Boundary layer control – preventing unwanted flow separation

Apart from the problem of wing stalling, there are several areas in the flow where we wish to prevent flow separation, or inhibit the build-up of thick low-energy boundary layers. Flow separation in air intakes of gas turbine engines is a particularly serious problem, since it is most likely to occur at high angles of attack on landing and take-off; just the time when it is least wanted. Stalling of the intake flow can cause the engine to lose power, or flame-out (switch-off) altogether, with potentially disastrous consequences. Some air­craft are even fitted with a device that automatically operates the starting igniters at high angles of attack.

In high speed flight, flow separations may also be caused by the interaction between shock waves and a thick boundary layer. Notice how the air intake of the supersonic Tornado shown in Fig. 3.15 is separated from the fuselage, to form a slot through which the fuselage boundary layer can pass, preventing its interfering with the intake flow.

In addition to the problem of air intakes, it is also important to prevent separation in the vicinity of control surfaces, since the last thing we want to lose in the approach to a stall, is the ability to control the aircraft.

One way to prevent local flow separation is to apply engine generated suction via small slots or openings in sensitive areas. An alternative pass­ive measure is the attachment of small tooth-like vortex generators on the surface. These are designed to give a highly turbulent surface flow, thus inhibiting separation. Figure 3.7 shows the vortex generators on the wing of

a Buccaneer. This type of vortex generator may be seen on many early swept – wing aircraft, where they were used to try to overcome the problems described below.