Flow-control approaches to drag reduction in aerodynamics: progress and prospects.

Flow control—the control of turbulence and turbulent flows, in particular— may be likened to the exercise of cupping water between two arthritic hands: frustratingly difficult and, at best, only partially successful. Harder still, innovative control concepts that are eventually brought to fruition in a laboratory setting face the additional, often major, obstacle of being translated into practical systems that operate effectively, efficiently and reliably over a wide range of conditions. Closing this gap is conditional on an ambitious collaboration—a meeting of minds—between university-based scientists and engineers in industry, with each group recognizing and responding creatively to the priorities and constraints of the other. Most flow-control challenges arise simply from the fact that the stability characteristics of fluid flows—for example, those governing transition—are very difficult to influence using energy-efficient, fast-response sensor–actuator systems. Moreover, the highly complex, multi-scale nature of turbulence makes it very hard to target particular mechanisms with actuation devices that, necessarily, operate within a narrow range of scales, or even a single scale. An example in the latter area is the reduction of frictional drag, requiring control over both the small-scale streaky structure in the viscosity-affected near-wall layer and the large-scale structures (‘super-streaks’) in the outer, fully turbulent region—and this needs to be achieved using only a small number of fixed wall-mounted sensors and actuators. The principal driver of flow control is the pressure to reduce fuel consumption and hence CO2 emissions, especially in civil aviation, but also in road and shipping transport, which together are responsible for around 30 per cent of global CO2 emissions. Global aviation-fuel consumption alone is estimated to rise from 155 million tonnes in 2002 to almost 400 million tonnes in 2030 [1,2], with consequent CO2 emissions rising to 1250 million tonnes. A 1 per cent reduction in drag on a jet airliner in cruise conditions translates roughly to a 0.75 per cent reduction in fuel consumption, implying a potential reduction in emitted CO2 of nine million tonnes per 1 per cent of drag reduction. In the case of a modern airliner, friction (viscous) drag amounts to about 60 per cent of the total drag, the remainder being mostly shape, induced and trim