# Knudsen number

The Knudsen number is defined as

$Kn\equiv \frac{\lambda}{L}$

where

• $\lambda$ is the mean free path of the molecules
• $L$ 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:

$Kn = \frac{Ma}{Re} \; \sqrt{ \frac{\pi \gamma}{2}}$

where $\gamma$ is the ratio of specific heats.