odd values for output force - RAW_WITH_POS_ID

Asked by Kahlil Fredrick Cui

Hello all!

I am trying to output the forces between two contacting particles. As per the tutorial, I used the vector interaction field saver with the data type set to RAW_WITH_POS_ID. However, I noticed that some of the forces are 0. How can this be when the contact positions are recorded? And is there a particular reason between the arrangement of particle IDs, or is it random?

Thanks in advance!

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Dion Weatherley
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Best Dion Weatherley (d-weatherley) said :

Hi Kahlil,

Interaction field savers output the current data associated with all actual and potential particle pair interactions. For unbonded interactions (such as NRotFrictionPrms, NRotElasticPrms, etc.) the list of interactions at the current timestep will include particle pairs that are touching, as well as pairs that as likely to touch prior to the next update time of the Neighbour Table; as determined by the verletDist parameter of LsmMpi.initNeighbourSearch(..).

For a pair of particles that are close to one another but not touching, the interaction force is zero but the contact point is calculated as the midpoint between the centres of the two particles. Thus, when using the RAW_WITH_POS_ID output format, these pairs will have a zero force but non-zero "contact point".

Regarding the arrangement of particle IDs, it is best to assume it is random although it is not deliberately randomised. The order in which contacts are output, depends upon the order in which these were identified during a SearchNeighbours() step, as well as the spatial subdomain in which the pair of particles resides. Consequently, one may find that the order of the contacts is the same for multiple successive timesteps but it is not guaranteed to remain so. Hence it is best that you assume the contacts are listed in no particular order.

I hope this helps.



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Kahlil Fredrick Cui (physicsman98) said :

Ahhhhhh so it is safe to assume then that these zero contact forces can be discarded from the total contact force calculations right?
Thank you Dion.

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Kahlil Fredrick Cui (physicsman98) said :

Thanks Dion Weatherley, that solved my question.

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Dion Weatherley (d-weatherley) said :

Yes Kahlil.

It is fine to disregard zero contact pair forces obtained using RAW_WITH_POS_ID fieldsavers.



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Kahlil Fredrick Cui (physicsman98) said :

Hi again!

I would like to raise a related question. if the arrangement or particle Id's are somewhat 'random', can we determine which among the pair exerts the force? From whose vantage point is the force direction determined?

Thank you!