Supersonic flow

There are fundamental differences between supersonic flow and subsonic flow, and perhaps these differences are best illustrated by the different ways in which the two kinds of flow turn corners, or – what comes to much the same thing – pass through contracting or expanding ducts.

Although we may not have put it in that way we have already studied this in the case of subsonic flow; and perhaps we may sum up the results by saying –

1. That subsonic flow anticipates the corner or whatever it may be, and so the pattern of the flow changes before the corner is reached.

2. That the change of flow takes place gradually on curved paths.

3. That at what we might call a concave comer, or in a contracting duct, the flow speeds up and the pressure falls.

4. That at what we might call a convex corner, or in an expanding duct, the flow slows down and the pressure rises.

There are of course complications (as, for instance, if we overdo the sudden­ness of the change and the flow breaks away from the surface), but in the main we have established these four principles, and have seen numerous examples of how they are applied in practice.

Now let us look at supersonic flow.

We have already made it sufficiently clear that the first principle is different – and we have explained why it is different. Supersonic flow does not, and cannot, anticipate a corner or anything else that lies ahead, because there is no means by which it can know that it is there.

What, then, of the other three principles?