
From:  Wink, Markus 
Subject:  [ESPResSousers] Simulation of fluid using LBmethod 
Date:  Mon, 27 Jan 2014 10:54:24 +0000 
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): Using that I get for the parameters in MDunits: 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*^105 [x]/[t], which corresponds to 0.02 m/s in SI units (T. Krueger
et al., arXiv:1311.3650v1, formula 2.6).
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
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