[petsc-users] Neumann BC with non-symmetric matrix

[Mohammad Mirzadeh]

Mark,

On Fri, Feb 26, 2016 at 8:12 AM, Mark Adams <mfadams at lbl.gov> wrote:

>
>>    4-4) Along the same lines, I tried a couple of other PCs such as
>> {jacobi, sor, gamg, ilu} and none of them were able to converge with bcgs
>> as the KSP. However, with gmres, almost all of them converge with the
>> exception of gamg.
>>
>
> Note, I'm not sure why you need the null space of A^T, you want the null
> space of A.
>
>
So the idea was to provide nullspace of A^T to make sure the true residual
also converges to zero by projecting the RHS onto the range of A. It
however looks like that GMRES (and sometimes BiCGSTAB) converge in the
least-square sense for which you only need the nullspace of A and not A^T.

And for singular systems like yours you need to use a pseudo inverse of the
> coarse grid because it is singular -- if you represent the null space
> exactly.
>
> GAMG is use for AMR problems like this a lot in BISICLES.
>

Thanks for the reference. However, a quick look at their paper suggests
they are using a finite volume discretization which should be symmetric and
avoid all the shenanigans I'm going through! I think it would actually be a
good idea for me to swap my solver with a conservative one and see if it
makes things better.


> You need to use an 'svd' coarse grid solver, or an appropriate iterative
> solver. LU is the default.
>
>
I see. How can I change the GAMG coarse grid solver? Is there an analogue
of "-pc_hypre_boomeramg_relax_type_coarse"?

Mark
>

Thanks,
Mohammad
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