ABSTRACT
The main difference between the flow behavior of incompressible and compressible fluids, and the equations that govern them, is the effect of variable density, for example, the dependence of density upon pressure and temperature. Sound is a small amplitude compression pressure wave, and the speed of sound is the velocity at which this wave will travel through a medium. Because the gas viscosity is not highly sensitive to pressure, for isothermal flow the Reynolds number and hence the friction factor will be very nearly constant along the pipe. For isentropic flow, as the downstream pressure drops, the mass velocity increases until it reaches a maximum. For adiabatic flow of an ideal gas in a constant area duct, the governing equations may be formulated in a more generalized dimensionless form that is useful for the solution of both subsonic and supersonic flows.
