yade-users team mailing list archive

[chiara modenese <c.modenese@xxxxxxxxx>]

On 25 March 2010 11:57, Anton Gladky <gladky.anton@xxxxxxxxx> wrote:

> 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.
>

Hi Anton,

the local damping (alias the one defined used in NewtonIntegrator law) is
working properly in the shear direction as well as in the normal. This time
I have not properly checked with a closed solution (it is a little bit more
complicated than with viscous damping) but numerically I get the expected
behavior (so I see an energy dissipation and a tendency to an equilibrium
position).
I do not know about your specific problem, maybe you can try to use the
viscous damp and see what happens (but wait till Sergei will have updated
his class).

cheers, Chiara

______________________________
>
> 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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>