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

[Ulf Schiller]

Hi Markus,

apart from the inherent performance of the implementation, there are 
some possibilities to speed up the simulation by tweaking the 
parameters. Since the basic units are completely arbitrary, you can try 
to tune them such that the simulation runs faster while the physics 
stays the same. In your case, for example, you can change the spatial 
resolution (which you had in your script already I think), and you can 
change the time resolution (mass is probably irrelevant unless you are 
looking for inertial effects).

I typically prefer a length, viscosity, and temperature as the basic 
parameters of LB. The time scale is set by the viscosity (some time ago 
the input parameter for LB in Espresso was the dynamic viscosity - 
please double check).

The time unit is h = \nu_sim * a^2 / \nu_phys where a is the (physical) 
grid spacing, and \nu_sim is the simulation parameter and \nu_phys the 
physical value of the kinematic viscosity. So if you increase the 
simulation parameter for the viscosity, you increase the physical time 
unit. Consequently, you need a smaller number of steps to simulate the 
same physical time. In practice, you may have to deal with stability, in 
particular when your initial conditions generate oscillating transients. 
Also, you need to change other input parameters accordingly, to keep the 
Reynolds and Peclet number etc. the same.

While this type of tweaking may help a bit, eventually you can't beat 
the physics. Your system will need the same physical time to reach 
steady state regardless of the parameters.

Hope this helps,
Ulf

On 01/27/2014 11:54 AM, Wink, Markus wrote:
> 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 <http://arxiv.org/abs/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


--
Dr. Ulf D. Schiller                        Building 04.16, Room 3006
Institute of Complex Systems (ICS-2)       Phone:   +49 2461 61-6144
Forschungszentrum Jülich, Germany          Fax:     +49 2461 61-3180

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