Re: [Libmesh-users] ex11

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

On Tue, 5 Sep 2006, Hae-Won Choi wrote:

> Hi, I noticed that ex11 (stokes problem) does not converge
> as I have monitored convergence  history by adding
> PetSc flag: -ksp_view -ksp_smonitor.
> I have changed max iter number to 250 to 10000
> but it still not converge.
> I think this problem is due to zero diagonal entities in Kpp matrix.
> Physically Kpp matrix has no entities but in order to solve
> mixed finite element problems diagonal entities in Kpp matrix should
> be filled with small values (let say 1.0e-6 or smaller) as penalty.
>
> Please correct me if I'm wrong

You are correct that with an unpenalized pressure matrix the Stokes
problem is singular, but the null space of the operator is only one
dimensional, and even the default PETSc options we use (GMRES + ILU)
should handle that fine.  Is it possible that your PETSc installation
is configured to use a different, less flexible (or less efficient)
solver?  CG and/or LU break down for me, for instance.
---
Roy