Knudsen Number

The Knudsen Number is a dimensionless number defined as the ratio of the molecular mean free path length to a representative physical length scale. This length scale could be, for example, the radius of a body in a fluid.
The Knudsen Number helps determine whether statistical mechanics or the continuum mechanics formulation of fluid dynamics should be used to model a situation. If the Knudsen number is near of greater than one, the mean free path of a molecule is comparable to a length scale of the problem, and the continuum assumption of fluid mechanics is no longer a good approximation. In such cases, statistical methods should be used.
The following formula is used: `"Kn"=lambda/L`, where:

• `lambda` = mean free path
• `L` = representative physical length scale

References

Wikipedia (https://en.wikipedia.org/wiki/Knudsen_number)