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Re: [lammps-users] Poisuille flow

# Re: [lammps-users] Poisuille flow

 From: m n Date: Fri, 6 Oct 2017 18:58:10 +0000 (UTC)

Dear Axel,

According to your last comment, in order to simulate a steady flow at a given velocity, I initialized the flow with a velocity command and added a small force, to compensate for the dissipation of energy through interaction with the walls:

velocity      flow set 1.0 0.0 0.0
fix    1 flow addforce 0.01 0.0 0.0

In order to define "temperature" for the flowing medium, I used "compute temp/com" command to exclude the kinetic energy of the center of mass of the flowing medium:

compute hot upper temp
compute flow argon temp/com
compute                   cold lower temp

Moreover, for controlling the temperature of the walls I have used the following commands:

fix hot upper nvt temp 150 150 0.01
fix cold lower nvt temp 90 90 0.01
fix nve flow nve

The temperature of the flow region decreased after I used the "compute temp/com" from 1250 to 840 but there are still some biases.

I would really appreciate if you could help me to figure it out.

Best,
Santiago

On Wednesday, October 4, 2017 6:27 PM, Axel Kohlmeyer <akohlmey@...24...> wrote:

On Wed, Oct 4, 2017 at 6:18 PM, m n wrote:
Thanks Axel. That's totally right. Is there any other way to simulate a flow between parallel plates? In other words, is it more reasonable to use the "velocity flow set V_x 0.0 0.0" command in order to prevent from the continuous increase in temperature?

​well, first of all, you have to be certain about how you define "temperature" for your flowing medium.
typically, you don't want the kinetic energy of the center of mass of your flowing medium included.
i.e. you either look only at temperature orthogonal to that direction or you subtract out the center of mass motion (cf. compute temp/partial and compute temp/com).

then you have to decide what is exactly the state you want to simulate. if you want a steady flow at a given velocity, you can initialize that flow with a velocity command, but you also need to add a small force, to compensate for the dissipation of energy through interaction with the walls. so you need to monitor the overall center of mass velocity of your flowing medium and tweak your settings to reach the desired steady state.

with the differently thermostatted walls, you have another variable to control. that will add/subtract kinetic energy to your medium. so you have to observe that as well and equilibrate your setup into a steady state for that as well.

this is no rocket science, but it also is not as straightforward as a simple homogeneous bulk system. it is important that you carefully characterize and observe what your system is doing and that you confirm what is happening is actually what you are aiming to simulate. your initial question indicates, that you approaching your simulations with the right attitude, you just have to understand, that you are looking at something much more complex, and thus you have to be correspondingly more careful and diligent in your observations and considerations.

axel.

On Wednesday, October 4, 2017 4:54 PM, Axel Kohlmeyer <akohlmey@...43...4...> wrote:

On Wed, Oct 4, 2017 at 5:47 PM, m n via lammps-users wrote:
Dear LAMMPS users,

I am trying to simulate argon flow between two parallel metallic plates with different constant temperatures. In order to simulate the argon flow, I use the "fix ID group-ID addforce f_x 0.0 0.0" but after applying the force, the temperature of  flow region increases dramatically. When there is no force applied on the argon particles, the temperature is reasonable.

I would really appreciate if somebody could help me to figure out this issue.

​trivial. the stronger the force you add, the more you accelerate your flowing particles, the more they heat up. simple as that.

axel.

Santiago

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Dr. Axel Kohlmeyer  akohlmey@...24...  http://goo.gl/1wk0
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.

--
Dr. Axel Kohlmeyer  akohlmey@...24...  http://goo.gl/1wk0
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.