# yade-users team mailing list archive

## Re: [Question #681232]: Sphere goes through the facet wall in Harmonic vibration

```Question #681232 on Yade changed:

Jan Stránský proposed the following answer:
Hello,

> Then the piston does harmonic vibration. I find when the vibration
amplitude goes to high, the spheres go through the piston, which is
constructed by facet. I want to know the reason and fix it.

There might be several issues:
a) the piston period is to short w.r.t O.dt that the piston simply "overjumps" the particles, not interacting with them at all
b) the stiffness/mass/timestep combination is such that the repulsive force acts so little time, has so liitle magnitude etc. that the piston is not able to "push away" the particles.
c) Linear contact law also does not help b) as for really high penetration, the force is "too low"

> In addition, the density is amplified by a factor of 1e9 to increase
the calculation speed.

by increasing density, you can get higher critical time step. BUT you also change the physics of the system (see point b above).
It always depends if and which values are acceptable. 1e9 * density sounds really too much..
If you want to get "realistic" results of a dynamic system, **in my opinion** this is not the way to go..

Somewhat less drastic scenario is to increase mass of some fraction (e.g. 5% of mass) of the smallest particles.
It increases time step (determined by the smallest particles), but preserves dynamic properties of the "most" of the system.

> And also I want to know how to define the word of "rigid" in yade. In my case, the box is rigid and the spheres are relatively soft.
> Does it make sense if i set the young's modulus of the box material as 100 times of that of the sphere material?

the stiffness of interaction is computed as a harmonic average of stiffness of individual bodies, something like
E=2*E1*E2/(E1+E2)
The limit for E2->infinity is E=2*E1. For E2=100*E1, E=1.98, so it could be "close enough" to rigid.

cheers
Jan

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