Knudsen number
= Knudsen number
{c}
{title2=$\mathrm{Kn}=\ell_{\mathrm{mfp}}/L$}
The ratio of the <mean free path> to a macroscopic length compares collision and transport scales. Small Knudsen number motivates the <hydrodynamic limit of the Boltzmann equation>; large values favor a direct kinetic description. The scaling of time, velocity and space must also be stated when using it as a coefficient in a kinetic equation.