Perspective on hypersonic nonequilibrium flow

Introduction O as well as new hypersonic flow programs continue to challenge the aerospace engineer. These programs are often of consuming difficulty, involving the synthesis of chemical kinetics, quantum mechanics, and radiation physics with fluid dynamics. To further complicate matters, the flowfield is often rarefied; thus the Knudsen number requires independent consideration along with the Mach and Reynolds numbers. Although these parameters are related, different regions of the flow, such as the interior of a bow shock, depend differently on them as compared with other regions. Because of its complexity, recent research has focused on the modeling of more realistic, complex hypersonic flows with the intensive use of computational fluid dynamics (CFD). Of necessity, the large codes used in this effort must resort to various empiricisms and approximations. Our objective in this Survey is to discuss some of these empiricisms and approximations within the context of hypersonic nonequilibrium flow. The issues and advances examined are primarily chosen for their relevance to theoretical gasdynamics; however, this Review makes no claim to being comprehensive. The books by Anderson and Park provide background material for the discussion of current issues, including lessons learned from the design and operation of the Space Shuttle. Our presentation is partly based on a previous report, some of which was published elsewhere. An alternative discussion of some of the same material can be found in an article by Tirsky whose discussion is especially valuable for emphasizing hypersonic research by Russian workers, with greater emphasis given to engineering heat transfer predictions. Some of this work is not as well known in the western community as it should be. The importance of finite rate chemistry and the problem of fuel-air mixing to hypersonic airbreathing propulsion cannot be overemphasized. These topics, however, are omitted from our discussion in as much as they fall under combustion gasdynamics and ought to be examined in this context.' The treatise on hydrogen combustion presented in Ref. 8 should be of interest to scramjet propulsion researchers. A broad perspective on hypersonic airbreathing vehicle design may be found in a survey (to be three volumes) edited by Bertin et al. In the next section, nonequilibrium aerothermodynamics is discussed. Much of this section is concerned with energy exchange that involves the vibrational modes of diatomic molecules. The subsequent section reviews the modeling of rarefied hypersonic flows and their continuum extension. There is no shortage of approaches that are available, as well as open issues. These approaches encompass the direct simulation Monte Carlo (DSMC) method, NavierStokes (NS) equations, Burnett, or augmented Burnett equations, and the 13-moment equations. The review concludes with a few brief remarks.