This part of the book concerns gasdynamics. One might think that the term “gasdynamics” could refer to any sort of flow of any sort of gas. However, by tradition, unless specifically stated otherwise, the terms gasdynamics or compressible flow refers to a relatively simple type of gas flow, affected only by pressure and flux, neither too dense nor too rare. A more precise definition appears in Chapter 2.

The treatment of gasdynamics found in Part I varies from the traditional gasdynamics treatment in several ways, due mainly to the demands of the numerical approximations studied later in this book, as opposed to the demands of the simple hand calculations studied in traditional gasdynamics texts. For example, traditional gasdynamics texts consider linearized potential flow approximations such as the the Prandtl–Glauert equation; while many people still use linear approximations, modern computing power has made them increasingly unnecessary. This book will not discuss linearized approximations. For another example, traditional gasdynamics texts focus mainly on steady two-dimensional flows, whereas this book focuses mainly on unsteady one-dimensional flows. These two model problems are equally difficult: Both model problems involve the same number of dependent and independent variables; and many solutions to one problem have an analogous solution in the other problem such as, for example, the steady two-dimensional expansion fan versus the unsteady one-dimensional expansion fan. The steady two-dimensional model problem has one major positive aspect: Many gas flows of practical interest are approximately steady and two dimensional. Unfortunately, the steady two-dimensional model problem has at least two critical cons.