Overview

In this book we shall be concerned with the motion of compressible fluids, i.e. gases. They will be considered as continuous media or, simply, continua. This means that we assume that each point of the domain occupied by the fluid represents exactly one fluid particle. Moreover, we assume that functions describing the behaviour and motion of the fluid are continuous or even, according to the mathematical means used, sufficiently smooth. These assumptions are used mainly for the derivation of the equations and relations of fluid dynamics. In further considerations the strong assumptions are removed in the so-called weak formulations.

Depending on their compressibility and expansibility, fluids are divided into liquids (e.g. water, oil) and gases (e.g. air, steam). The characteristic feature of the gases is the substantial change of the volume under the influence of the pressure. This property is called the compressibility and the gases are therefore called compressible fluids. On the other hand, as a consequence of their expansibility, the gases fill up the whole space into which they can penetrate. Here we shall be concerned with the dynamics of gases only.

The behaviour of fluids is determined by intermolecular forces and the molecular mean free path characterizing heat phenomena. A fluid offers a relatively small resistance against forces causing it to change its shape. The property of resisting this change of shape is called viscosity. The viscosity force tries to reduce the difference between the velocities of neighbouring fluid layers. It...