ksp tutorial ex22

[Wee-Beng Tay]

Hi Ryan,

Thanks alot.

There's still some parts I don't understand. It'll be great if someone can
help:

In ex22f.f90, there's a :

MatStencil   row(4),col(4,7)

I understand that the 7 is due to the 7-pt stencil. However, why is there a
"4"? In that eg., coarsest grid has dimension 3 with 3 lvl. it seems that in
eg. ex29.c, it is only given as "row", not array

As for Neumann B.C.s, it's mentioned there's null space of the coefficient
matrix A. looking at ex29,

if (user->bcType == NEUMANN) {
MatNullSpace <http://www.mcs.anl.gov/petsc/petsc-as/snapshots/petsc-3.0.0/docs/manualpages/Mat/MatNullSpace.html#MatNullSpace>
nullspace;

KSPGetNullSpace
<http://www.mcs.anl.gov/petsc/petsc-as/snapshots/petsc-3.0.0/docs/manualpages/KSP/KSPGetNullSpace.html#KSPGetNullSpace>(dmmg->ksp,&nullspace);
MatNullSpaceRemove
<http://www.mcs.anl.gov/petsc/petsc-as/snapshots/petsc-3.0.0/docs/manualpages/Mat/MatNullSpaceRemove.html#MatNullSpaceRemove>(nullspace,b,PETSC_NULL
<http://www.mcs.anl.gov/petsc/petsc-as/snapshots/petsc-3.0.0/docs/manualpages/Sys/PETSC_NULL.html#PETSC_NULL>);
}

I don't really understand what it is trying to do. My BC has du/dx=0 and
du/dy = 0 and I believe it's the same as that of ex29.c

if I do it in Fortran, is it the same?

Thanks again!



On Sat, Dec 12, 2009 at 12:25 AM, Ryan Yan <vyan2000 at gmail.com> wrote:

>
>
> On Fri, Dec 11, 2009 at 1:44 AM, Wee-Beng Tay <zonexo at gmail.com> wrote:
>
>> Hi,
>>
>> I'm also looking at the fortran version of ex22. I have some questions:
>>
>> 1. M,N,P are global dimension in each direction of the array. I'm abit
>> confused. Do you mean the no. of grid pts at the coarse level? Hence, if my
>> problem has 27x27 grid pts, do I use M=N=9 with 3 multigrid levels or M=N=3
>> with 9 lvls? What is the guideline in choosing the number of multigrid
>> levels?
>>
>> 2. What is stencil width (s)? Seems to be 1 in both ex22 and ex29.
>>
>> 3. What are arrays containing the number of nodes in each cell along the
>> x/y/z coordinates? Is it ok to just use the default PETSC_NULL_INTEGER?
>>
>> 4. There's a subroutine ComputeJacobian which computes the jacobian.
>> However, what is it actually calculating? Is it simply calculating the
>> coefficient arising from the discretization of the matrix?
>>
>>
> I just realized that you are looking at Fortran version ex22f.F for this
> question.
> In fact, my answer still applies.
> You may want to make a comparison between:
>
> http://www.mcs.anl.gov/petsc/petsc-as/snapshots/petsc-current/docs/manualpages/DMMG/DMMGSetSNES.html
>
> http://www.mcs.anl.gov/petsc/petsc-as/snapshots/petsc-current/docs/manualpages/DMMG/DMMGSetKSP.html
>
>
>
>> 5. My objective is to solve for the poisson equation arising from the
>> Navier Stokes equations. ie div grad P = F, which F can be calculated
>> separately. The boundary conditions are Neumann which is dp/dx, dp/dy=0. Is
>> this a good example to follow? Or is ex29 a better example, although I need
>> time to digest the C syntax.
>>
>> thanks alot!
>>
>>
>> On Fri, Dec 11, 2009 at 4:14 AM, Ryan Yan <vyan2000 at gmail.com> wrote:
>>
>>> Hi Barry,
>>> Thank you very much for the confirm.
>>>
>>> Yan
>>>
>>>
>>> On Thu, Dec 10, 2009 at 3:12 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
>>>
>>>>
>>>> On Dec 10, 2009, at 2:09 PM, Ryan Yan wrote:
>>>>
>>>>  Hi all,
>>>>> I was looking at the ex22 of ksp.
>>>>>
>>>>> If I call:
>>>>> ...
>>>>> ierr = DMMGCreate(PETSC_COMM_WORLD,3,PETSC_NULL,&dmmg);CHKERRQ(ierr);
>>>>> ierr =
>>>>> DACreate3d(PETSC_COMM_WORLD,DA_NONPERIODIC,DA_STENCIL_STAR,-3,-3,-3,PETSC_DECIDE,PETSC_DECIDE,PETSC_DECIDE,1,1,0,0,0,&da);CHKERRQ(ierr);
>>>>>  ...
>>>>>
>>>>> How many grid pts should I have on the finest level? Is it 27,
>>>>> considering I am set 3 as the global dimension in each direction of the
>>>>> array?
>>>>>
>>>>> When I run ex22 with -ksp_view, I saw that  on the finest level there
>>>>> are 9 pts on each direction.
>>>>>
>>>>> Is that because I have 3 level?
>>>>>
>>>>
>>>>   Yes. The DA you create initially is for the coarsest grid, finer ones
>>>> are automatically created by the DMMG.
>>>>
>>>>   Barry
>>>>
>>>>
>>>>> Yan
>>>>>
>>>>>
>>>>
>>>
>>
>
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