There are a number of experimental aerodynamic facilities that allow testing and research to be done at velocities considerably above those achieved in tunnels of the types described in the preceding chapters. Usually in these facilities the high velocities are achieved at the expense of some other test parameter,such as Mach number,pressure,and/or run time.
Generally speaking,these facilities have come into being because the aerodynamicist does not believe that the aerodynamic problems of high-speed flight are completely answered by tests in hypersonic wind tunnels, where the tunnel operating temperature is only high enough to avoid liquefaction. There are several reasons for this skepticism. In order for the static temperatures and pressures in the test section of a wind tunnel to equal values at some altitude in the atmosphere at the same time that the velocity in the wind tunnel equals the flight velocity of an aircraft at that altitude, the total temperatures and pressures in the wind tunnel must be very high. The importance of duplicating static temperature, static pressure,and velocity in the test section arises from the fact that only in this way will the temperatures and pressures in the vicinity of a model (behind shock waves and in boundary layers) correspond to conditions for the vehicle in flight.‘Having the proper temperatures and pressures in the vicinity of the model is considered important because at bigh temperatures,the characteristics of air are completely different from those at low temperatures. The enthalpy no -longer increases linearly with temperature and the gas law p = pRT is no longer valid. It is noted that enthalpy at high temperatures is dependent on pressure as well as temperature.
The process taking place in air at high temperatures which causes the gas law p = pRT to be invalid and which has a major effect on the enthalpy is called dissociation. Dissociation is the term given to the process in which diatomic molecules of oxygen and nitrogen (O2 and N2) are broken up into atomic oxygen and nitrogen(( and N). At some temperature,which is dependent on pressure,the internal energy level of the air becomes high enough that the two
atoms of the diatomic molecules begin to loose their bonds and fly apart.'This occurs after the vibrational degree of freedom is fully excited.Considerable energy is required for breaking the diatomic molecules into atoms. This explains the unusual variation of enthalpy with temperature at high temperature levels. The gas law is not valid when dissociation is present because the number of molecules present is increased. Each molecule of O2 that is dissociated becomes two molecules of O.