The Knudsen number, often denoted as Kn, is a dimensionless quantity used in fluid dynamics and gas dynamics to characterize the relative importance of molecular mean free path length to a characteristic length scale of the system. It is named after the Danish physicist Martin Knudsen.
The Knudsen number is defined as the ratio of the molecular mean free path (λ) to a characteristic length scale (L) of the flow:
where:
- λ is the average distance a molecule travels between collisions.
- L is a characteristic length scale of the system, such as a characteristic size of an object or a characteristic length of a flow passage.