Thanks,
> Shilpa 

Then I suggest first finding out which cells those points lie in, then
then for each cell with a point get the expansion coefficients within
that cell, then multiply those coefficients with the values of the
basis functions at the points.

The basis functions are available from the ufc::finite_element.

We can implement a suitable interface for this at some point but
until then, you can do it manually.

-- 
Anders


> On Feb 13, 2008 8:42 PM, Anders Logg <logg@xxxxxxxxx> wrote:
> 
>     On Wed, Feb 13, 2008 at 08:28:03PM +0100, Dag Lindbo wrote:
>     > > Shilpa Khatri wrote:
>     > >> I would also like to know how this is done (We are using the c++
>     > >> interface).
>     > >> Thanks,
>     > >> Shilpa
>     > >>
>     > >> On Feb 13, 2008 4:09 PM, Kristen Kaasbjerg <cosby@xxxxxxxxx
>     > >> <mailto:cosby@xxxxxxxxx>> wrote:
>     > >>
>     > >>     Hi,
>     > >>
>     > >>     I have a rather simple question - having obtained a FEM
>     > >>     solution "u" of a given PDE, how does one get the
>     > >>     function value at an arbitrary point "x" (I'm using the
>     > >>     python interface) ?
>     > >>
>     > >>
>     > > This functionality existed in DOLFIN a while ago, but it needs updating
>     > > to new interfaces. Take a look at the old implementation:
>     > >
>     > > http://www.fenics.org/hg/dolfin?f=50aee5576fed;file=src/kernel/function
>     /DiscreteFunction.cpp
>     > >
>     > > The point evaluation function is:
>     > >
>     > > real DiscreteFunction::operator()(const Point& p, uint i)
>     > >
>     > > The significant change that needs to be done is how to compute basis
>     > > functions. This functionality now exists for general finite elements in
>     > > the UFC interface (evaluate_basis()).
>     > >
>     > >   Johan
>     >
>     > This is good news. As far as I can tell, the search is handled by a
>     > IntersectionDetector which is mainly a wrapper for GTS. Has the
>     _idetector
>     > member been removed from the DiscreteFunction, or does it come from some
>     > base class?
>     >
>     > Dag
> 
>     Why do you need to evaluate functions at arbitrary points?
> 
>     I can understand it's nice to be able to do this, but in many cases
>     there are other solutions (which are much more efficient).
> 
> 
> 
> 
> 

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