# Kinematic viscosity

Hi everyone;
I just have started using LBM modelling. By reading the code examples in Palabos, I saw some examples using the kinematic viscosity is 0.001 (I found it in example externalFlowAroundObstacle). However, normally, the viscosity of water and gas are 1E-6 and 1E-5. So I am wondering that does the author intentionally use the small value or it is just a random value.
Thank you

Hello,

as with all CFD what is important is the value of the non-dimensional quantities. Here we are giving examples which are not necessarily based on any “real life” situations.

Hi Nina

For air (1E-5), it also runs good in LBM, but for so high Reynolds number flow, you have to use a turbulence model, for example Smagorinsky SGS model. I have tried the SmagorinskyMRTdynamics for air (set nu=1.5E-5), and the result is good.

best wishes,

steed188

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Hi steed188
You tried the SmagorinskyMRTdynamics for air , but I want to know whether the model is incompressible? And the number of grid in a unit?

best wishes,
Lin

Hi Lin,
As you know LBM is a compressible method, therefore I think it is still weakly-compressible when addressing the low speed airflow problem, regardless of SGS model dynamics. The density still changes during the simulation, but you can control it varies in a small range so we see it as imcompressible.

best wishes,
steed188

Hi steed188
Thanks for your reply. I tried the SmagorinskyIncBGKdynamics and SmagorinskyBGKdynamics for air , but two results of force are difference and computational convergences are different(when Re are beyond 800, the number of grid is large and and divergence of calculation results are easier in SmagorinskyIncBGKdynamics)(SmagorinskyIncBGKdynamics is easier to diverge and need more grids refinement than SmagorinskyBGKdynamics),so I think we need to think about the compressibility for air.
best wishes,
Lin