Flow of a Compressible Fluid

Fluids have the capacity to change volume and density, that is, compressibility. Gas is much more compressible than liquid. In a situation where an extreme change in pressure occurs, such as in a water hammer, the compressibility is taken into account. Gas has large compressibility; however, when its velocity is low as compared with the sonic velocity, the change in density is small and it is then treated as an incompressible fluid. This chapter briefly discusses thermodynamic characteristics, followed by the effects of sectional change in isentropic flow, flow through a convergent nozzle, and flow through a convergent–divergent nozzle. It also describes the adiabatic but irreversible shockwave and explains adiabatic pipeflow with friction (Fanno flow) and pipeflow with heat-transfer (Rayleigh flow). When a minute disturbance develops in a gas, the resulting change in pressure propagates in all directions as a compression wave (longitudinal wave, pressure wave), which is felt as a sound; its propagation velocity is called the sonic velocity.