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[Dominik Boemer <question264191@xxxxxxxxxxxxxxxxxxxxx>]

Question #264191 on Yade changed:
https://answers.launchpad.net/yade/+question/264191

Dominik Boemer posted a new comment:
Hi everyone,

thank you for your messages!  After adding some "printf"s in my version
of the code, I made the following findings.  Excuse me for the large
number of attributes used in the following explanation; they are
necessary to eliminate any ambiguity as best as possible:

- if the normal projection of the center of the sphere on the plane of
the facet is inside of the triangular facet, the penetration depth is
simply determined by calculating the distance between the center of the
sphere and its normal projection on the plane of the facet.

- if the projection of the center of the sphere on the plane of the
facet is not inside of the triangular facet, the sphere might be in
contact either with an edge or a vertex of the facet.  To simplify the
explanation, the projection of the center of the sphere on the plane of
the facet will be called the potential contact point with the plane:

= edge contact: each edge has an outward-pointing normal in the plane of
the facet.  The potential contact point with the edge (not with the
plane) is then obtained by projecting the potential contact point with
the plane on the closest edge along its outward-pointing normal.  The
penetration depth is then determined by calculating the distance between
the center of the sphere and this potential contact point (with the
edge).  The normal direction of the interaction is given by the vector
going from the potential contact point with the edge to the center of
the sphere.

= vertex contact: if the sphere is potentially in contact with a vertex
of the facet, the potential contact point is obviously the respective
vertex.  The normal of the interaction is given by the vector going from
this vertex to the center of the sphere.

The most important consequence in Yade DEM simulations with triangular
surface meshes is that a sphere can simultaneously be in contact with
two facets (edge between facets) or even more facets (depending on the
number of facets which share a vertex, usually 3).  This behavior might
induce a local stiffening of the contact.

Feel free to correct any errors in my explanation.  
Thanks,
Dominik

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