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[Václav Šmilauer <question137230@xxxxxxxxxxxxxxxxxxxxx>]

Question #137230 on Yade changed:
https://answers.launchpad.net/yade/+question/137230

    Status: Open => Answered

Václav Šmilauer proposed the following answer:
Hi Giulia, let me try to answer. I should say at start that it is
possible that we will find some bugs in LawTester, as it was written
only recently.

1. Local coordinate system; the 0th component is normal (between
sphere's centers), 1st and 2nd components are 2 shear directions,
perpendicular to the normal. Local system definition depends on IGeom
class that is in use. If you use ScGeom (which only knows normal, i.e.
local x axis), the other two axes will be chosen arbitrarily at first,
then tracked following motion of both particles.

2. That is correct; if there is no rolling, then shear can be created in
geometrically different ways, although the resulting shear will be the
same -- imagine (i) one sphere fixed, the other one rotating around an
axis going through its center and perpendicular to the interaction [that
is rotWeight==1] (ii) one sphere fixed, the other one translated around
the other one on circular arc centered at the first sphere's center
(distance does not change), without underoging any rotation [that is
rotWeight==0].

You can assign other values to rotWeight as well (including those ouside
the 0...1 range) and the result should still be the same.

Note that rotWeight is ignored when mutual rotations (bending/twisting)
are considered by the IGeom, since then, shear must be applied in such a
way that it does not create bending and vice versa (that takes away that
degree of freedom expressed by rotWeight).

3. Oh yes, they should be the same... I will have a look tomorrow, hope
that is fine for you.

If you use ScGeom6D: I will have to add support for it to LawTester (I
have not done it yet, and I did not expect someone will use it that
soon); OTOH it is not complicated and I will have a look tomorrow
morning probably.

Also I did some changes in LawTester today to make it more intuitive
(hopefully); your old code will work, and some warnings will tell you
how to change it then.

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