The subject of e ow control, particularly reactive e ow control, is broadly introduced, leaving some of the details to other papers in this special volume of the Journal of Aircraft . The ability to manipulate a e owe eld actively or passively to effect a desired change is of immense technological importance. In general, methods of control to achieve transition delay, separation postponement, lift enhancement, drag reduction, turbulence augmentation, and noise suppression are sought for both wall-bounded and free-shear e ows. An attempt is made to present a unie ed view of the means by which different methods of control achieve a variety of end results. The important advances in the e eld of e ow control that took place during the past few years are discussed. Spurred by the recent developments in chaos control, microfabrication and neural networks, reactive control of turbulent e ows is now in the realm of the possible for future practical devices. HEability to manipulate a e owe eld actively or passively to effect a desired change is of immense technological importance, and this undoubtedly accounts for the subject being more hotly pursuedbyscientistsandengineersthananyothertopicine uidmechanics.The potential benee ts of realizingefe cient e ow-controlsystems range from saving billions of dollars in annual fuel costs for land, air,and seavehiclesto achieving economically andenvironmentally more competitive industrial processes involving e uid e ows. Methodsofcontroltoeffecttransitiondelay,separationpostponement,lift enhancement, drag reduction, turbulence augmentation, and noise suppression are considered. Prandtl 1 pioneered the modern use of e ow control in his epoch-making presentation to the Third International Congress of Mathematicians held at Heidelberg, Germany. In just eight pages, Prandtl introduced the boundary-layer theory, explained the mechanics of steady separation, opened the way for understanding the motion of real e uids, and described several experiments in which the boundary layer was controlled. He used active control of the boundary layer to show the great ine uence such control can exert on the e owpattern. Specie cally, Prandtl used suction to delay boundary-layer separation from the surface of a cylinder. NotwithstandingPrandtl’ s 1 success,aircraftdesignersinthethree decades following his convincing demonstration were accepting lift anddragofairfoilsaspredestinedcharacteristicswithwhichnoman could or should tamper. 2 This predicament changed mostly due to the German research in boundary-layer control pursued vigorously shortly before and during World War II. In the two decades following the war, extensive research on laminar e ow control, where the boundary layer formed along the external surfaces of an aircraft is kept in the low-drag laminar state, was conducted in Europe and the UnitedStates,culminatinginthesuccessfule ighttestprogramofthe X‐21,wheresuctionwasusedtodelaytransitiononasweptwingup to a chord Reynolds number of 4 :7£10 7 . The oil crisis of the early 1970s brought renewed interest in novel methods of e ow control to reduce skin-friction drag even in turbulent boundary layers. In the 1990s, the need to reduce the emissions of greenhouse gases and to constructsupermaneuverablee ghterplanes,faster/quieterunderwater vehicles, and hypersonic transport aircraft, for example, the U.S. National Aerospace Plane, provides new challenges for researchers in the e eld of e ow control.

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