[petsc-users] Periodic boundary conditions in DMPlex

[Matthew Knepley]

On Fri, Oct 15, 2021 at 7:31 AM Pierre Seize <pierre.seize at onera.fr> wrote:

> It makes sense, thank you. In fact, both ways seems better than my way.
> The first one looks the most straightforward. Unfortunately I do not know
> how to implement either of them. Could you please direct me to the
> corresponding PETSc functions ?
>
> The first way is implemented for example in DMPlexCreateBoxMesh() and
DMPlexCreateCylinderMesh(). The second is not implemented since
there did not seem to be a general way to do it. I would help if you wanted
to try coding it up.

  Thanks,

    Matt

> Pierre
>
> On 15/10/21 13:25, Matthew Knepley wrote:
>
> On Fri, Oct 15, 2021 at 7:08 AM Pierre Seize <pierre.seize at onera.fr>
> wrote:
>
>> Hi,
>>
>> I'm writing a code using PETSc to solve NS equations with FV on an
>> unstructured mesh. Therefore I use DMPlex.
>>
>> Regarding periodicity, I manage to implement it this way:
>>
>>    - for each couple of boundaries that is linked with periodicity, I
>> create a buffer vector with an ISLocalToGlobalMapping
>>
>>    - then, when I need to fill the ghost cells corresponding to the
>> periodicity, the i "true" cell of the local vector fills the buffer
>> vector on location i with VecSetValuesBlockedLocal, then
>> VecAssemblyBegin/VecAssemblyEnd ensure each value is send to the correct
>> location thanks to the mapping, then the i "ghost" cell of the local
>> vector reads the vector on location i to get it's value.
>>
>>
>> It works, but it seems to me there is a better way, with maybe PetscSF,
>> VecScatter, or something I don't know yet. Does anyone have any advice ?
>>
>
> There are at least two other ways to handle this. First, the method that
> is advocated in
> Plex is to actually make a periodic geometry, meaning connect the cells
> that are meant
> to be connected. Then, if you partition with overlap = 1,
> PetscGlobalToLocal() will fill in
> these cell values automatically.
>
> Second, you could use a non-periodic geometry, but alter the LocalToGlobal
> map such
> that the cells gets filled in anyway. Many codes use this scheme and it is
> straightforward
> with Plex just by augmenting the map it makes automatically.
>
> Does this make sense?
>
>   Thanks,
>
>      Matt
>
>
>> Pierre Seize
>>
> --
> 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
>
> https://www.cse.buffalo.edu/~knepley/
> <http://www.cse.buffalo.edu/%7Eknepley/>
>
>
>

-- 
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

https://www.cse.buffalo.edu/~knepley/ <http://www.cse.buffalo.edu/~knepley/>
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