Introduction

As I was writing chapter 13 I thought that I would include the pitôt-static tube when used to measure speeds of supersonic flow. When I tried to describe it I found that I needed to take a much closer look at what happens around bodies like bullets and aeroplanes. It proved to be rewarding.

 

I think that the only thing that is generally well known about objects moving at supersonic speed is that shock waves are produced and that somehow these make sonic booms. Such shock waves will be propagated from the body and the shape of these waves will depend to some extent on the shape of the object. The size and intensity of the waves will be greater for larger objects and one would not expect a bullet to produce the same wave pattern as a supersonic aeroplane. We require an explanation of these waves but not to the depth required by someone working in this field.

 

The investigation of high speed flow round solid bodies would be a lot easier if we had some means of visualising the flow like those used for low speed flows. Occasionally aeroplanes produce interesting clouds of water vapour and a good photograph of this will be viewed over and over again rather like the Tacoma Narrows Bridge but the fact is that these odd photographs do not tell us very much at all. We have to bring supersonic flow indoors into wind tunnels and find ways of “seeing” the flow and of investigating systematically.

 

There are snags. Small supersonic tunnels often do not run continuously and have obvious limitations. On the other hand a supersonic tunnel having a cross section of, say, 5 square metres running at Mach 2 will require powers measured in megawatts and can only be supplied with power at night. It is an expensive facility. There will be extensive instrumentation to measure speeds, pressures, temperatures etc. and means to observe and photograph the density patterns produced round models in the tunnel. None of the laboratory work is easy to carry out and, if experimental data is to be interpreted correctly, the levels of uncertainty in test data makes it very important to have good mental models of the physics and of the observable behaviour of air moving at very high speed.

 

It seems that there is no new physics for this flow and we shall need to use the physics that has been established for nozzles and shock waves. I want to try to create a mental model of supersonic flow and that will mean combining this physics with an understanding of pictures taken using schlieren equipment.

 

So I must start with the schlieren apparatus.