Hi all,

I wanted to compare MD results for a confined fluid flow through two parallel plates to analytical solutions. I applied some interaction parameters between fluid and wall so that no-slip B.C is happen.

Now, Could you tell me how to relate the external force which is given by LAMMPS with the pressure gradient? As you know in analytical solution we have pressure gradient and fluid viscosity, and to compare MD results with exact solution, I need to related external force applied to each atom with pressure gradient through the channel.

Any helps would be really appreciated.

Thanks,

Omid

Hi all,

I wanted to compare MD results for a confined fluid flow through two

parallel plates to analytical solutions. I applied some interaction

parameters between fluid and wall so that no-slip B.C is happen.

Now, Could you tell me how to relate the external force which is given by

LAMMPS with the pressure gradient? As you know in analytical solution we

have pressure

"as you know" is a strong statement in this community. most people

probably do not know (and care) about continuum model solutions.

gradient and fluid viscosity, and to compare MD results with exact

solution, I need to related external force applied to each atom with

pressure gradient through the channel.

two comments on this statement:

- *both*, your analytical model and MD are computing *exact* solutions.

they are just different models with different resolution based on different

assumptions about the nature of matter.

- relating microscopic to macroscopic properties is not exactly a simple

thing because of the difference in representation and assumption of length

(and time) scales. i suspect you need to spend much more time to study the

literature describing how to do that. a starting point could be to review

the papers related to the atoms-to-continuum package (AtC) in LAMMPS and

specifically the fundamental publication or textbook knowledge used in it.

axel.

Axel, thanks for replying soon.

I want to only compare the results for a no-slip B.C problem with MD in non-dimensional form.

So the final results should be the same, and MD should have similar results as analytical if not that might not be a reliable method!

I have already worked with MD in LAMMPS so much, and validated results with previous studies. But, this time I want to validate MD results in a very basic example with analytical solution in pressure driven flow. My problem is to relate the applied gradient pressure along the channel to the external force term applied to each atom in MD-LAMMPS.

Do you know any primary MD study validated with analytical solutions in basic flows?

Omid

Axel, thanks for replying soon.

I want to only compare the results for a no-slip B.C problem with MD in

non-dimensional form.

So the final results should be the same, and MD should have similar

results as analytical if not that might not be a reliable method!

i disagree with that assessment.

I have already worked with MD in LAMMPS so much, and validated results with

previous studies. But, this time I want to validate MD results in a very

basic example with analytical solution in pressure driven flow. My problem

is to relate the applied gradient pressure along the channel to the

external force term applied to each atom in MD-LAMMPS.

you are basically just repeating what you stated before. thus i will save

you repeating my answer. just re-read my previous e-mail.

Do you know any primary MD study validated with analytical solutions in

basic flows?

that is something that *you* should have researched with a proper

literature search *before* even starting a simulation. this is just a

basic good practice in research to survey the available literature before

re-inventing wheels or repeating mistakes.

axel.

Yes, you are right that I should have checked primary steps in MD validation with analytical solutions (in continuum at least)

I started my research from validating my scripts results with MD papers, not anayltical ones.

But for a simple case of pressure driven flow in no-slip B.C and non-dimensional form in which continuum assumption might be correct, then MD results should be same as analytical. Am I right?

Yes, you are right that I should have checked primary steps in MD

validation with analytical solutions (in continuum at least)

I started my research from validating my scripts results with MD papers,

not anayltical ones.

But for a simple case of pressure driven flow in no-slip B.C and

non-dimensional form in which continuum assumption might be correct, then

MD results should be same as analytical. Am I right?

this is yet another variant of the same question and an ill-formed

question to boot. my initial answer also applies to this version of your

inquiry. before asking yet another version of the same question, please go

and do your homework and research in the published literature or suitable

text books how to properly relate microscopic behavior to macroscopic

expressions. i still have the impressions that you are underestimating how

different those two views of condensed matter are.

axel.