yade-users team mailing list archive

[chiara modenese <c.modenese@xxxxxxxxx>]

Hi,

not sure to understand. I mean, what you say it is correct and valid as far
as we look at the relative situation (to me is like being in a local
coordinate system). To put it better, if you exchange the id of the
particles, your relative velocity will have same direction but different
sense (right?). From this we understand that we get the increment in a local
system (and that is fine) but the shear force is supposed to be expressed in
the global system (that is why we rotate it).
So, would not be better to define a sort of tangential vector in order to
fix a reference also for the shear part as we for the normal one?

Chiara



On 18 March 2010 15:09, Sergei D. <dorofeenko@xxxxxxxxx> wrote:

>
> > sorry but I cannot get this point:
> >
> > ***
> > normal = body2_pos - body1_pos,
> > so we should have
> > relative_vel = body1_vel - body2_vel
> > in order to rel_vel>0 if loading and rel_vel<0 if unloading
> > ***
> >
> > How can you derive that? First the relative velocity is a vector so
> > you cannot say that has a sign (positive or negative). In case it
> > holds a sense.
> normal is the vector from the body1 to body2.
> But rel_vel is the velocity of the _body1_ relative body2.
> So, if normal.Dot(rel_vel)>0 => body1 moves towards the body2.  In this
> sense.
>
> Next. Contact force acts from body1 towards body2 (as the normal),
> so shearForce acts on body2, so it have same direction as shearVelocity,
> because shearVelocity is the velocity of _body1_, so body2 have
> -shearVel velocity,
> and shearForce acts in opposite direction of body2's velocity, as should
> be friction force.
> So, we have
> shearForce += ks*shearVelocity
>
> Hope, that help...
>
> S.
>
>
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