ABSTRACT
The main difference between the flow behavior of incompressible and compressible fluids, and between the equations that govern them, is the effect of variable density, e.g., the dependence of density upon pressure and temperature. At low velocities (relative to the speed of sound), relative changes in pressure and associated effects are often small and the assumption of incompressible flow with a constant (average) density may be reasonable. I t is when the gas velocity approaches the speed at which a pressure change propagates (i.e., the speed of sound) that the effects of compressibility become the most significant. I t is this condition of high-speed gas flow (e.g., "fast gas") that is of greatest concern to us here.
