Dear all,

I have a few questions regarding granular reduced units in lammps, and

their relation to si units.

* I understand that the length unit is the diameter d of the grains,

that can be specified in the fix pour command. Is that correct?

* I understand that the mass unit is the mass of one grain m = dens *

(pi/6) * d^3. In that case, the grain mass can be set to one by

specifying a density of 6/pi ~ 1.91 in the fix pour command. Is that

correct?

* But for time I'm confused since their is no obviously defined energy

scale as for LJ units. Time appears when Newton's law is applied,

namely acceleration is set to force/mass. Then if one refers to the

gravity force g/m, the time unit will be sqrt(d/g); but if one

considers the pair forces, then the time unit will be defined as

sqrt(rho*d^3/Kn). One can also define the time unit through the

inverse of gamma_n...

My concerns are that:

1. these different time scales evolve differently with the grain size.

Is one of them more relevant to granular MD simulations?

2. I don't know which one to use to translate the results into real

units.What would be the smart way to do this?

I'm asking all these questions because my student is facing strange

problems (in particular a huge increase of computing times) when the

grain size is multiplied by two, and every other paramter is hopefully

rescaled accordingly -using a time unit of sqrt(d/g).

Looking forward to your help,

Best regards,

Laurent Joly