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Re: [Question #698365]: non zero incident velocity on sticking contact


Question #698365 on Yade changed:

    Status: Open => Answered

Jan Stránský proposed the following answer:
> I did not think it would be helpful in this case, since this is about
output. Here it is, modified from Bruno's triax example:

such code is indeed not much helpful :-(
What I asked is a MWE [1], M=minimal.
For this case it could be (by words):
I have these two spheres, I set this and that velocities and run one step. Velocities are this and that, forces are this and that, incidentVel is this but I expect that or zero.

> 'non sliding' or 'sticking' contacts ... are for me the ones ...

a "naive" definition could be what? Fs<<<Fn? Fn<<<Fs?

> the tangent of the friction radius

tangent of friction angle?

> The relative velocity I get from the body velocities is exactly the
same as the one returned by incidentVel.

Thanks for clarification.

> I would expect no sliding at such a contact.
> For contacts where I would expect no relative velocity, it is of the order of magnitude of the velocities of the bodies, here 10^{-3}. I would expect it to be significantly smaller, even if not zero.

Why do you expect this?

In Yade, there are a few cases where you can force no sliding:
- clumps (rigid body set of a few particles)
- fixing ("blocking DOFs") of certain particles and prescribe their motion such that they move like rigid body

Otherwise, it is not possible to force two particles to "stick" or "no sliding".
And actually it is even not reasonable.
Image two "sticking" particles A and B. Now a particle C impacts to B, which would normally make it rotate.
Now should B rotate due to the impact? Should it do nothing doe to the sticking with A? ...

One of the basic concepts of Yade is the "simulation loop" [2]. In this case there are important the blue and the green part.
1) Based on position and velocities, forces are computed.
2) Based on forces and current velocity and position, new velocity and position is computed
1) ...
2) ...

So one interaction only partially influences its bodies motion. Body motion is a result of many interactions.
Maybe this may be source of the discrepancy of expectation and results?

Also, the "sticking" contacts according to your "ratio definition" is no
special from the loop point of view, just the forces has some different
values than in other cases. No "simulation loop" reason for the contact
to be non sliding..


[1] https://www.yade-dem.org/wiki/Howtoask
[2] https://yade-dem.org/doc/introduction.html#function-components

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