Re: [ESPResSo-users] Lattice Boltzmann External Force

[Georg Rempfer]

Hello Wolfgang,

thank you for your effort. Owen Hickey and I were actually working on 
tracking down and fixing this bug yesterday. We agree with you on the 
fact that it is not a simple discretisation artifact since it does not 
occur in the gpu implementation.

Also as far as I understand the LB populations are accessible from the 
TCL level via the "lbnode x y z print pop" command in the CPU 
implementation. Unfortunately the GPU implementation does not contain 
this feature.

Dominic (if you are on this mailing list): I wanted to ask you anyways 
whether you could implement that for us. Is this possible or would it 
take too much time?

Greetings,
Georg Rempfer


Am 09.02.2012 14:28, schrieb Wolfgang Riefler:
> Hello everyone.
>
> First of all thanks for all your quick answers.
>
> I probably should have said before that the offset I was getting was in
> the range of many times the maximum velocity (a maximum velocity of 0.04
> had an offset of 0.5), so I don't think its because of the discrete
> boundaries. Just to be sure I checked the simulation again for a very
> small viscosity and I still get the offset.
>
> At the moment I am working with the version 3.0.2.
>
> Coming back to the original problem, it was actually the PhD thesis of
> Ulf "Thermal fluctuations and boundary conditions in the lattice
> Boltzmann method" that caught my eye and made me change the source code,
> since it says in equation (4.65):
>
>                                j = j' + 0.5 * g * tau
>
> where j is the hydrodynamic momentum density, g is the volumetric force
> and tau the lattice boltzmann time step.
>
> Implementing this line, I get a much better result, with only a little
> offset, which is probably due to the discrete boundary.
>
> Thanks again for your help,
>
> Wolfgang