Ground Testing for Hypervelocity Flow, Capabilities and Limitations

Abstract : In this document the requirements for ground simulation of hypervelocity flows are set out on the basis of the similarity parameters of the problem. This, together with the thermodynamical properties of the gas in question, the consequent heat loads on the facility and large power requirements, leads to the two most successful devices, the reflected shock tunnel and the expansion tube. After a description of the operation and the thermodynamics of these devices, their essential limitations are explained. Scale effects of these limitations are discussed. On this basis the range over which they can be applied for flow simulation is delineated. The term high-enthalpy or hypervelocity flow is used to distinguish those flows in which the velocity is so large that the conditions after the bow shock on a body are such as to cause the molecular components of the gas to dissociate. The fields of human endeavor where high-enthalpy flows are of importance are those in which an object traverses the atmosphere of one of the planets of the solar system. Typically this could be associated with transport to or from space in man-made vehicles, but high-enthalpy flows also occur naturally, e. g., when a meteorite enters a planetary atmosphere. The term hypersonic flow is used to describe situations where the flow speed is large compared to the free-stream speed of sound. Such high-Mach-number flows can, of course, be generated by lowering the speed of sound by lowering the temperature. The low temperature limit is the boiling point of the gas. In such cold hypersonic flows, the important dissociative and other real-gas effects of hypervelocity flows do not occur. In order to understand the intricacies of flows in which the chemistry of the gas is activated by the kinetic energy of the flow, it is necessary to simulate high-enthalpy flows in the laboratory.