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[Chareyre <question136034@xxxxxxxxxxxxxxxxxxxxx>]

Question #136034 on Yade changed:
https://answers.launchpad.net/yade/+question/136034

Chareyre proposed the following answer:

> achieve quasi-static equilibrium. I am used to scale up the mass, but when I
> employ high inter-particle stiffnesses, then the system is quite unstable
> and reducing the time step does not help. There are so many parameters
> involved, like mass, loading rate, damping and so on.

1- Is the system stable (i.e. unbalanced force monotonicaly decreasing down to 1e-10 or
so) under isotropic (or gravitational - I don't know what you are doing exactly) loading?
	2- If yes, do you find a proportionality between loading rate and unbalanced force during
the loading?
		3- If yes, unbF/rate is most probably scaled with damping. Do you really need damping?
			4- If yes, find the optimal damping in terms of CPU time (most probably around 0.1-0.2).
			5- else get rif of damping
		6- else, there is a problem in the formulation (either contact law or boundary
conditions control). It needs to be fixed.
	7- else, same as 6.


> I could not find a
> good paper where they suggest a "precise" way of calibration to reach
> quasi-static equilibrium. For instance, can we use a smaller sample to
> calibrate those parameters? Would be the calibration still representative?

I don't think such paper exists, especially if you use Hertz-Mindlin.
Generally, no, you will not get exactly the same behaviour with a different number of
particles. Numerical waves have a velocity of 1 particle/timestep in explicit scheme,
whatever dt and size of particle. Hence they will be different with different size of packing.
OTOH, if there is a problem in the formulation (6,7), it is a good idea to investigate it
in small packings.

In triaxial tests, I found different ratio between unbalanced force and peak stress error
with different number of particles.

Bruno

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