measure force in periodic boundary

Asked by jamespaul on 2019-02-22

Hi,

For the periodic boundary, how does it measure the forces on each surface?

James

Question information

Language:
English Edit question
Status:
Solved
For:
Yade Edit question
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Solved by:
Jan Stránský
Solved:
2019-02-23
Last query:
2019-02-23
Last reply:
2019-02-23
Jan Stránský (honzik) said : #1

Hi James,

using periodic boundary conditions (I prefer to call it periodic contact detection), the sample is periodic and there is no surface..
You can compute average stress [1] and using periodic cell dimensions [2], you can extract something like "surface force":
###
stressTensor = getStress()
s_xx = stressTensor[0,0]
s_yy = stressTensor[1,1]
s_zz = stressTensor[2,2]
dx,dy,dz = O.cell.refSize
a_xx = dy*dz
a_yy = dx*dz
a_zz = dx*dy
f_xx = s_xx / a_xx
f_yy = s_yy / a_yy
f_zz = s_zz / a_zz
###

but **personally** I would use just the stress values..

cheers
Jan

[1] https://yade-dev.gitlab.io/trunk/yade.utils.html#yade._utils.getStress
[2] https://yade-dev.gitlab.io/trunk/yade.wrapper.html#cell

jamespaul (jamespauljames) said : #2

Thanks Jan,

> **personally** I would use just the stress values..

Do you means you would use triax.stress[]?

>A second question:

In Peri3dController[1] ,goal=(20e-4,-6e-4,0, -2e6,3e-4,2e6),stressMask=0b101100.
For a periodic cube, according to Newton's second law, the forces on the opposite sides should be the same.Why set six goals instead of three as PeriTriaxController.Did I misunderstand the meaning of these 6 values?

[1] https://github.com/yade/trunk/blob/master/examples/PeriodicBoundaries/peri3dController_example1.py

Best Jan Stránský (honzik) said : #3

> Do you means you would use triax.stress[]?

yes, for example.

> A second question:

next time, please open a new question for a new question.
Apart form 3 normal components, Peri3dController also controls 3 shear components, therefore in total there are 6 values.

cheers
Jan

jamespaul (jamespauljames) said : #4

Thanks Jan Stránský, that solved my question.