Doubt on the boundary condition settings in the example Poiseuille flow

Attached is an input script you can find at lammps-9Dec14/examples/flow/in.flow.pois, and in it there is a piece of commands defining the boundary conditions as below:

Poiseuille flow

velocity boundary set 0.0 0.0 0.0
fix 3 lower setforce 0.0 0.0 0.0
fix 4 upper setforce 0.0 NULL 0.0
fix 5 upper aveforce 0.0 -1.0 0.0
fix 6 flow addforce 0.5 0.0 0.0
fix 7 all enforce2d


My questions are:

(1) I think the upper and lower boundaries should be both set as “no-slip” zero-velocity, as the force for should also be “0.0 0.0 0.0”. However, as you see, the upper bound is prescribed to an averaged force “0.0 -1.0 0.0”. I have no idea why such settings is given here, though there should be some reason. Any comment?

(2) How can I post-process the dump file in order to plot the velocity profile? Here I want to compare the numerical solution with the analytical solution.

Thanks for shedding any light!

Best regard

in.flow.pois (1.46 KB)

(1) This force (on the upper boundary atoms) is orthogonal to the flow (i.e. slip) direction. Think about what that may or may not be enforcing.

(2) Look at the documentation for averaging fixes.

Thanks for the reply. I got to understand the basics through reading and trying.

Also, may I ask for one more pertinent questions? The real purpose of learning LAMMPS in my case is to use DPD for modeling fluid flow, like in the very first case – Poiseulle Flow. However, when I substituted the original LJ pair style & coefficients with DPD, I observed the flow particles “fly” out of walls. Indeed, I have found similar post requiring the proper setups of no-slip wall boundary for Poiseulle flow in the case of DPD simulation, but there is no answer in this user list (see below)

http://lammps.sandia.gov/threads/msg01712.html

So it is possible who is experienced to give an exemplary input script for it?

Thanks for any help on my question.

Best regard,

Yidong