[ESPResSo-users] Simulation of fluid using LB-method

[Wink, Markus]

Dear all,

 

first of all hello. I am a novice in ESPResSo. At the moment I am trying to simulate a simple fluid confined by two plates and exposed to a constant force. My source code is inspired by “lb_planar.tcl” I found at espressomd on github. I aimed at simulating a system, consising of two plates, seperated by a distance of 50 micrometers, and a force acting on the fluid, so that the maximum velocity corresponds to 0.02 m/s. Thus, in SI units I have the following parameters:

channel width = 50 micrometers, T =300K, density of fluid = 1000 kg/m^3, kin. Kin. viscosity = 10^-6 m^2/s, force density = 64000 kg/m^2/s^2.

 

For the MD units I chose (inspired by the bachelor thesis of Georg Rempfer):
[x] = 1 micrometer, [E] = kbT= 4.21*10^-21J, [t] = 2.96 ns, [m] = 2,99*10^-26 kg (mass of one water molecule).

 

Using that I get for the parameters in MD-units:

Channel width = 50[x], density = 3.3*10^10 [m]/[x]^3, kin. Viscosity = 2.69*10^-3 [x]^2/[t], force = 15.5 [m]/[x]^2/[t]^2

 

The expected maximum velocity should be around 5.5*^10-5 [x]/[t], which corresponds to 0.02 m/s in SI units (T. Krueger et al., arXiv:1311.3650v1, formula 2.6).

Still two questions arise, which I could not answer yet and to which I did not find any answer neither in the mailing/archive/user’s manual nor in the tutorials, so probably one of you could help me (the source code one can find attached):

 

1) For the lbfluid command I set the frictional parameter to unity. Is that justified? As far as the user’s manual is concerned, this parameter couples particles/soft matter to the fluid. But I don’t have any particles other than water molecules, right? So, honestly I am not sure what to do with that parameter.

 

2) In my script, I integrated 1000 times with an integration step of 1000. I get a parabolic velocity profile, so far so good. Anyway, the maximum velocity is about 4.91*10^-5[x]/[t], corresponding only to 0.018 m/s. The maximum velocity saturated, so longer integration times did not help.

 

3) Even though, the velocity profile saturates, it takes pretty long (time scale of days). The saturation sets in at about 350 times the “integration 1000” command. This corresponds to a time scale of ms in Si units. Besides using OpenMpi or the GPU impementation, is there anyway to speeden up the simulation? For example by adjusting the parameter tau in the lbfluid command?

 

Thanks in advance for your help and I am looking forward to hearing from you.

 

Best Regards

Markus Wink

 

 

 

flow.tcl
Description: flow.tcl