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KNUDSEN NUMBER

DOI: 10.1615/AtoZ.k.knudsen_number

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.

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