Re: [ESPResSo-devel] LB fluid with variable viscosity

[Michael Kuron]

Hi Ivan,

to implement this, you would need a field that stores all four
relaxation times individually (shear, bulk, ghost even, ghost odd) for
each cell. This of course increases the LB's memory footprint and
reading from a field is slower than accessing a constant, so this code
should only be used when variable viscosity is actually needed.

I don't know how well LB with non-uniform viscosity actually works. All
applications that I have seen so far only do that to make the algorithm
more stable in certain extreme situations, but I don't know whether it
produces any reasonable physics. So unless you already know that it
works for object-in-fluid, I'd be careful with non-constant
viscosities. The one thing that comes to my mind is Geier et al.'s
claim that "[c]hanging the relaxation rates locally usually results in
non-constant transport coefficients" (
https://doi.org/10.1016/j.camwa.2015.05.001), though I don't know how
bad that is. Since you don't want transport between your two phases of
different viscosity, you might actually be fine. In Shan-Chen-like
multiphase models, different viscosities are commonly used, but they do
something more advanced at the interface.

Regards,
Michael

On Tue, 2018-11-20 at 16:55 +0100, Ivan Cimrak wrote:
> Dear developers,
> 
> I plan to implement the possibility of variable viscosity in LB
> fluid. We need this feature in the object-in-fluid module, where the
> inner part of a flowing object has different viscosity than the outer
> part.
> 
> I would like to discuss this issue with someone that is familiar with
> LB implementation in ESPResSo. Could anyone give me hint whom to ask?
> 
> 
> Thanks, 
> Ivan Cimrak