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The Knudsen number (Kn) is a dimensionless number defined as the ratio of the molecular mean free path length to a characteristic physical length scale. It is used to determine whether statistical mechanics or the continuum mechanics formulation of fluid dynamics to be used in any application. If the Knudsen number is near or greater than one, the mean free path of a molecule is comparable to a length scale of the problem and continuum assumption is no longer valid. In this case statistical methods must be used. On the other side if Knudsen number is much less than one, Continuum assumption is valid and Fluent can be used to simulate the problems of those length scales.