It is a common approach in physics to try to simplify representation of material properties as much as possible. It is called modeling. For example, an ideal fluid means a fluid with vanishing diffusion coefficients of both viscosity and thermal conduction. This is useful to represent fluid motions in which transfer phenomena due to molecular motion can be neglected. This enables separation of macroscopic motions from microscopic irreversible processes due to molecular thermal motion. In an ideal fluid, the motion is adiabatic and the stress tensor is isotropic, i.e. stress force being normal to any surface element. As a result, many useful expressions can be given concisely (Chapters 5 8).

An ideal gas, or equivalently a perfect gas, is one which is governed by the equation of state (for a unit mass):

where R = 8 .314 10 ⁷ [erg/deg mol] is the gas constant, and p, ?, T, V are respectively the pressure, density, temperature and specific volume ( V = 1 / ?) of the gas, and ? _m is the molecular mass. In an ideal gas, the interaction force between molecules is so weak to be negligible.

For a gram-molecule of an ideal gas,

The sound speed c in an ideal gas is given by

where ? = C _p /C _v is the specific heat ratio, called the adiabatic index. C _p and C _v are specific...