dolfin team mailing list archive

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

kent-and@xxxxxxxxx wrote:
> > 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
>
> So the essential bc that you refer to is the essential bc for the
> classical Poisson problem
> which ends up as natural in the mixed formulation. So you have something
> like:
>
> L = w*f*dx + w*g*ds

Exactly.

Garth

> I'll have a look.
>
> (But as already said, I completely agree on breaking up)
>
> Kent