yade-users team mailing list archive

[Anton Gladky <gladky.anton@xxxxxxxxx>]

Hi, Chiara!

Recently I have got some problems with damping in shear directions, but I
don't know the reason.

When my RockPM had only normal forces, it worked normally. When I added
shear forces, bodies started to "hang" in the air. If I reduce dampingCoef,
it become better, but too low dampingCoef brings other problems.

I use only NewtonIntegrator for damping purposes, so I have to check it
there.
______________________________

Anton Gladkyy


2010/3/25 chiara modenese <c.modenese@xxxxxxxxx>

> 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;
> 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). 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.
> 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.
>
> Cheers, Chiara
>
>
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