Hypersonic flow

We have examined the way in which the flow changes as the speed of sound is exceeded and in Chapter 8 we will look at the way aircraft have developed to operate at speeds up to about twice the speed of sound (Mach 2). Some aircraft, however, have to operate at very much higher Mach numbers, particularly re-entering satellites and space shuttles. We find that a number of problems are associated with flight at these very high Mach numbers (up to about 27). Some of these are direct aerodynamic problems associated with the extreme speeds. Some are primarily structural and material problems caused by the high temperature induced by the flow. The SR-71 spy-plane used expansion joints in the structure, which lead to conspicuous, but not dangerous, leakage of fuel when on the ground. Other problems are due to the height at which such flight conditions are most likely to be made and are caused by the very low density encountered. For realistic flight conditions these problems begin to be felt at Mach numbers above about six, and so flight above this rather imprecise demarcation line is known as hypersonic.

What, then, are the aerodynamic problems associated with this flight regime? Initially nothing particularly dramatic occurs. All the main features of the supersonic flow are there, such as the bow shock wave and expansions. As would be expected from the increase in Mach number, the bow shock wave is more acutely swept to the free-stream direction. It is when we come to look at the details of the flow that we find the important changes that have taken place.

We have already seen that the shock wave is quite a traumatic experience for the flow passing through it. Pressure, density and temperature all increase dramatically over a very short distance. However, the basic composition of the air passing through the wave does not change in supersonic flow. It still consists of a mixture of roughly 70 per cent nitrogen, 20 per cent oxygen, 9 per cent carbon dioxide, with a few rare gases thrown in for good measure. The molecules of each of the various constituents are in their usual form with the nitrogen and oxygen molecules both being diatomic (i. e. with two atoms to each molecule). All the constituents are also electrically neutral, the electrons in each molecule exactly balancing the charge in the molecular nucleus.

As the Mach number increases and the shock wave gets stronger this situ­ation changes and the so-called real gas effects become important. The relatively simple relationships between gas properties which occur under moderate con­ditions of temperature and pressure break down. The two atoms in the gas molecules become detached from each other, a process known as dissociation and energy is released into the flow. This dissociation may also be present in the high temperature regions of the boundary layer near the surface of the vehicle.

A further problem occurs due to the fact that molecules may become elec­trically charged, or ionised. This means that electrical forces may further com­plicate the fluid motion. This may not necessarily be a bad thing, and schemes have been suggested to use this feature to control the flow or even provide a propulsion system.

Yet another complication arises when we consider flight at extreme altitude. For normal aircraft operation, the air molecules are very close together. The average distance between molecular impacts (the mean free path at sea level) is about 6.6 x 10-5 mm. At 120 km altitude, this distance increases to 7 m, a distance that is quite large when compared with the size of the vehicle travel­ling through the air. In this case we can no longer think of the air as being a continuous fluid, but must consider the action of individual molecules, and average their effect.

From the above, it will be appreciated that the theoretical prediction of such flows becomes very difficult. Experimental work under such extreme condi­tions is also an arduous and costly undertaking. For further information, the interested reader is referred to Cox and Crabtree (1965).