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Turning the flow the other way – the expansion
We have now seen how the flow can be turned by a shock wave resulting in an increase in pressure, density and temperature of the air as the flow is almost instantaneously slowed by the shock. If we turn the air in the opposite direction
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Fig. 5.16 Expansion The flow accelerates around the corner through an expansion fan. Pressure decreases so pressure gradient is favourable for the boundary layer, which remains attached |
(Fig. 5.16) we find that the pressure decreases as do both density and temperature, while the speed increases. If we look at the process in greater detail we see that the process of expansion is not sudden as in the case of the shock wave compression, but takes place over a well defined area.
It is interesting to observe that at supersonic speed the flow is much better able to negotiate this type of corner than it is at subsonic speed where boundary layer separation would almost certainly result (Chapter 3). In fact the degree of turn that can be achieved by a supersonic stream is quite surprising.
At first sight it seems strange that the faster flow is better adapted to making sudden changes in direction, but the clue to why this should be so has already been given in Chapter 3. The problem in subsonic flow is that the boundary layer separates and one of the primary causes of this is an increasing pressure in the direction of the flow; an adverse pressure gradient. If we now look at the change in pressure around the corner for the subsonic case we see that there is, indeed, an adverse pressure present. In supersonic flow, however, the pressure gradient around the corner is in the favourable sense and acts to prevent boundary layer separation (Fig. 5.16).
This difference in the ability of subsonic and supersonic flows to turn corners is not just of academic interest. The supersonic aerofoil section shown in Fig. 5.17 is perfectly adequate for use at its design speed, but will have a very
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Fig. 5.17 Supersonic aerofoil section (double wedge) This section has good supersonic but poor subsonic performance |
poor subsonic performance. As we shall see in Chapter 8, this makes the designer’s life much more difficult since, with the exception of some missiles, most aircraft have to land and take off and must therefore be capable of satisfactory operation at both subsonic and supersonic speed.
