system Damping(wall)

Dear Lammps Users:

I’m facing up with a problem. For my system, it’s

Orthoganal Cubic Box, with x,y periodic and Z shrink-wrapped
I constructed a rough wall(particles) my self in xy plane(normal to z direction) both at top and bottom.
The box is fulfilled with fluid particle.
For the initial condition I have set, It will lead to expand the box (only in z direction because x,y are periodic)
Now I need to constraint the wall movement, because the fluid inside is pushing the wall hardly that the wall just blow up.

of cause, I can apply external force onto the wall to avoid blow up. But what I want is to reach a equilibrium of the whole system, in other words, I want reach a final state that average force on wall is fluctuating about 0.

I used to do it with my in-house code, which is a bit like “Fix Spring/self” command. but I need to update the initial position of the wall particles. As I go through the manual, I find that for the command “Fix Spring/self” the initial position of each particle is set as the input value and never updated. thus cannot satisfied my need.

Can anyone give possible solution for the problem

Thanks A Lot

See the examples/flow/in.poiss script
which adjusts the positon of an upper wall of
particles due to the pressure of a fluid.

Fix aveforce is what is used.


Hi, Steve

Thanks to your answer.

Fix aveforce is exactly one way that should work, but I have to calculate the initial repulsion force on wall, then given an equal external force to offset the expanding. By using the variable function, external force can gradually decrease to 0, that is actually the equilibrium state of my system.

Thank you
I will try it out, and I am still expecting if there is other way to do it