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Re: twist Moment and Bending Moment in cohesionlessMomentRotation


Luc Sibille said:     (by the date of Wed, 20 Oct 2010 09:55:26 +0200)

> However, Plassiard considers in his paper "and in his thesis" only the 
> rolling part of the relative rotation of particles (and thus only 
> bending moment), whereas in the code rolling and twist part of the 
> relative rotation are considered (and thus bending and twist moment).
> My question: is there a particular reason for that? What is the 
> motivation of the person who wrote this code? I don't say that is bad or 
> good, but I would like to have an idea about advantages and 
> disavantages, and physical meaning for considering a twist moment.

I see that there is my formula, which I derived when I was working on
snow last year. I still remember discussing with Bruno about this
exact problem :)

/* Moment Rotation Law */
Quaternionr delta( b1->state->ori * phys->initialOrientation1.conjugate() *phys->initialOrientation2 * b2->state->ori.conjugate()); //relative orientation

It calculates rotation difference between two spheres using
quaternions. The motivation to use both bending and twisting is
because quaternions cover ALL possible types of rotations. And in
fact the resulting rotation must be further decomposed into bending
and twisting. And whether you want to use the twisting component or
not, is just up to you.

> I am still investigating the cohesionlessMomentRotation.
> It is written in the sources files that this code has been "verified 
> with the paper of Plassiard in GM".

Who verified this with his paper? Not me :)

I wrote that formula and verified it on a beam (made of spheres formed
into a line).

best regards
Janek Kozicki                               http://janek.kozicki.pl/  |

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