he Knudsen Number represent the ratio between the mean-free-path of the molecules (lambda) and the problem characteristic length (L):
Kn = lambda/L
This number is used in fluid dynamics to define the limits of the continuum approximation. For Kn<<1 a fluid can be assume to be a continuum, while at Kn close to 1 or larger we have free molecular flow. Also, in the range of values 0.01<Kn<0.1 we have the slip regime, where the fluid at the wall has a different velocity than the solid.
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