More about oblique shock waves – turning the flow

Because an oblique shock wave is able to impose a sudden change in the direc­tion of an oncoming air stream (Fig. 5.12), the necessary flow deflection around an aerofoil with a sharp leading edge can be achieved with an attached shock wave system (Fig. 5.13) in which the bow shock waves emanate from the lead­ing edge itself.

However, there is a limit to the angle through which a flow can be deflected. This depends on the Mach number of the flow. If this critical angle is exceeded, the shock wave becomes detached (Fig. 5.14) and looks very much like the bow shock wave of the blunt aerofoil described earlier (Fig. 5.1).

So far we have only considered sudden changes in flow direction. If the flow is turned gradually (Fig. 5.15), the picture looks slightly different. Near to the surface the flow compresses and turns without a shock wave, but one is observed further away from the surface. The reason for this is that as the flow compresses its temperature rises. The speed of sound therefore increases and if we draw ‘Mach lines’ to indicate the extent to which each point on the surface can influence the oncoming flow, we see that these get progressively steeper and eventually run together to form the shock wave.

The compression near the surface is known as a ‘shockless compression’ and we will see later how this type of compression can be exploited in practical design as it involves no wave drag.