The high enthalpy gas flows, associating high velocities, and high temperatures, are the scene of physical and chemical processes such as molecular vibrational excitation, dissociation, ionization, or various reactions. The characteristic times of these processes are of the same order of magnitude as aerodynamic characteristic times so that these reactive media are generally in thermodynamic and chemical non-equilibrium. This book presents a general introductory study of these media. In a first part their fundamental statistical aspects are described, starting from their discrete structure and taking into account the interactions between elementary particles: the transport phenomena, relaxation, and kinetics as well as their coupling are thus analysed and illustrated by many examples. The second part of the work is devoted to the macroscopic aspects of the reactive flows including shock waves, hypersonic expansions, flows around bodies, and boundary layers. Experimental data on vibrational relaxation times, vibrational populations, and kinetic rate constants are also presented. Finally, experimental aspects of reactive flows, their simulation in shock tube and shock tunnel are described as well as their applications, particularly in the aero-spatial domain.

A theoretical model based on a quasi-one-dimensional formulation is developed which allows determination of the shock stand-off distance at the stagnation point of blunt bodies in hypersonic non-equilibrium flows. Despite the simple ideal dissociating gas model implemented in the theoretical approach, it gives insight into the main physics governing the shock stand-off problem. More detailed and precise data are obtained by a numerical simulation where vibrational and chemical relaxation processes as well as their interactions are taken into account. The physical modelling of these processes is based on a kinetic approach and on a generalized Chapman–Enskog method of solving the Boltzmann equation. Explicit formulae for rate constants and vibrational energy consumption are derived and incorporated into the general conservation equations. Good agreement between theoretical, numerical and experimental results is achieved which ensures a reliable and mutual validation of the different methods.

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