[petsc-users] question about Neumann boundary condition

[Hailong Xiao]

Thank you.

Hailong

On Mon, Dec 5, 2011 at 10:14 PM, Jed Brown <jedbrown at mcs.anl.gov> wrote:

> 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
>>
>


-- 
Hailong
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