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

[Ryan Yan]

Hi Matt,
Thank you very much for the comment, agreed on 29.c.

Yan



On Sat, Dec 12, 2009 at 5:47 PM, Matthew Knepley <knepley at gmail.com> wrote:

> On Sat, Dec 12, 2009 at 4:44 PM, Ryan Yan <vyan2000 at gmail.com> wrote:
>
>> 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?
>>
>
> No, its FD. Homogeneous Neumann conditions in this case mean fix nothing.
>
>   Matt
>
>
>> 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
>>>
>>
>>
>
>
> --
> 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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