ksp ex29.c B.C.s and Forcing terms.

[Ryan Yan]

Hi Matt,
I am not fixing the potential for Newman constraints, and that's why we get
only 3 or 2 fluxes in and out "the boundary cell" instead of 4 fluxes for
the full cell.

Maybe I should ask this question first, are we using Finite Volume
discretization for ex29.c?
Thanks,
Yan



On Sat, Dec 12, 2009 at 5:35 PM, Matthew Knepley <knepley at gmail.com> wrote:

> On Sat, Dec 12, 2009 at 4:34 PM, Ryan Yan <vyan2000 at gmail.com> wrote:
>
>> Hi Barry,
>> Since the ex32.c has been brought up to make comparison with ex29.c. Can I
>> just get a confirm that, in ex29.c, the instance with Neumann B.C.s is the
>> case where one has cell centered unknown for the u, with vertex centered f.
>>
>
> No, Neumann constraints mean that we fix the normal derivative of the
> potential rather than the potential itself on the boundary.
>
>   Matt
>
>
>> Feel free to criticize this comment please, and this is where I get very
>> confused.
>>
>> Thanks,
>>
>> Yan
>>
>>
>>
>> On Sat, Dec 12, 2009 at 2:57 PM, Barry Smith <bsmith at mcs.anl.gov> wrote:
>>
>>>
>>> On Dec 12, 2009, at 12:31 PM, Tahar Amari wrote:
>>>
>>>  Hello,
>>>>
>>>> SOrry. I took this very interesting discussion a bit late, but one of my
>>>> major problem is to understand if all this is valid for node centered
>>>> variables for u or cell centered.
>>>> Usually the best way of solving Neuwman BC for this equation is having
>>>> cell centered unknowns to have derivatives
>>>> given on the domain boundary.
>>>> This then have some impact on handling ghost values transfer between
>>>> subdomains with MPI.Is all this transparent for
>>>> us and PETSC handle it . I guess no, right ?
>>>>
>>>
>>>  If you have only cell centered unknowns the ghost point updates are the
>>> same. See src/ksp/ksp/examples/tutorials/ex32.c  Having some Neuman and some
>>> Dirichlet boundary conditions is then trickery with cell centered.
>>>
>>>   If you have some cell-centered and some vertex centered unknowns
>>> (staggered grid), DA does not handle this well, you have to "cheat" to match
>>> up variables on the cells and vertices.
>>>
>>>
>>>   Barry
>>>
>>>
>>>
>>>> Tahar
>>>>
>>>>
>>>> Le 12 déc. 09 à 02:28, Barry Smith a écrit :
>>>>
>>>>
>>>>> On Dec 11, 2009, at 7:18 PM, Ryan Yan wrote:
>>>>>
>>>>>  Hi Matt,
>>>>>> Thank you very much for the reply.  Now, I got the Neumann part. But I
>>>>>> am still a bit confused about the Dirichlet part. Please see the following
>>>>>> quote.
>>>>>>
>>>>>> Yan
>>>>>>
>>>>>>
>>>>>> For the Dirichlet B.C.s, I did not understand the coefficients given
>>>>>> below. Isn't correct to set the v[0]=Hx*Hy here?
>>>>>> if (i==0 || j==0 || i==mx-1 || j==my-1) {
>>>>>> if (user->bcType == DIRICHLET) {
>>>>>> v[0] = 2.0*rho*(HxdHy + HydHx);
>>>>>> }
>>>>>> }
>>>>>>
>>>>>> This is the proper scaling.
>>>>>>
>>>>>> the Dirichlet B.C.s:
>>>>>> Which  scaling  do you think is proper, "v[0]=Hx*Hy" or 2.0*rho*(HxdHy
>>>>>> + HydHx)?
>>>>>> If it is 2.0*rho*(HxdHy + HydHx), can you say a little bit more about
>>>>>> why is this one? I only see a factor of Hx*Hy when we set up the  RHS. Did I
>>>>>> miss something?
>>>>>>
>>>>>>   Ryan,
>>>>>
>>>>>   You can scale the equations for Dirichlet boundary conditions anyway
>>>>> you want; you could multiply them by 1,000,000 if you want. The answer in
>>>>> exact precision with direct solvers will be the same. The reason we use the
>>>>> given scaling is to make the scaling work well with multigrid. If you use a
>>>>> different scaling the Dirichlet boundary conditions on the coarser grid
>>>>> matrices would have a different scaling then the interior equations and this
>>>>> would slow down MG's convergence rate. For toy problems people usually
>>>>> eliminate the Dirichlet boundary conditions, then there is no scaling issue
>>>>> for multigrid.
>>>>>
>>>>>
>>>>>  Barry
>>>>>
>>>>>
>>>>>> Matt
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>> --
>>>>>> What most experimenters take for granted before they begin their
>>>>>> experiments is infinitely more interesting than any results to which their
>>>>>> experiments lead.
>>>>>> -- Norbert Wiener
>>>>>>
>>>>>>
>>>> --------------------------------------------
>>>> T. Amari
>>>> Centre de Physique Theorique
>>>> Ecole Polytechnique
>>>> 91128 Palaiseau Cedex France
>>>> tel : 33 1 69 33 42 52
>>>> fax: 33 1 69 33 49 49
>>>> email: <mailto:amari at cpht.polytechnique.fr>
>>>> URL : http://www.cpht.polytechnique.fr/cpht/amari
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>
>>>
>>
>
>
> --
> What most experimenters take for granted before they begin their
> experiments is infinitely more interesting than any results to which their
> experiments lead.
> -- Norbert Wiener
>
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