[petsc-users] question about Neumann boundary condition

[Hailong Xiao]

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
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