yade-users team mailing list archive

["Sergei D." <dorofeenko@xxxxxxxxx>]

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?

> 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...

> 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.

Thanks for you very deep insight work!

==
Best regards,
Sergei D.