The Knudsen number K is usually defined for a gas as the ratio

(1)

where 1 is the *mean free path* and L is any macroscopic dimension of interest. The importance of this number is that, when K << 1, the gas behaves as a continuum fluid on the length scale L. The regime in which K << 1 is known as the hydrodynamic regime while that in which K >> 1 is known as the Knudsen regime. In the former, the gas obeys the *Navier-Stokes equations * of hydrodynamics while in the latter, *rarefied gas dynamics* apply.

#### REFERENCES

Truesdell, C. and Muncaster, R. G. (1980) *Fundamentals of Maxwell's Kinetic Theory of a Simple Monatomic Gas*. Academic Press. New York.