yade-dev team mailing list archive

[Chareyre <bruno.chareyre@xxxxxxxxxxxxxxx>]

> OK, my list of weird people has 2 items now:
>
> * gomechanicians
> * contact law freaks
>
>   
Haha!
I fell like a contact law freak more than a geomechanician. The contact 
law freak asks Vaclav : what the hell continuum mechanics as to do with 
interactions in multibody systems? ;)

There are two different logics and I think both of them are right.
Vaclav convinced me that there was a consistent reason for his sign 
convention. However, he didn't persuaded me that his logic was more 
intuitive (read post #4).
The alternative is :
1. We want the same signs as in continuum mechanics for 
tension/compression. As a consequence, we need to define the shear force 
as applied by b2 on b1 (right?).
2. We use the convention that an interaction is "from 1 to 2", which 
apply for the definition of the normal as well as for contact forces, 
which represent forces applied by b1 on b2, defined in the global 
coordinate system.

In logic 2, there is no inward or outward normal. The sign of fn in 
traction is the (unfortunate) consequence, not the objective. I agree 
with Chiara on the fact that logic 2 is at the same time more common, 
and more intutive (for the majority, not for Vaclav).

Perhaps make a vote in yade users to evaluate what is the most popular 
convention?

-- 
NormalShearInteraction::{normalForce,shearForce} has undefined orientation
https://bugs.launchpad.net/bugs/493102
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Status in Yet Another Dynamic Engine: Confirmed

Bug description:
NormalShearInteraction::{normalForce,shearForce} should contain forces on the interaction, which can be used for various purposes by other functions. However, different functors use them with different sign

ef2_Spheres_Elastic_ElasticLaw (a.k.a. ElasticContactLaw):

   applyForceAtContactPoint(-currentContactPhysics->normalForce-shearForce, ... )

Law2_Dem3DofGeom_CpmPhys_Cpm:

   applyForceAtContactPoint(BC->normalForce+BC->shearForce, ...)

This is a convention issue, but must be fixed. Should normalForce and shearForce be oriented as it applies on id1 (first case) or id2 (second case)? 2nd case is nicer WRT applyForceAtContactPoint syntax, but that one is arbitrary as well.