[lammps-users] One question about the example poisseuille flow

Dear all,

In the examples–poisseuille flow, there are four sentences:
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
In my opinion,this means to fix the “lower”, and give the “upper” a pressure downwards. I am puzzled about it. Why can’t use these sentences:
velocity lower set 0.0 0.0 0.0
velocity upper set 3.0 0.0 0.0
fix 3 boundary setforce 0.0 0.0 0.0
instead of those?

Thank you !
kelly Lee

Because you have no fix aveforce which provides
the pressure downwards.

Steve

2008/10/4 楠 李 <[email protected]>: