Flow control is a major rapidly evolving field of fluid dynamics. It implies a small change of a configuration serving an ideally large engineering benefit, like drag reduction, lift increase, mixing enhancement or noise reduction. This change may be accomplished by passive or active devices. Passive devices, like turbulators or roughness elements, are steady and require no energy by definition. Active control requires actuators which may be driven in a time-dependent manner and require energy. Examples are valves and plasma actuators. The actuation command may be pre-determined (open-loop control) or be dependent on sensors monitoring the flow state (closed-loop control).
Airplane wing performance has a substantial effect on not only the runway length, approach speed, climb rate, cargo capacity, and operation range but also the community noise and emission levels. The wing performance is often degraded by flow separation, which strongly depends on the aerodynamic design of the airfoil profile. Furthermore, non-aerodynamic constraints are often in conflict with aerodynamic restrictions, and flow control is required to overcome such difficulties. Techniques that have been developed to manipulate the boundary layer, either to increase the lift or decrease the drag, and separation delay are classified under the general heading of flow control. Flow control methods are divided into passive, which require no auxiliary power and no control loop, and active, which require energy expenditure. Passive techniques include geometric shaping, the use of vortex generators, and the placement of longitudinal grooves or riblets on airfoil surfaces. Examples of active flow control methods include steady suction or blowing, unsteady suction or blowing, and the use of synthetic jets.
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