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Re: dissipating all energy in a packing


2. I could try to make cundall's damping to always diminish the force
iff the coefficient is <0 (.e.g damping==-.3 would mean to always reduce
the force to ×.7), what do you think?

This is a very interesting problem, but honestly I thought to this hard (and I tried many different dampings on 1D oscillators) and couldn't find any good idea. I always end with the basic Cundall's damping being the most efficient and versatile damping.
I'm still open to suggestions though.

Note that these 2-steps oscillations are sensitive to the time increment, so it is just one more aspect of numerical instability with inapropriate timesteps (and in fact, I think Cundall's damping is so that if the undamped scheme is stable, the damped scheme will be stable too, I have no instant-proof of that though). The only question is : can damping revert the velocity to a greater opposite value in one (or even two) step? In principle it can not.

If I understand the idea above correctly, you would multiply all forces by 0.7 whatever the direction relative to velocity? Well, in one sense, it would just mean a change in timescale (slowing down all motions by a factor of 0.7...).
The problem is it would damp no energy at all.
In terms of stability, it would have exactly the same effect as decreasing the time increment.

What value are you using for damping?
In order to decrease kinetic energy even more (you need to normalise kinetic energy with elastic energy btw, or you can't tell if the value is "high" or "small", why do you say it's not small enough?), I would set it to something around 0.1, and I would try reducing the timestep by a factor of ~0.8.

It is useless to increase the damping to higher values in most cases. The PFC default : 0.7, is some of the worst idea they had if you ask me.

Higher (than the optimal value) damping means slower convergence to equilibrium when "convergence" implies some rearangements in the packing (instead of just damped oscillations in a completely elastic problem). When you are close to equilibrium, you always have this last grain that will move a little, then trigger a local instability that will spread to all the sample, which leads to small changes in each particle's equilibrium state. This is the sort of events that can be a lot slower with a large damping.


Regards, Vaclav

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