In the present book we shall deal only with specific phenomena of gas dynamics, i.e., such that have no analogies in the mechanics of an incompressible liquid. We shall not dwell on those subjects in which gas dynamics and the consideration of compressibility give only slight correction for the conventional formulas of hydrodynamics of an incompressible liquid. The emphasis in the present book will be on the careful definition of the fundamentals of gas dynamics, of the fundamental laws, and of the methods for solving the simplest problems, rather than on the computational methods of gas dynamics, the methods of numerical integration of complex two- and three-dimensional flows, etc. We shall proceed here from the simple to the complex, rather than from general problems to particular ones. Instead of writing first the equations of gas dynamics in their most general form (taking into consideration all the factors), searching for general solutions and then, by simplifying these solutions, going on to the particular solution of simple cases, we shall solve simple, elementary problems that describe certain aspects of some phenomena, and then, by means of these individual partial solutions piece together the solution of more complex problems.
We can outline the following, fundamental fields of application of gas dynamics. The first, which today is the better known and more developed one, comprises problems of flow around bodies moving at great speeds. This involves, first of all, the corrections in ordinary formulas of resistance and lift for bodies moving at subsonic speeds, i.e., corrections that are already applicable to contemporary aviation. A radical change in flow around bodies occurs when we deal with velocities exceeding the speed of sound. These speeds are involved in ballistics, i.e., the science of the motion of missiles and projectiles, and also in the study of rocket aircraft of the near future.
This application of gas dynamics to the problem of the motion of a body in a gas at speeds of the order of the speed of sound or exceeding it is dealt with in detail in text books, hence we shall deal with it only marginally here.
The second, extremely important field is that of the motion of a gas in ducts, such
