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[Chareyre <1070744@xxxxxxxxxxxxxxxxxx>]

A very interesting work program!
Clumping spheres and boxes would be awesome.

What we have:
- boxes can rotate already.
- 6D version of box-sphere interactions is almost here since we have 6D sphere-sphere and box-sphere interactions (or let's say plane-sphere)  are in the end represented as sphere-sphere contacts. It's only a matter of additional functors, but no real addition in terms of equations.

What we don't have (at all):
- box-box interactions
- box-sphere on edges or vertices

For box-box interactions, there is a serious background in 3-DEC from
itasca, where arbitray polyhedral shapes are modelized. It should apply
to boxes.

My only question is if it is really necessary to modelize angular vertices instead of rounded shapes. Angularity is causing major geometrical trouble, and in reality surfaces are never completely angular. Then the other option would be to have Minkowsky sum of boxes and spheres (or rounded boxes, let's say).
Somehing like this: 
http://www.google.fr/imgres?hl=en&safe=off&sa=X&biw=1438&bih=801&tbm=isch&prmd=imvns&tbnid=I4ZhvDXNCA5--M:&imgrefurl=http://liris.cnrs.fr/m2disco/rapport_2007_en.html&docid=EWZqGLpwGDTYOM&imgurl=http://liris.cnrs.fr/m2disco/morpho.png&w=1252&h=596&ei=TOuHUILiJPTB0gXpyoCABA&zoom=1&iact=rc&dur=504&sig=112569331993381553393&page=1&tbnh=124&tbnw=260&start=0&ndsp=24&ved=1t:429,r:19,s:0,i:126&tx=186&ty=38

In this case you would clump spheres and rounded boxes. François Keib is
currently working on something a bit similar (toward "rounded"
triangulated surfaces), so it would be good to speak  with him if you go
for the "rounded" way.

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https://bugs.launchpad.net/bugs/1070744

Title:
  boxes with torsional moment

Status in Yet Another Dynamic Engine:
  New

Bug description:
  Hi all,

  I thought about extending boxes ability by including torsional moment
  and box-box interactions. And I want to know from you, if this a good
  idea.

  The reason is the following: I want the model more realistic sand
  grains. I can create complex shapes by using clumps, but there are 2
  disadvantages. First, the number of spheres is getting too high, which
  corresponds to high calc. times. Second, by clumping just spheres,
  there are no real egdes, just rounded ones. To override this I want to
  create clumps consisting of one sphere and one or more boxes inside
  the sphere. Only the edges of the box are outside the sphere, so one
  can create an angular particle. With one box maximal eight edges can
  be created.

  For implementation, boxes must be extended by following abilities:

  - boxes can rotate (torsional moment must be implemented)

  - boxes can interact with each other (Ig2_Box_Box_ScGeom will be
  needed)

  - all interaction types must be handled:
   * box vertex - box plane interaction
   * box vertex - box edge interaction
   * box vertex - box vertex interaction
   * box vertex - sphere interaction ----- see figure below
   * box edge - box plane interaction
   * box edge - box edge interaction
   * box edge - sphere interaction
   * box plane - box plane interaction

  example box vertex - sphere interaction:

  ```````````````````````````******
  ````````````````````````*''''''''''''''''' *
  `````````````````````*'''''''sphere''''''*
  ```````````````````*'''''''''''''''''''''''''''''''''''*
  ################'''''''''''''''''''''''*
  ___________________*''''''''''#'''''''''''''''''''''''*
  _____________________*''''''#''''''''''''''''''''*
  ________box___________ *'#''''''''''''''' *
  _________________________#'******

  What do you think about this? I know it will be a lot of work, but in the end it will be a very big extension in possibilities of DEM simulations with yade (not just angular particles!).
  Are there other DEM codes, where something similar it is implemented?
  What about the details of implementation? Where do you see problems in coding?

  Christian.

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