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

[Matthew Knepley]

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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