Re: [Libmesh-users] error about solving darcy equation with mixed FEM

[Roy S. <roy...@ic...>]

On Tue, 11 Sep 2007, Shengli Xu wrote:

> Hi, I use the mixed galerkin finite element to sovle darcy equation.
> pressure is linear interpretation, velocity is quadratic interpretation¡£ The
> domain is square. The result is correct when I apply no-slip b.c on top and
> bottom side, zero pressure on right-bottom corner, Ux=1.0 on left side. But
> I can not get correct result if I apply no-slip b.c on top and bottom side,
> zero pressure on the right side, P=1 on the left side. Why can I get correct
> result with this b.c.?

Unless someone has seen a similar Darcy-flow problem before (John?),
which is a long shot, you're not likely to find someone who can answer
this.  If you've already got the problem boiled down to a simple
benchmark, I'd try running it on a single square element and comparing
the numerical linear system to what you can compute by hand.  Doing
so for even that bare minimum (22x22 in your case, I suppose) system
can be tedious, but in the past it's been the only way I've been able
to find one or two subtle bugs in application codes.
---
Roy