[petsc-users] question about Neumann boundary condition

[Jed Brown]

Ah, okay. With petsc-dev, just call MatSetNullSpace(). With earlier
versions, SNESGetKSP, KSPSetNullSpace.
On Dec 5, 2011 8:10 PM, "Hailong Xiao" <xiaohl1986 at gmail.com> wrote:

> Hi Jed
>
> I am not going to set Dirichlet conditions.
> I just look at the webpage (maybe you wrote ) someone recommended
>
>
> http://www.cfd-online.com/Forums/main/15709-singular-linear-system-pressure-poisson-equation.html
>
> "You need to remove the null space. The best way is to tell the Krylov
> solver about this (see KSPSetNullSpace(...) in PETSc) but you can also fix
> the pressure at one point. That is, pick any row of your matrix and set it
> to a row of the identity and put 0 on the right hand side. This should also
> make the matrix nonsingular, but it disrupts the spectrum so may effect
> convergence rates (highly preconditioner dependent). "
>
> Since I am studying the performance of the preconditioners, it would be
> great for me that the spectrum would not be disrupted.
> It seems like KSPSetNullSpace(...) could better handle it.
> My code only has the application of the matrix not forming the matrix
> explicitly.
>
> Maybe I just fix one value is 1.
> Hailong
>
>
> On Mon, Dec 5, 2011 at 9:36 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:
>
>> There is not a PETSc function to set Dirichlet conditions. There are many
>> ways to do it, you should pick a way they works well for your problem.
>>
>> I think you might misunderstand KSPSetNullSpace.
>> On Dec 5, 2011 6:59 PM, "Hailong Xiao" <xiaohl1986 at gmail.com> wrote:
>>
>>> Hi
>>>
>>> I am solving an elliptic problem with Neumann boundary condition
>>> with dmda routines by snes solver.
>>> I set up the residue by DMDASetLocalFunction.
>>> Besides the way like fix the first value to be one by myself, is there
>>> way to do it by petsc routine? like the KSPSetNullSpace() rountine does?
>>>
>>> --
>>> Hailong
>>>
>>
>
>
> --
> Hailong
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.mcs.anl.gov/pipermail/petsc-users/attachments/20111205/8508b30e/attachment.htm>