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Re: [lammps-users] Errors in lubricate/poly ?
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Re: [lammps-users] Errors in lubricate/poly ?

From: "Heine, David R" <HeineDR@...233...>
Date: Wed, 28 Jun 2017 13:51:31 +0000



I didn’t get the attached pdf when the message was forwarded, so maybe sending it directly to me will help me understand the issue better. 


In general, I followed the equations in Chapter 9 to incorporate polydispersity into pair_lubricate.  As I was discussing with Tim Najuch, the text assumes you have particle A approaching another particle B, so being consistent with them, the separation distance is scaled by the radius of particle A.  In the lubricate implementation, the forces on A and B are calculated separately, hence the requirement that “newton” is set to off.  The grand resistance matrix approach in Chapter 11 that Tim was working on assumes the particles are approaching each other at the same speed, which may be a better approximation, but I don’t have a sense of how big the difference is when modeling things like highly filled systems as opposed to semi-dilute solutions.  If you haven’t already talked to Tim about the grand resistance matrix implementation, maybe that will address some of your issues.


I don’t see the issues about specific terms you mention below, but again, maybe I need the pdf attachment to see your explanation.  If you have a means of making this more generally applicable than what is provided in Kim and Karilla, then I am all in favor of it.


Best regards,




From: Steve Plimpton [mailto:sjplimp@...24...]
Sent: Wednesday, June 21, 2017 10:23 AM
To: Ranga Radhakrishnan; Heine, David R; Bolintineanu, Dan Stefan (-EXP)
Subject: Re: [lammps-users] Errors in lubricate/poly ?


I'm CCing Dan Bolintineanu and Dave Heine who can likely

answer these Qs.



On Wed, Jun 21, 2017 at 4:54 AM, Ranga Radhakrishnan <r.radhakrishnan@...652...> wrote:


I think that I have a few issues with the lubricate/poly implementation in LAMMPS based on my reading of Microhydrodynamics book by Kim and Karilla [1].

1) The gap-distance (h_sep) between the particles should be scaled by (radi+radj)/2 and not as radi, where radi, radj are the radii of the two particles.

2) The first term in the squeeze force seems to be missing a prefactor of 2.

3) \omega^\infty seems to have the wrong units. It is because "h_rate" has the units of length.

4) The pump term is also incorrect for particles of different sizes. Briefly, specific cases of calculation of torques in Ref. [1] cannot be used to write down a generalized version.

Please look at the attached pdf for a more detailed explanation on why I raised these concerns, and how to implement a "corrected" lubrication force if you agree with my concerns. Just to be clear, I have looked at previous messages in the mailing list before I send this message, and I don't think any of the previous messages have answered my concerns.



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