# Modeling squeeze film effect in the free molecular flow regime on an axi-symmetric geometry

Hi,

This is my first time I’m attempting to run a molecular dynamics simulation, so please excuse me if I ask questions which might appear trivial to you.

I’m trying to simulate the squeeze film effect between two disks whose diameter is 1.9mm. The disks are separated by a distance of 30-500 nm. The gas in the gap between the disks is Argon, and the ambient (=initial) pressure is approximately 20mbar.
While the bottom disk is fixed, I’d like to apply a force/displacement to the top disk and measure its displacement/force acting on it, accordingly.

• I did notice that in most cases the boundary conditions that are used in LAMMPS are periodic bcs. In my case since there is flow in the radial direction, I can’t use these boundary conditions, and thus I was wondering how am I supposed to prescribe the ambient pressure on the circumference (diameter=1.9mm) of the control volume which represents the gap?

• How accurate is it to model the problem as a 2D problem, and does LAMMPS support axisymmetric boundary conditions?

Thanks very much,
Peleg

Hi,

This is my first time I’m attempting to run a molecular dynamics
simulation, so please excuse me if I ask questions which might appear
trivial to you.

simpler, already published, problems and learn how to reproduce them​. thus
you can build your skills and gather required practical experience before
starting a simulation on a new and complex problem. otherwise you are
setting yourself up to needlessly repeat many beginner mistakes and wasting
a lot of time, since you'll never know whether unexpected behavior is
caused by the physics of your problems or mistakes of your setup.

I’m trying to simulate the squeeze film effect between two disks whose
diameter is 1.9mm. The disks are separated by a distance of 30-500 nm. The
gas in the gap between the disks is Argon, and the ambient (=initial)
pressure is approximately 20mbar.
While the bottom disk is fixed, I’d like to apply a force/displacement to
the top disk and measure its displacement/force acting on it, accordingly.

​LAMMPS has options for each, you can use, e.g. fix move for displacement,
or fix addforce or fix aveforce for adding​ a force. similarly, you can
define groups of atoms and access the center of mass location or forces via
the corresponding variable functions or for other properties, set up a
suitable compute reduce.

- I did notice that in most cases the boundary conditions that are used in
LAMMPS are periodic bcs. In my case since there is flow in the radial
direction, I can’t use these boundary conditions, and thus I was wondering
how am I supposed to prescribe the ambient pressure on the circumference
(diameter=1.9mm) of the control volume which represents the gap?

​if you don't have periodic boundaries _and_ want to contain atoms, you'll
have to have some kind of wall. these can be realized with wall​ potentials
via a fix wall variant, or with explicit, immobilized particles.

- How accurate is it to model the problem as a 2D problem, and does LAMMPS
support axisymmetric boundary conditions?

​sufficient accuracy is in the eye of the beholder. LAMMPS supports the
boundary conditions mentioned on the boundary command documentation. no
more, no less.​

​please note, that most of your questions are not really LAMMPS questions,
but questions about how to realize your simulation and thus could be
applied to any MD code. therefore, you should keep in mind, that it is
_your_ job to ensure, that everything you are doing is justifiable and
​meaningful. people here may give an opinion, if it is easy, but they
cannot fill in for having competent personal advice and supervision, as
required if you are new to a field.

​axel.​

Dear Axel,

About the question regarding the ambient pressure boundary condition on the circumference, I would not want to contain atoms in the control volume as that boundary is open and molecules flow out of the control volume when the top disk is moving down. Is there a way in LAMMPS to prescribe ambient pressure at that opening?

Thanks so much!
Peleg

Dear Axel,

​too bad you are not actually _following_ the advice i've been giving you.

About the question regarding the ambient pressure boundary condition on
the circumference, I would not want to contain atoms in the control volume
as that boundary is open and molecules flow out of the control volume when
the top disk is moving down. Is there a way in LAMMPS to prescribe ambient
pressure at that opening?

what you are asking for is very tricky to get right. overall, ​you seem to
be looking at this from a conti​nuum mechanics and/or CFD point of view. in
my experience it is pretty much pointless to discuss such intricate details
before you even have a suitable understanding for MD basics including
relevant chunks statistical mechanics and thermodynamics.

axel.

Hi Axel,

Point taken. My background is indeed in continuum mechanics, but I’m eager to learn MD. Could you please point me to some good books which could help me ramp up?

Thanks,
Peleg