Before beginning a discussion of the special effects brought about by the energy and momentum of radiation, we review ideal gas dynamics as it exists without these sources. We will define the variables we use and present the main equations that will be modified later.

A quite good introduction to fluid mechanics is the volume Fluid Mechanics in the Course of Theoretical Physics by Landau and Lifshitz (1959). This does not spend much time on the microscopic picture of fluids, but is very strong on the physical applications. The approach is entirely analytic. Mihalas and Mihalas (1984) describe kinetic theory in some detail, and the basis of viscosity, in addition to some of the basic results of gas dynamics. The chapters on viscous effects and relativistic flows are valuable.

Ideal fluid description: p, p, u, and e

A fluid is, as the name suggests, free to flow, which distinguishes it from an elastic solid. The solid can deform, but as it does stresses are produced that depend on the displacements. In a fluid the stress is primarily (i.e., apart from a small correction due to viscosity) an isotropic pressure, and this depends on the local temperature and density of the matter, and is independent of how far a parcel may have moved from its starting point. So density is the parameter that expresses how the kinematics will change the state of the matter. The density evolves as the fluid moves, and the volume occupied by a parcel of material changes.