yade-users team mailing list archive

[chiara modenese <c.modenese@xxxxxxxxx>]

I am not sure that this is just a convention (to me it is no more a
convention once we define the normal vector). I mean, the normal vector
holds both a direction and a sense (btw, sorry if I did a little bit of
confusion switching sign with sense, but I think we understand each other
anyway). Moreover perhaps we should talk about repulsive and/or eventually
attractive forces at the interaction level (compression or tension are fine
if we refer to the normal vector but according to each particle, whether it
is id1 or id2, we deal with repulsive forces).
Therefore my question is still valid (but I could be wrong): how can we
apply the forces to the bodies in a different way for ScGeom and Dem3DofGeom
if we have previously defined the normal vector exactly in the same way?

Thanks for your answer (and thanks Vaclav for the answer about the rotation
of the shear force, now it is very clear and make sense!)

Cheers, Chiara



2010/3/18 Václav Šmilauer <eudoxos@xxxxxxxx>

> > Saying that "it is better do define interaction force as it applies on
> > id2", you mean, in the case of ScGeom:
> >
> > Vector3r f = phys->normalForce + shearForce; (POSITIVE)
> > ncb->forces.addForce(id1,-f); (NEGATIVE -> compression)
> > ncb->forces.addForce(id2,f); (POSITIVE -> tension)
> > ncb->forces.addTorque(id1,-c1x.Cross(f));
> > ncb->forces.addTorque(id2,(c2x).Cross(f))
> >
> > right?
> Almost. f is a Vector3r, and it will be positive/negative with respect
> to the normal: normal.dot(f) negative for compression and positive for
> tension. (Geotechnicians have opposite convention, which might have been
> the source of the inverse; but they are persuaded now ;-) ).
> > So it is clear that the convention between ScGeom and Dem3DofGeom is
> > different, but why? The normal vector in both cases is defined in the
> > same way. Where is the difference that matters in terms of signs? It
> > is just to know, at the moment I am using ScGeom but it is useful to
> > understand such basic things and where they do come from. Actually I
> > had a look at Dem3DofGeom but is quite hard to understand although it
> > is probably better defined than ScGeom. It would be great to have the
> > same convention because this could cause a bit of confusion at least
> > at a first look.
> As Luc already mentioned, interaction of 2 bodies defines direction, but
> not sense. It is purely conventional which way it is, but I agree it
> should be uniform. I will fix that next week, I hope.
>
> Thanks for your quesions/remarks on this.
>
> Cheers, Vaclav
>
>
>
> _______________________________________________
> Mailing list: https://launchpad.net/~yade-users<https://launchpad.net/%7Eyade-users>
> Post to     : yade-users@xxxxxxxxxxxxxxxxxxx
> Unsubscribe : https://launchpad.net/~yade-users<https://launchpad.net/%7Eyade-users>
> More help   : https://help.launchpad.net/ListHelp
>