about LBM with lattices unit

what are fifferents between computing the macro-flow and simulating micro flow when i use the lattices units.
For example i want simulate pipe flow use LBM,i think macro pipe flow and micro pipe flow are the same,so what are different between macro and micro? but in the class the teacher tell me macro and micro are different. who can tell me why?

Dear zhrleo,

The microflows limit implies that the continuity approach is not valid any more. That means your Knudsen number is not small, it is around unity. Therefore, the Navier-Stokes equation can’t simulate the microfluids and microflows.

You know also that LBM can restore Navier-Stokes equation in a small Knudsen number. But Navier-Stokes equation is not valid for microfluidics. It’s interesting to point out, that kinetic equation describes microflows behavior. However, the kinetic equation is valid for microflows, in BGK case upto Knudsen number around unity (there are some papers about it). MRT does better job for microflows.

Hopefully it will help,