# add force to atoms in one specified region and periodic setup

Dear all,

I have 2 problems using Lammps as follows:

1. There r 2 regions in my simulation, one up and one down, close to each other. The 2 regions are full of fluid particles moving randomly and may move to the other region back and forth. I want to add force to fluid atoms in the upper region all through the simulation. The idea is :" When the atoms move into upper region, they should receive this force; and when they leave the upper region, the force is gone. These forced atoms contains 3 types: may come from lower region moving into upper region; or may come from upper region moving to lower region but back to upper region; never leave the upper region.

2. In the same domain, the periodic flow directions is on X axis; But on Y axis, the lower side of lower region is fixed; But the upper side of lower region is periodic; While the lower side of upper region, which is close to upper side of lower region, is periodic. Namely, the 2 regions exchange atoms through the border they share. And the upper side of upper region is periodic, which means the lost atoms from upper region will be pull back to the upper region during simulation.

How to realize above idea using Lammps? Please see attachment for a picture view of the problem.

I really appreciate your help! Have a good day!

Bests,

Eric

Dear all,

I have 2 problems using Lammps as follows:

1. There r 2 regions in my simulation, one up and one down, close to each other. The 2 regions are full of fluid particles moving randomly and may move to the other region back and forth. I want to add force to fluid atoms in the upper region all through the simulation. The idea is :" When the atoms move into upper region, they should receive this force; and when they leave the upper region, the force is gone. These forced atoms contains 3 types: may come from lower region moving into upper region; or may come from upper region moving to lower region but back to upper region; never leave the upper region.

this one is *really* obvious:
you need to use the region option to fix addforce.

2. In the same domain, the periodic flow directions is on X axis; But on Y axis, the lower side of lower region is fixed; But the upper side of lower region is periodic; While the lower side of upper region, which is close to upper side of lower region, is periodic. Namely, the 2 regions exchange atoms through the border they share. And the upper side of upper region is periodic, which means the lost atoms from upper region will be pull back to the upper region during simulation.

i don't see a problem here.

How to realize above idea using Lammps? Please see attachment for a picture view of the problem.

this is straightforward to set up. just follow the
documentation and do it step by step.

what you seem to be asking is that somebody
else should be doing *your* work for you. this
is very unlikely to happen.

axel.