dolfin team mailing list archive

["Garth N. Wells" <gnw20@xxxxxxxxx>]

kent-and@xxxxxxxxx wrote:
> > kent-and@xxxxxxxxx wrote:
> > > > kent-and@xxxxxxxxx wrote:
> > > > > > One or more new changesets pushed to the primary dolfin repository.
> > > > > > A short summary of the last three changesets is included below.
> > > > > >
> > > > > > changeset:   4730:6548c25c33352492c9279c035509a139caab323b
> > > > > > tag:         tip
> > > > > > user:        "Garth N. Wells <gnw20@xxxxxxxxx>"
> > > > > > date:        Tue Sep 09 13:08:58 2008 +0100
> > > > > > files:       dolfin/fem/Assembler.cpp dolfin/pde/LinearPDE.cpp
> > > > > > description:
> > > > > > Revert to old assembly in LinearPDE due to bug in symmetric assembly.
> > > > > >
> > > > > > There is a problem with exterior facets in the symmetric assembly.
> > > > > > The
> > > > > > code needs to be broken up to make debugging easier.
> > > > > Agree that it should be broken up. But it will take some effort.
> > > > > Do you have an example where the bug is apparent ?
> > > > I was using a RT0 element (not one of the demos).  It should be
> > > > reproducible by adding a non-zero Dirichlet bc (given be a Function) to
> > > > the mixed-poisson demo.
> > > >
> > > > I had a quick look, but I couldn't find the problem, so I thought it
> > > > better to wait until the code is broken up.
> > > >
> > > > Garth
> > > Strange, I see no reason for this not to work.
> > > (since the code involving only cell integrals is pretty clean, but ...)
> > >
> > > Anyway, I noticed that with my setup, CG is used. CG can not be used
> > > in this case since the problem is not positive. Did you use CG ?
> > No, I used an LU solver. I first noticed the problem when the LU solvers
> > return a message that the system was singular. I only looked at the
> > computation of the RHS vector which was zero when it  shouldn't have been.
>
> Ok, and I guess the system is singular unless you have a mix of essential
> and natural
> bc.  But the rhs should not be zero.

The essential bcs appear in the form for this problem, so the assembler 
shouldn't be doing anything extra for the bcs (bcs.size() = 0 in 
Asssembler.cpp). Makes the bug rather strange.

Garth


> > The application of bcs on external is within the loop over all cells.
> > Shouldn't it be within a loop over all boundary cells?
>
> In the present code I only run over each cell once. After the computations
> of the various integrals is performed, the BC is enforced.
>
>
> Kent