[Dune] [#975] outerNormal for outside entity

[Dune]

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A new Flyspray task has been opened.  Details are below. 

User who did this - Andreas Dedner (dedner) 

Attached to Project - Dune
Summary - outerNormal for outside entity
Task Type - Feature Request
Category - Dune Core Modules
Status - New
Assigned To - 
Operating System - Unspecified / All
Severity - Low
Priority - Normal
Reported Version - 2.0
Due in Version - Undecided
Due Date - Undecided
Details - For surface grids one in general does not have matching outerNormals, e.g., 
  N_{T'} != -N_T
where T and T' are neighboring elements.

On an intersection one now needs to find the corresponding intersection on outside to obtain n_{T'} through iteration over all intersection and some matching. That is quite difficult and expensive especially if the normals are not constant.

Suggestions (with increasing complexity for everyone):

A: add outerNormalInOutside(x) to intersection

B: implement some sort of reverseIntersection
   method on the Intersection class which provides the matching intersection from the outside entity.

C: Have method to obtain normal on the entity:
   outerNormal(i,x) (x in codim=1 or codim<0> coordinates).
   One could then use something like:
   inter.outside()->outerNormal(inter.numberInOutside, inter.geometryInOutisde.global(x))

D: have the intersection provide an insideIntersection() and outsideIntersection() method returning a class with methods like geometry() (the same as geometryInInside/Outside) and so on. Code like (*iter).geometryInInsde(x) would then have to be changed to (*iter).insideIntersection().geometry(x). The Intersection class would then have the bool methods (neighbor/boundary...) and the geometry method while all other methods would be moved to the class returned by insideIntersection() and outsideIntersection()

Because I think D would be too intrusive and not worth it, I favor A or B.

But perhaps somebody has a better suggestion?

More information can be found at the following URL:
http://www.dune-project.org/flyspray/index.php?do=details&task_id=975

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