Applicability of the theory of continuous media to unsteady viscous gas flows

The idealized medium conventionally assumed in gas dynamics possesses the property of continuity or compactness, whereas real gases are composed of particles discretely distributed in space. In the present paper, it is pointed out that for unsteady viscous gas flows, the Knudsen number (i.e., the ratio of the molecular mean free path to a certain linear scale defined by the nonstationarity of the flow) may figure as the continuity criterion, rather than the ratio of the mean time between molecular collisions to the characteristic time of the problem.