[petsc-users] explanations on DM_BOUNDARY_PERIODIC

[Matthew Knepley]

On Thu, Apr 27, 2017 at 3:46 AM, neok m4700 <neok.m4700 at gmail.com> wrote:

> Hi,
>
> I am trying to change my problem to using periodic boundary conditions.
>
> However, when I use DMDASetUniformCoordinates on the DA, the spacing
> changes.
>
> This is due to an additional point e.g. in dm/impls/da/gr1.c
>
> else if (dim == 2) {
>     if (bx == DM_BOUNDARY_PERIODIC) hx = (xmax-xmin)/(M);
>     else hx = (xmax-xmin)/(M-1);
>     if (by == DM_BOUNDARY_PERIODIC) hy = (ymax-ymin)/(N);
>     else hy = (ymax-ymin)/(N-1);
>
> I don't understand the logic here, since xmin an xmax refer to the
> physical domain, how does changing to a periodic BC change the
> discretization ?
>
> Could someone clarify or point to a reference ?
>

Just do a 1D example with 3 vertices. With a normal domain, you have 2 cells

  1-----2-----3

so each cell is 1/2 of the domain. In a periodic domain, the last vertex is
connected to the first, so we have 3 cells

  1-----2-----3-----1

and each is 1/3 of the domain.

   Matt


> Thanks
>



-- 
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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