[lammps-users] viscosity calculation

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1

Hi all

this is in reference to a mail in the LAMMPS archives titled Thermal Conductivity Calculation usingGreen-Kubo Relations
( http://sourceforge.net/mailarchive/message.php?msg_id=33ea1b0a0904170737m6f8fd8abt53fbe3ce3572c2e4%40mail.gmail.com)
i came across a sentence in the mail
“1) Because GK simulations are done with the system in equilibrium, there is no imposed driving force and so the system is always in the linear-response regime.”
my doubt is regarding calculating viscosity for a fluid flowing through nanotubes. which method in LAMMPS is suitable for this problem since in this case there is a driving force for the flow?

thanking you in advance

unni

2

You can use a non-equil method like SLLOD for
viscosity of sheared flows. See fix nvt/sllod and fix deform.

Steve

3

Are you driving the flow through the tube? If so, linear response could be a bad assumption - I think the fluid is going to be sheared as it is driven past the tube walls. If it is in the linear response regime, why even bother with the flow (NEMD)? Just do an equilibrium simulation and use GK. If I was doing this, I would do it all post processing. Dump positions and velocities, and write your own code to analyze the momentum flux.
Matt

4

hi
thank you for spending your valuable time for me.
yes…its pressure driven flow through the tube…so i think i will have to go for NEMD…
but in some of the papers in literature they have gone for GK for the same problem…this is again confusing me…
can you please give your suggestions on this?
thanking you in advance

unni

5

If you are assuming linear response, you don't need to do an NEMD simulation. Why complicate it? Just do a second simulation without the flow and use GK. Not having seen the papers you are talking about I can only guess, but I would imagine if they are using GK in a flow they would need to adjust for the contribution of the translation of the center of mass to the kinetic part of the stress tensor. LAMMPS can do this through thermo_modify. You could just follow their formulation. But again, I wouldn't bother with an NEMD simulation. By using GK you are making the assumption that the response of the system is the same as that of an equilibrium system - that is, linear response. The flow is an unnecessary complication. The flow is going to heat the system, and the way you thermostat will alter the viscosity.
Matt