While the velocity field V is the most important fluid property, it interacts closely with the themodynamic properties of a fluid. The three most common such properties are:
These three are constant companions of the velocity vector in flow analyses. Four other themodynamic properties become important when work, heat, and energy balances are treated:
All nine of these quantities are true thermodynamic properties that are determined by the thermodyanmic condition or state of the fluid. For example, for a single-phase substance such as water or oxygen, two basic properties such as pressure and temperature are sufficient to fix the value of all the others:
ρ = ρ( p, T ) h = h( p, T ) μ = μ( p, T )
and so on for every quantity in the list. Note that the specific volume, so important in thermodynamic analyses, is omitted here in favor of its inverse, the densityρ.
Thermodynamic properties describe the state of a system—that is, a collection of matter of fixed identity that interacts with its surroundings. In most cases here the system will be a small fluid element, and all properties will be assumed to be continuum properties of the flow field: ρ = ρ(x, y, z, t), and so on.
Thermodynamics is normally concerned with static systems, whereas fluids are usually in variable motion with constantly changing properties. From a statistical argument, the properties retain their meaning in a fluid flow that is technically not in equilibrium. In gases at normal pressure (and even more so for liquids), an enormous number of molecular collisions occur over a very short distance of the order of 1 μm, so that a fluid subjected to sudden changes rapidly adjusts itself toward equilibrium. It is therefore assumed that all the thermodynamic properties just listed exist as point functions in a flowing fluid and follow all the laws and state relations of ordinary equilibrium thermodynamics. There are, of course, important nonequilibrium effects such as chemical and nuclear reactions in flowing fluids.
