yade-users team mailing list archive

[chiara modenese <c.modenese@xxxxxxxxx>]

On 25 March 2010 14:13, Sergei D. <dorofeenko@xxxxxxxxx> wrote:

> 25.03.2010 14:29, chiara modeneseт:
> > Hi Sergei,
> >
> > I think that the global damping (the one at the contact level) as it
> > is now implemented in Yade (class ViscoelastiPM) is wrong in the shear
> > direction.
> >
> > At the moment we do the following (I only refer to the shear direction):
> >
> > First we rotate Fs_tot(old);
> > Then:
> > deltaFelastic=ks*deltaUs;
> > Fvisc=cs*deltaVrel_n;
>
> May be deltaVrel_s here?
>

Oh yes.

>
> > Fs_tot(new)=deltaFelastic+Fvisc+Fs_tot(old);
> >
> > Then we check Mohr-Coulomb on Fs_tot(new);
> >
> > The wrong thing (I suppose) is that we store Fs_tot including the
> > viscous component and then we go for the next step. Instead we should
> > only store the elastic part and then add the viscous part if we pass
> > the Mohr-Coulomb criterion (Bruno was right in pointing this out).
>
> So, if no moving then no viscous friction... I agree, of course...
>

Exactly.

>
> > Otherwise the final effect is that we are not dissipating energy but
> > only changing the amplitude and the frequency of the oscillation. I
> > did a comparison between the analytical solution, Yade code and what I
> > coded for the shear direction (I took a simple example to do that). I
> > attach the comparison.
>
> Very good!
>
> > If you think in the normal direction we do exactly the same. We work
> > out the normal elastic force as:
> >
> > Fn_tot_elastic=kn*Un_tot;
> > Fvisc=cn*deltaVrel_n;
> > Fn_tot=Fn_tot_elastic-Fvisc; (minus or plus depending on how we work
> > out the relative velocity)
> >
> > Next step we get a new Fn_tot_elastic that does not include the
> > hystory of the viscous force, and then we simply add the incremental
> > current viscous force.
> >
> > This is a total formulation but we could use the incremental one also
> > for the normal part (as in Bruno's notes). So you see that in the
> > normal direction there is no history of the viscous force. And this is
> > correct, in fact Un_tot (as well as Us_tot) includes the damping
> > effect since it is a result of the motion.
> >
> > I wrote a new class that adjusts the implementation of the damping in
> > the shear direction as explained above. Should I commit it? Or would
> > you prefer to modify your existing one (ViscoelasticPM)? If you agree
> > with me, of course.
> > Any comments would be appreciated.
> >
>
> I can modify ViscoelasticPM class.
> Please, add your test scripts (analytical solution and so on...) into
> scripts/test directory.
>

I derived the analytical solution by hand (no script, I have just used excel
to plot the results over time).
Once your class is ready I will update/commit my test script so I can
include your class in my example and plot shear displacements and so on to
show it works.
cheers, chiara

>
> Thanks for you very deep insight work!
>
> ==
> Best regards,
> Sergei D.
>
>
>
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