In these environments, the mean-free-path between molecule collisions approaches that of the characteristic length of the investigated configuration. Most hypersonic flows are observed in the upper atmosphere, in the stratospheric and the lower altitude of the mesospheric layer. Meanwhile, these theories have withheld the test of time and set excellent standards for basic research in science. The collective accomplishments contribute to the fundamental knowledge of aerodynamics and have opened avenues in search of better understanding and innovation. These invaluable classic theories such as the rarefied gas dynamics, Newtonian flow theory, hypersonic similitude, law of cross-section, blast wave analogue, as well as, the Mach wave and vorticity interactions at the leading edges are developed from remarkable insights and firmly established by rigorous analyses. In order to better understand the realistic hypersonic flows, all pertaining disciplines such as the nonequilibrium chemical kinetics, quantum physics, radiative heat transfer, and electromagnetics need to bring forth. Therefore, the hypersonic flows are no longer a pure aerodynamics subject but a multidisciplinary science. When the air mixture is ionized to become an electrically conducting medium, the governing physics now ventures into the regimes of quantum physics and electromagnetics. Under this circumstance, the theoretic linkage between the microscopic particle dynamics and macroscopic thermodynamics properties of gas is lost. However, most hypersonic flows encounter strong shock-wave compressions resulting in a high enthalpy gas environment that always associates with nonequilibrium thermodynamic and quantum chemical-physics phenomena. The hypersonic flow is quantitatively defined by the Mach number independent principle, which is derived from the asymptotes of the Rankine-Hugoniot relationship. Nearly all illuminating classic hypersonic flow theories address aerodynamic phenomena as a perfect gas in the high-speed range and at the upper limit of continuum gas domain.
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