[BUG] Inconsistency using fix wall/gran with the granular pairstyle about lammps HOT 3 OPEN

Hi Kevin,
Thanks for the very detailed description.

In short, I agree with everything you pointed out, especially that the solution (short-term, and arguably even long term) is a clarification in the documentation.

Slightly longer answer: the original intent/philosophy was to have material properties (Young's modulus, etc.) not necessarily associated with particle types, but rather associated with pairwise interactions, but use mixing rules based on Hertz theory when needed/unspecified. This is partly for consistency with how all other pair styles work in LAMMPS, and partly for additional flexibility to the user. So, with pair coeff, the intent is to allow the user to manually override the mixing rules for different types, e.g. pair coeff 1 2 would override the mixing of the type 1 and type 2 coefficients, even though that may not be consistent with the effective modulus from Hertz theory. For walls, the idea is similar, where the modulus is not the modulus of the wall, but the effective modulus associated with the interaction of the wall and the particle specified in the wall fix. As you correctly point out, if the effective modulus associated with Hertz contact is desired, the user can simply compute this for a given combination of a wall material modulus and particle type, and use multiple fix wall/gran commands for different particle types. While that can be slightly clunky in a script, it would not incur a noticeable performance penalty (and personally I've never needed more that 2 or 3 particle types in a granular simulation).

Please let me know if you have a specific suggestion about the rewording for the doc page, otherwise I can come up with something to clarify that the wall coefficients are associated with the wall-particle interaction rather than the material of the wall.

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Hi Dan,

I see the intent — that makes sense. A clarification of the doc page would be the best solution then. I had a go at drafting an addition to the doc page, though it may be a little wordy. Please edit in any way you see fit, of course:

When fstyle granular is specified, the associated fstyle_params are taken as those for a wall–particle interaction. For example, for the hertz/material normal contact model with $E$ = 960 and $\nu$ = 0.2, the effective Young’s modulus for a wall–particle interaction is computed as $\frac{960}{2(1-0.2^2)}$ = 500. A mixing rule is not applied which implicitly takes the particles' properties into consideration.

If the simulation contains particles with different Young's moduli, and the correct effective moduli for wall–particle interactions are sought, the user may define multiple fix wall/gran commands: one (or more) for each atom type. The fix wall/gran command(s) corresponding to each atom type would be specified with the Young's modulus that would yield the desired wall–particle effective Young's modulus.

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Hi both,

Just fell on this issue while noticing the same problem as @kjhanley. Altough maybe obvious when stated, I think this:

As you correctly point out, if the effective modulus associated with Hertz contact is desired, the user can simply compute this for a given combination of a wall material modulus and particle type, and use multiple fix wall/gran commands for different particle types.

probably deserved to be written as well, especially for new users of lammps as myself.

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