Discontinuity of the velocity distribution function in a rarefied gas around a convex body and the S layer at the bottom of the Knudsen layer

Discontinuity of the velocity distribution function in steady flows of a rarefied gas around a convex body is discussed, and its examples in an evaporating flow from a cylindrical condensed phase, in a slow uniform flow past a sphere, and in a flow induced around a sphere in a gas with a temperature gradient are presented. The same type of discontinuity is shown to be absent in a gas around a concave body. The S layer in the bottom of the Knudsen layer (Y. Sone, Phys. Fluids 16, 1422 (1973)) is attributed to this discontinuity, and its influence on the Knudsen layer and the slip boundary condition is discussed, with a numerical example.