The different types of high speed flow

We have spent some time in looking at the differences between flows at high and low speed. It is worth emphasising that, for both the duct flow and the ‘external flow’, although Bernoulli’s equation becomes inaccurate as speed increases, it is still true that an increase in speed is accompanied by a decrease in pressure, irrespective of whether the flow is sub – or supersonic.

We find that our criterion for high speed, introduced above (the speed at which density changes first become apparent) is related to the Mach number. For an aircraft this usually occurs at flight Mach numbers above about 0.5. Rather than a single measure of what constitutes a ‘high speed’ we can now begin to identify Mach numbers at which distinguishing features of high speed flow begin to appear (Fig. 5.5).

This figure shows the Mach numbers at which we will obtain our typical low subsonic and fully developed supersonic flows. It also shows a number of other features, which we will discuss shortly, such as the intermediate stage between these flows, the transonic speed range. The advent of important heating effects caused by the passage of the aircraft through the air is also shown.