Introduction

It has become apparent in recent years that no single experimental device fulfills the demand for complete scaling of model experiments in hypersonic flow. As a consequence, a variety of ingenious testing devices and associated instrumentation were developed, in each of which at least some aspects of hypersonic similarity may be reproduced. The reason for this state of affairs is that, in addition to the traditional similarity parameters of Mach number and Reynolds number, the high enthalpy level of hypersonic flows must be reproduced. This high enthalpy level in turn causes excitation of molecular vibrations, and chemical and electrical transformations of the air or gas mixtures of interest. Since these changes of the thermodynamic state occur at finite rates, ideally one ought to model these relaxation processes in addition. Finally, many of the problems in hypersonics require relatively low Reynolds numbers, or density in the flow, a situation in which all lag processes are accentuated.