Shock waves

Let us see if we can find out a little more of what actually happens during the change from incompressible flow to compressible flow, and so discover the cause of the mounting error in making the assumption of incompressibility. Let us also investigate the ‘shock’, together with its cause and effects.

As the speed of airflow over say a streamline body increases, the first indi­cation that a change in the nature of the flow is taking place would seem to be a breakaway of the airflow from the surface of the body, usually some way back, setting up a turbulent wake (Fig. 11.3). This may occur at speeds less
than half that of sound and has already been dealt with when considering the boundary layer. It will, of course, cause an increase of drag over and above that which is expected at the particular speed as reckoned on the speed- squared law.

As the speed increases still further, the point of breakaway, or separation point tends to creep forward, resulting in thicker turbulent wake starting forward of the trailing edge.

This happens because, when we reach about three-quarters of the speed of sound, a new phenomenon appears in the form of an incipient shock wave (Figs 11.4 and 11A, overleaf). This can be represented by a line approximately at right angles to the surface of the body and signifying a sudden rise in pressure and density of the air, thus holding up the airflow and causing a decrease of speed of flow. There is a tendency for the breakaway and turbu­lent wake to start from the point where the shock wave meets the surface which is usually at or near the point of maximum camber, i. e. where the speed of airflow is greatest.