Pressure Propagation and Critical Flow (Compressible Fluid Flow)
In the previous chapters, discussion was devoted mainly to a fluid for which there is no need to consider the fluid compressibility. In this chapter, we consider flows in a wide range of temperatures and pressures and treat each gas as a compressible fluid. In such a case, the sound velocity plays a unique role, and the Mach number of the flow becomes an important parameter that determines the flow characteristics. In addition to air, a wide variety of gases, including steam and combustion gases, are considered here. Most of these gases are not ideal gases; however, since they may be regarded as approximating ideal gases due to the high temperature, the computational formulas for ideal gases are described, unless otherwise noted. This treatment is an approximation; however, it is sufficient to present qualitative features and provide theoretical results with good reliability in many cases. In addition, the flow is assumed to be steady. Another treatment must be considered when an unsteady flow has an important role; however, since that leads to a complicated analysis and significantly limits the generality of the conditions considered, no explanation is given here.