High temperature effects : The kinetic energy of the high speed, hypersonic flow is dissipated by the effect of friction within the boundary layer (Fig. 4.6.1-d). The extreme viscous dissipation can result in substantial increase in temperature (~10000 K) exciting the vibration within the molecules and can cause dissociation, ionization in the gas. Typically, in the range of 2000K-4000K, the oxygen molecules start dissociating and with increase in temperature, dissociation of nitrogen molecules takes place. Further increase in temperature (> 9000 K), ionization of both oxygen and nitrogen can start. This leads to chemical reaction within the boundary layer. As a result, the gases within the boundary layer will have variable specific heat ratio and gas constant which are functions of both temperature and pressure. Therefore treatment of air or any fluid flowing with hypersonic speed over any configuration should be done properly by incorporating all the microscopic changes which essentially leads to change in thermodynamic properties with temperature. If the vibrational excitation and chemical reactions takes place very rapidly in comparison to time taken by the fluid element to move in the flow field, then it is called as equilibrium flow . When there is sufficient time lag, then it is treated as non-equilibrium flow . All these phenomena are called as high temperature real gas effects . The presence of high temperature reacting plasma in the vicinity of the flight vehicle influence the aerodynamic parameters, aerodynamic heating and subsequently, communication is blocked. Flight parameters like pitch, roll, drag, lift, defection of control surfaces get largely deviated from their usual estimate of calorically perfect gas. The presence of hot fluid in the vicinity of vehicle surface induces heat transfer not only through convection but also through radiation. Communication waves which are necessarily radio waves get absorbed by free electrons formed from ionization of atmospheric fluid. This phenomenon is called as communication blackout where on board flight parameters and ground communication is lost.

Low density flow : At standard sea level conditions, all the fluids are treated as continuum so that the global behavior is same as that of average fluid properties. In these conditions, the fluid contains certain desired number of molecules and the average distance between two successive collisions of the molecules is specified by its mean free path . Since, the hypersonic flows are encountered at very high altitude (~100 km), the density of the medium is very less and the mean free path may be in the order of 0.3m. So, the air is no longer a continuous substance, rather treated as individual and widely spaced particles in the matter. Under these conditions, all the fundamental equations based on continuum assumption break down and they are dealt with the concepts of kinetic theory. This regime of the aerodynamics is known as low-density flows . Further increase in altitude (~ 150 km), the air density becomes so low that only a few molecules impact on the surface per unit time. This regime of flow is known as free molecular flow . Thus, a hypersonic vehicle moves in different flow regimes during the course of its flight i.e. from a dense atmosphere to a rarefied atmosphere. The similarity parameter that governs different regimes of the flow for certain characteristic dimension L, is then defined as Knudsen number (Kn).

(4.6.4)

Large value of implies free molecular flow while small value of is the regime of continuum flow as shown in Fig. 4.6.1(e). In the inviscid limit, the value of approaches to zero while the free molecular flow regime begins with . In the low density regimes, the Boltzmann equation is used to deal with the fundamental laws.

Fig. 4.6.2: Characteristics features of hypersonic flow.

From these characteristics of hypersonic flows, it is clear that Mach number to be greater than 5 is the most formal definition of hypersonic flow rather it is desired to have some of the characteristics features summarized in Fig. 4.6.2. It is more important that one of these characteristics features should appear in the flow phenomena so that the definition becomes more appropriate. There are many challenges for experimental simulation of hypersonic flow in the laboratory. Understanding the challenges faced by hypersonic flight and driving solutions these problems on case to case basic are the most research themes on hypersonic flows.