Shock Waves in Compressible Flow: An Outline of the Phenomena Occurring in the Trans‐Sonic Speed Range

THE earlier classical treatises on aerodynamics concerned themselves with the properties of incompressible fluids. The theory developed on this basis gave an excellent theoretical background to the aeronautical engineer and made possible a scientific approach to the problems of aircraft flight. With the steady increase of aircraft speed, however, it soon became evident that the theory would have to be extended to take compressibility into account. One important result, brought out by Glauert's analysis, was the modification of the flow pattern with increasing Mach number. A more striking divergence of compressible from incompressible flow, first encountered at near sonic speeds, is the occurrence of shock waves. A shock wave, in the specialized aeronautical sense, is a pressure impulse travelling through the flow causing a sudden transition from supersonic to subsonic speeds (normal to the wave front) with an attendant increase in pressure and temperature. A brief statement of this sort, however, is of little or no value in giving an idea of the physical nature of the phenomenon. A considerable amount of attention is now focused on the repercussions of shock waves on aeroplane design. It is far easier to understand these design trends if one has a good grasp of the fundamentals underlying the problem. This article sets out to give a brief survey of these fundamentals. It is not easy also to give a complete physical picture of a shock wave but at least a discussion of their formation, propagation, etc. goes a long way towards clarifying one's ideas.