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This paper reviews and highlights recent developments of certain aspects of flow control concerned with reducing the drag of, and delaying flow separation on, wings and bodies over which the flow is turbulent. The study is restricted to devices that extend beyond the viscous sub-layer but are on a smaller scale than geometric features of the aircraft (e.g. wing chord).The review is mainly concerned with developments within the UK, although significant developments in other countries are discussed.The review discusses types of flow that need to be controlled, basic features of flow control devices and applications. It concludes with recommendations for future research.

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The effect of streamwise slots on the interaction of a normal shock wave with a turbulent boundary layer has been investigated experimentally at a Mach number of 1.29. The surface-pressure distribution for the controlled interaction was found to feature a distinct plateau. This was caused by a change in shock structure from a typical unseparated normal shock-wave/boundary-layer interaction to a large bifurcated lambda-type shock pattern, which led to a reduction of total pressure losses. A strong spanwise variation of boundary-layer properties was observed downstream of the slots, whereas the modified shock structure was relatively two-dimensional. Surface flow visualization confirmed that the slots introduced a region of recirculation into the boundary layer, similar to passive control with uniform surface ventilation. Surface flow visualization revealed the presence of a pair of counter-rotating vortices, confirmed by crossflow velocity measurements. Because of the reduction of total pressure losses, streamwise slots can reduce aircraft wave drag at transonic cruise while incurring only small viscous penalties. A similar control device can also be of use in supersonic intakes where total pressure losses limit engine performance. The introduction of streamwise vorticity can be beneficial in delaying boundary-layer separations often encountered in intakes and on transonic wings. The device is also thought to be capable of delaying buffet onset. Nomenclature M = Mach number P = pressure U = streamwise velocity x = streamwise distance, mm y = vertical distance, mm z = spanwise distance, mm δ = boundary-layer thickness Subscripts S = surface static value 0 = total value 1 = upstream value 2 = downstream value

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