[Dune] centerUnitOuterNormal() in UGGrid

[Oliver Sander]

Hi Eike,
no worries.  In fact, I should have thought of an orientation problem much earlier.
Also, one may argue that UGGrid should give an error message when an element is
oriented wrongly, rather than tacitly producing wrong results.  Maybe that's easy
to do.
Best,
Oliver

Am 18.09.2013 11:27, schrieb Eike Mueller:
> Hi Oliver,
>
> thanks for looking into this and confirming that it is not a bug but rather a problem with my grid - several of my elements did indeed have the wrong orientation, i.e. the vertices were not in the
> correct order. The problem did not show up in ALUGrid as there the grid was 2d instead of 3d.
> I now fixed my code which generates the .dgf grid files such that it writes out correctly oriented elements, and then centerUnitOuterNormal() shows the expected behaviour.
>
> Apologies for the confusion,
>
> Eike
>
> On 16/09/13 16:24, Oliver Sander wrote:
>> Hi Eike,
>> that sounds like a bug.  You are right in expecting a method call '*OuterNormal'
>> to return an outer normal.  Can you send a complete test case that reproduces
>> the problem?
>> Cheers,
>> Oliver
>>
>> Am 16.09.2013 16:30, schrieb Eike Mueller:
>>> Dear DUNE-list,
>>>
>>> is centerUnitOuterNormal() for the (3d-) UGGrid guaranteed to give the OUTER normal in all cases?
>>> in other word, should the following code, which calculates the scalar product of the outer normal and the distance between the center of the cell and center of its the intersection with the
>>> neighbouring cell always print out positive values? [Ok, of course one could construct some weird mapping from the reference elements to 3d such that this is not the case, but for the grid I'm looking
>>> at the cells are approximately prisms].
>>>
>>> iend = gv.template end<0,Dune::Interior_Partition>()
>>> for (Iterator it =gv.template begin<0,Dune::Interior_Partition>();it!=;++it) {
>>>      Entity &en = *it;
>>>      typename Entity::Geometry::GlobalCoordinate    centerEn = en.geometry().center();
>>>      IntersectionIterator isend = gv.iend()
>>>      for (IntersectionIterator is = gv.ibegin(en);is!=isend; ++is)
>>>      {
>>>          typename Entity::Geometry::GlobalCoordinate    normal = is.centerUnitOuterNormal();
>>>          typename Entity::Geometry::GlobalCoordinate    centerIs = is.geometry().center();
>>>          centerIs -= centerEn;
>>>          std::cout << " Normal * (centerIs - centerEn)" << normal*centerIs << std::endl;
>>>      }
>>> }
>>>
>>> I found that I sometimes get negative values in a cell (and all it's children which share the same boundary).
>>> In my case, I actually need the dot product between the outer normal and the distance between the cell and it's neighbours, and I can fix the problem by just adding an fabs(), but I was slightly
>>> surprised by this behaviour which I did not see in ALUGrid.
>>>
>>> Thanks a lot,
>>>
>>> Eike
>>>
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