# yade-users team mailing list archive

## Re: The convergence criterion in YADE

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```Feng, Chen said:     (by the date of Mon, 28 Nov 2005 22:21:02 -0500)

>     I am currently concentrated on the convergence criteria of the sphere
> system in YADE, for example, like in ITACSA's PFC2D software, the convergence
> criteria is that when the maximum unbalanced force is less than a certain
> value, the whole system is considered in equilibrium,

I'm not sure if I understand - you mean that for something to start
moving - a treshold force must be reached?

> or when the total
> relative displacement e.g., ||D(i)-D(i-1)||/||D(i-1)||<TOL,

so when a sphere _wants_ to move less than some treshold displacement -
it is not moving at all?

> the whole system is considered in equilibrium.

this sentence means - that nothing is moving, right?

> Is there any similar criterion that is already
> built within YADE? or I will have to implement it?

If my guesses above are correct, and if I understand you correctly -
then it is not in yade currently, but it is very easy to add. It's just
another kind of damping. Currently in yade we have only
CundallNonVoscousDamping, which has one nice advantage - it doesnt damp
velocity, just force/acceleration. So moving objects (due to velocity)
do not stop moving due to damping.

If you want your damping (how to call it? - ConvergenceDamping ?? ) you
can just make a copy of CundallNonViscousDamping, and modify it. It
would check PhysicalAction - a Force. If it is below your treshold force
- then it sets that force to zero. And that's all you need.

This damping method maybe will work for quasi-static systems. But
certainly not where more spheres are _slowly_ moving. Because The forces
(probably) will accumulate until they reach the treshold value, and then
(maybe) you will have unrealistic explosion.

> Since currently I can only use the number of calculation steps?

Sorry, I don't understand this sentence at all :/ What is the use of
number of iterations?

--
Janek Kozicki                                                         |
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