Two relaxation time

Hi everyone;
Due to the small viscosity and density, the relaxation time of my model is small (0.51). I have read the previous discussion in this forum, and the suggestion is to use MRT (multi relaxation time) or TRT (two relaxation time). The TRT seems easier to me. However, the problem is in order to get the stability
(tau_s-0.5) x (tau_a-0.5)= 1/4
in that my tau_s = 0.51 which means tau_a = 25.5 . I am not sure that tau_a can reach to that value?
Has anyone had experience in TRT and could give me a suggestion ?
Thank you

Hi Nina,

not sure, but I think it can, but it worth trying and see (try to check in the book “The lattice Boltzmann method: principles and practice” 2016 Kruger et al). But my concern is about the reason why you say that you are bounded to tau_s = 0.51.

In this sentence, the “small” viscosity and “density” have to be interpreted as “in lattice units”, not in physical units. In other words: low density and viscosity in the physical system do not automatically force you to low lattice tau_s.
Can you explain better why you need such low tau_s?



Hi, I have developed the code for melting of phase change material. The code is running but after some time it becomes unstable. I think the problem is in unit conversion. Can anyone tell me how to go for unit conversion for thermophysical parameters like Cp, latent heat, gb and all. Thanks

Hi V_Kishan,

I am running into the same problem as you did. Have you managed to resolve your problem? If so, I’d very gladly like to hear how you did it.



I am also having this same problem on unit conversions. Let me know if any of you would like to meet to discuss.