relaxation parameter

Hi dear all:
I am just a beginner for LB. My aim is to use LB to simulate some basic cases for prelimilary study, no need to fullly understand the whole mechanism. After trying some code, a problem confused me about relaxation parameter. Say, what’s the exact use for this parameter and how to evaluate it. Can anyone help me? Thanks!

the relaxation time (tau) or relaxation parameter (omega = 1/tau) are directly related to the kinematic viscosity nu of the fluid: nu = 1/3(tau-1/2). nu is measured in units of the lattice (the distance between two cells, and the time interval of one iteration are 1), and not in physical units.

Hello Everybody,
We know, the relaxation time (tau) is related to the kinematic viscosity (nu) of the fluid for momentum equation. But for solid particle (kinematic viscosity is not known but thermal diffusivity is known) and we want to solve the momentum equation for solid particle (Cu, Cuo etc).How can we calculate tau? I saw some papers they use thermal diffusivity for tau_energy. Can I use tau_energy both for momentum and energy equation for solid particle? As a new of this field, Please give me some tips or suggestions. Thank you very much and Happy New Year to all.

Yours Truly