Knudsen number
From CFD-Wiki
The Knudsen number is defined as
where
is the mean free path of the molecules
is the characteristic dimension
Knudsen number is a measure of the rarefaction of the flow. The various Flow regimes can be classifed based on Knudsen number.
The Knudsen number is related to the Mach number and the Reynolds number by the following relation:
where 
is the ratio of specific heats.
