High speed flow

Differences between high and low speed flows

Sound waves consist of a succession of weak pressure disturbances which pro­pagate through the air. The speed at which these disturbances advance through the air is called the speed of sound, and we find that this speed is of great significance in aerodynamics. The speed of sound is not constant but depends upon the square root of the absolute air temperature. Thus, at low altitudes, where the temperature is relatively high, the speed of sound is higher than it is at high altitudes where the temperature is less (see Chapter 7).

Figure 5.1 shows the difference between the flows over a simple aerofoil on an aircraft flying at (a) a speed below the speed of sound (subsonic) and

(b) a speed greater than the speed of sound (supersonic). A number of signi­ficant differences are apparent. Firstly in the low speed flow the air is disturbed a long way in front of the aerofoil, while, for the supersonic flow, the area of disturbance is strictly limited and ahead of this region the air is totally un­affected by the presence of the aerofoil. Secondly, the local direction of the flow varies relatively smoothly at the low speed, while at high speed there is a very abrupt change where the air is first disturbed.

More detailed examination of the flow also shows that there are cor­respondingly abrupt changes in speed, temperature and pressure along a streamline. The line along which these abrupt changes take place is known as a shock wave. As can be seen in Fig. 5.1, shock waves form both at the leading and trailing edges of our aerofoil. The formation of shock waves is of great importance in high speed flow and we shall be looking at them in greater detail shortly.