# Poiseuille flow by LAMMPS [closed]

I am trying to model 3D Poiseuille flow of an argon gas confined between two parallel platinum plates by using LAMMPS. I have one reference paper, as given in [1] below. I built my platinum structure correctly and I placed Ar atoms and run my simulation. At the end of simulation, I checked the density and it was totally wrong. I suspect that, I am not able to build sufficient number of atoms in my model. I try to use equations given below to obtain number of atoms but I couldn't decide the number of Ar atoms in my model, still. So, I need your help.

In the paper, they obtain the diameter of atoms by using the Knudsen number. After that I divided this into 2 to get the radius, then I obtained the lattice parameter by $$2 \sqrt2 r$$. Then, I build argon as lattice fcc a. However, the final result and density is not correct. Therefore, I suspect that, the number of atoms in my system is wrong.

So, my question is, how I can obtain lattice constant from the Kn number and obtain the correct density, for instance, for the Kn = 0.1 case in the reference paper [1] ?

• +1. Thank you for asking your question here, and welcome to our site!!! We hope to see much more of you here! Also, a recentl LAMMPS question got an answer very quickly, much because they provided an input file on GitHub, are you able to do that too? mattermodeling.stackexchange.com/q/1380/5 Jun 27 '20 at 22:01
• I have used the input file shared here: lammps.sandia.gov/threads/msg77307.html This input file ends up a simulation of two times of the velocities given in the reference paper I shared. I try to find the error here. I suspect that number of Argon atoms might be wrong. Jun 27 '20 at 22:17
• I see, so someone else asked a question 2 years ago on the LAMMPS mailing list, about this same type of calculation? Jun 27 '20 at 22:20
• Yes, you are right. I wrote my own Lammps input file and I couldn't obtain the density, velocity etc. Then, I decided to use this script. However, this input file also gives wrong results. I wonder why, that is why I analyze all the details of the script. Jun 27 '20 at 22:23
• I’m voting to close this question because: Abandoned since 2 days after the question was asked, which was 13 days ago. Can re-open if the user comes back to answer Magic_Number's question, or if someone comments saying they want to answer it. Jul 11 '20 at 5:49