[Dune] tranformed geometry across periodic boundaries

[Martin Nolte]

Hallo Matteo,

the problem is that discussion about periodic boundary conditions got stuck 
around 3 years ago.

The documentation of dune-grid describes the "official" interface for 
implementing perioditicy and is only implemented by YaspGrid.

AlbertaGrid, ALUGrid and SPGrid support periodic boundary conditions in a 
different (unofficial) way (as you already observed): They return the original 
element on the other side. Since the intersection reports itselft as boundary, 
you can now choose the appropriate boundary condition (e.g., periodic). Notice 
that this is interpreted as a special choice of boundary condition and you 
have to do something by hand, here.

To obtain the transformation, AlbertaGrid's intersection implementation has a 
method transformation (in case that helps you). Apart from that, you can use 
the geometryInOutside and the outside element geometry to obtain the  world 
coordinates of the intersection on the outside element.

The big problem is that only 1 developer (me) needed periodicity for a long 
time and, hence, the necessary interface discussion is still pending. I hope 
this clarifies the current situation, even if it does not really help.

So, you will have to decide which technique you like better and either use the 
existing grids implementing it or implement such a grid yourself.

Best,

Martin

On 05/16/2012 12:09 PM, Matteo Semplice wrote:
> Hi dune,
> I think I need some explanation on the exact meaning of the phrase
> "transformed geometry" in the documentation of Dune::Intersection
> <http://www.dune-project.org/doc/doxygen/dune-grid-html/class_dune_1_1_intersection.html#_details>
>
>
> (By the way, the Detailed Description mentions a method "outer()" which is not
> listed in the interface, so I assume that it means "outside()"...)
>
> Say I have a 2d grid with 9 1x1 cells organized as follows
>
> 7,8,9
> 4,5,6
> 1,2,3
>
> so cell 1 has center at [.5,.5], cell 2 at [1.5,.5] and so on.
> Suppose also that the x direction have periodic boundary conditions, i.e. my
> dgf file has the section
>
> PERIODICFACETRANSFORMATION
> 1 0 , 0 1 + 3 0
> #
>
>  From the phrase "outer() cell has a periodically transformed geometry (so
> that one does not see a jump or something like that)" in the section on
> periodic boundaries, I would expect that
>
> if the intersection "face" is the left side of cell 1, and I do
>
> EntityPointer pOut = face.outside();
> Entity out = *pOut;
>
> then mapper(out) points me to data related to cell 3, but that
> out->geometry().center()
> returned [-0.5,0.5], so that effetively I would see the data of cell 3 as if
> they were at the left of cell 1 and a PDE code does not need to know about the
> boundary transformation.
>
> As this is not the case (out->geometry().center() returns [2.5,0.5]) , what is
> the meaning of that phrase?
> And, given the current situation, does the grid implementation provide a
> method to transform the [2.5,.5] into [-.5,.5] ?
>
> Best regards,
> Matteo
>

-- 
Dr. Martin Nolte <nolte at mathematik.uni-freiburg.de>

Universität Freiburg                                   phone: +49-761-203-5630
Abteilung für angewandte Mathematik                    fax:   +49-761-203-5632
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