The surrounding air flow around a hypersonic vehicle behaves quite differently from supersonic flows. The kinetic energy is converted into internal energy which can increase the flow temperature and induce endothermic reactions near the vehicle surface. It is a challenge to develop flow diagnostic and aerodynamic measurement technologies with high precision for high-enthalpy wind tunnel tests. There are, generally, three types of measurement technologies widely used in exploring high-enthalpy flows, including heat-transfer measurement, aerodynamic balance, and optical diagnostic techniques. In this chapter, hypersonic tests with the aforementioned measurement technologies are summarized to demonstrate the progress on high-enthalpy flow experiments. Four kinds of experiments are included here, and the topics are aerodynamic force and moment tests, aerothermal heating measurements, hypersonic boundary-layer flow diagnostics, and supersonic combustion and scramjet engine tests. Actually, there are a lot of interesting topics, but these four are important not only to understand aerothermodynamic physics but also to support the development of hypersonic vehicles.

The achievable total enthalpy and the pressure level in a shock tunnel depend on its capability to generate strong shock waves. To produce a strong shock wave, high pressure and high sound speed are two key parameters for driver gases. There are various techniques to increase the driver gas sound speed, which are essentially different approaches in the way to raise the driver gas temperature. The first technique to increase the driver gas sound speed is by the use of a light gas, and the second one is by heating the light gas to a high temperature with gas heaters. The light-gas-heated shock tunnel is introduced in this chapter, and the electrical heaters are discussed in detail, including the relatively simple electrical resistance heaters and electric-arc heaters. Strictly speaking, the electric-arc heating is not a gasdynamic technique and it is not capable of completing flight-condition duplication for hypervelocity testing. However, it is selected because it can generate extremely high total enthalpies and is useful in certain applications.

In this chapter, the aerodynamic fundamentals for the working principles of shock tunnels are summarized. The moving waves, including expansion waves, shock waves, and contact surfaces, are introduced as the key issues and their theories are based on the unsteady one-dimensional flows in textbooks of aerodynamics. As unsteady one-dimensional moving waves are also critical for the design and operation of shock tunnels, their theories are also selected and summarized in this chapter for book completeness and readers’ convenience.

The free-piston driver is a powerful technique to increase both the driver gas sound speed and pressure. Therefore, it is capable of generating high-enthalpy flows and offering high performance among various gasdynamic shock drivers. So far, it has been implemented in a number of major reflected-shock as well as shock-expansion wind tunnels around the world. The free-piston driver has the advantage that a high driver gas pressure is automatically generated in the same process. On the other hand, the driver is far more complex mechanically and requires operation-tuning in order to operate effectively. Moreover, its test time is short and the test flow is not steady because the piston motion is difficult to control. In this chapter, the basic concepts of the free-piston driver are discussed. The analytical theory that describes the piston dynamics and the method for tuned piston operation are presented. Examples of major free-piston-driven test facilities as well as their applications in hypersonic testing are also summarized.

Hypersonic and high-enthalpy wind tunnels have been a challenge in ground testing facilities in aerospace research for decades. In regard to performance requirements, theories and methods for designing hypersonic flow nozzles at high enthalpy conditions are quite difficult, but very interesting topics, especially when dissociation of air molecules take place in test-gas reservoirs. In this chapter, fundamental theories and important methods for nozzle design are reviewed with the emphasis on two-dimensional axisymmetric nozzles for hypersonic high-enthalpy wind tunnels, including the method of characteristics, the graphic design method, the Sivells method, the theory for boundary layer correction, and computational fluid dynamics (CFD)-based design optimization methods. They were proposed based on several physical issues covering the expansion wave generation and reflection, boundary layer development, and real-gas effects of hypersonic flows. Difficulties arising from applications of these methods in high-enthalpy nozzle design are discussed in detail and state-of-the-art of nozzle design technologies that have been reached over decades are summarized with some brief comments.

Based on detonation-driven shock tunnels, key issues that play important roles in extending the test time are introduced in this chapter, and the corresponding solutions are proposed, evaluated, and discussed in detail, including the tailored-interface condition, the shock–boundary-layer interaction at the end of the driven section, and the precursor shock damping in the vacuum tank. The research work on these issues was carried out to find out the flow physics and application methods to improve the high-enthalpy shock tunnel for meeting the test time requirement for supersonic combustion and scramjet engine experiments. With application of the aforementioned theories and methods to the high-enthalpy shock tunnel, a large-scale detonation-driven hypersonic flight-duplicated shock tunnel (the JF-12 shock tunnel) was successfully developed, which can provide test times of more than 100 microseconds and is capable of duplicating hypersonic flight conditions for Mach numbers of 5–9 at altitudes of 25–50 km.

The detonation-driven shock tunnel is one of three important classes of hypersonic and high-enthalpy ground testing facilities that are based on the shock-heated principle. The theory and methods for developing the detonation-driven shock tunnels aiming at hypervelocity flow generation are summarized in this chapter. At first, the primary concepts for detonation drivers are presented to demonstrate their unique advantages for aerodynamic ground-based testing. The difficult problems arising from the development of hypervelocity shock tunnels for simulating flight conditions are identified and discussed in detail to address critical issues underlying the high-enthalpy shock tunnel design. Then, three kinds of detonation-driven shock tunnels are introduced, and their key techniques and performances are reviewed and discussed in detail. Finally, some experiments are summarized to demonstrate the capability of the detonation-driven hypersonic shock tunnel and the importance of the measurement techniques for hypersonic and high-temperature flow experiments. Both are the frontiers of high-enthalpy flow research for developing hypersonic vehicles.

In order to introduce hypersonic ground testing facilities, background information in hypersonics is presented to show to readers what we want to do, where we have been, and where we are going to go. These will provide with a good indication of the research needs that are called as hypersonic vehicle ground testing. It is of fundamental importance that a vehicle design must be carefully evaluated in ground test facilities before flight testing can proceed. Indeed, the development of hypersonic vehicles is related to the capability development of hypersonic ground testing facilities.