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[Jan Stránský <question694664@xxxxxxxxxxxxxxxxxxxxx>]

Question #694664 on Yade changed:
https://answers.launchpad.net/yade/+question/694664

    Status: Open => Answered

Jan Stránský proposed the following answer:
Hello,

> I use this model for triaxial compression test
> the example figure could be find at [1]

please provide the code [2]

> I guessed it is that there was no contact detection between these tiny
particles and the top wall so that these particles passed through the
top wall.

it is the most reasonable explanation

> And I use GlobalStiffnessTimeStepper
> Then I set the timestepSafetyCoefficient

GlobalStiffnessTimeStepper, as the name suggest, set time step according
to stiffness, not particle sizes/velocities

> (1) The higher the timestepSafetyCoefficient, the more safe the dt is?
Or the lower the timestepSafetyCoefficient, the more safe the dt is?

GlobalStiffnessTimeStepper computes something, which is then multiplied by GlobalStiffnessTimeStepper to get actual dt [3].
So the lower value, the more safety.
But again, safety w.r.t. stiffness, not particle sizes/velocities. So for this problem, go another way than timestepSafetyCoefficient

> (2) Is there any other way to let the missed contacts being detected
correctly during the simulation?

of course. For example you can:
- set maximum wall velocity
- compute respective time step such that the maximum wall displacement during one time step is some fraction of tiny particles size
- set this time step as maxDt [4].

cheers
Jan

[2] https://www.yade-dem.org/wiki/Howtoask
[3] https://gitlab.com/yade-dev/trunk/-/blob/master/pkg/dem/GlobalStiffnessTimeStepper.cpp, search timestepSafetyCoefficient
[4] https://yade-dem.org/doc/yade.wrapper.html#yade.wrapper.GlobalStiffnessTimeStepper.maxDt

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