Recent Progress in Rarefied Gasdynamics

Abstract

S of research developments in the mechanics of rarefied gases through 1956 are contained in (1 and 2). The present paper thus will concentrate mostly on work in the field since that date, except for a few brief introductory remarks. The general field of rarefied gasdynamics is concerned with phenomena related to the molecular or noncontinuum nature of a gas which may occur at sufficiently low densities. The relative importance of such effects is determined by the ratio of the molecular mean path X to a characteristic dimension I of the field. It has been found convenient to distinguish between flows which are slightly rarefied (X/l ^ 0.01 — 0.1), moderately rarefied (X/l ^ 0.1 — 10) and highly rarefied (X/l > 10). These are usually referred to as transition flow and free molecule respectively. There is by now a rather extensive literature—mostly, but not entirely, experimental—dealing with aerodynamic force and heat characteristics of a variety of geometrical shapes immersed in a low density flow. Geometries so far investigated include: Flat plates, spheres, cylinders, cones, wedges, conecylinders, sphere-nosed cones, and some special developmental configurations. The data of this type available in 1956 are summarized in (3); more recent data are included in the third section of the present report. A number of additional experimental investigations have been carried out for situations of basic scientific interest, such as Couette flow, shock structure, and the dispersion and absorption of ultrasonic sound waves. This type of investigation is summarized in (2,4,5 and 6). The general conclusions to be drawn from these investigations are: 1 The flow regime is dominated by continuum viscous effects describable in terms of the normal NavierStokes formulation. The only important noncontinuum effect appears to be a slight modification in the boundary conditions for the gas past a solid surface, where slip velocity and temperature jump phenomena occur. These generally result in relatively small correction terms to be applied to standard continuum viscous results. 2 The free molecule flow regime seems to be quite satis-