Active Control of Separation on a Wing with Oscillating Camber

Abstract

Introduction L OW-REYNOLDS-NUMBER effects are signiŽ cant in the aerodynamics of low-speed airfoils, aircraft intended to operate in low-density environments, and small-scale lifting surfaces such as insect and bird wings. Current aerodynamic applications includemicro-aerialvehicles and unmannedaerial vehicles, as well as aircraft operating at high altitudes or low-density atmospheres other than Earth’s. The primary difŽ culty with the operation of a wing at low Reynolds number is that the  ow over the suction surface encounters an adverse pressure gradient at a point at which the boundary layer is quite likely to still be laminar. Because a laminar boundary layer is incapable of negotiating any but the slightest adverse pressure gradient, the  ow will inevitably separate. The separated  ow then transitions to turbulence, entrains  uid, and reattaches to form a turbulent boundary layer. The resulting structure is the laminar separation bubble, which has been described by Lissaman.1 A number of different  ow-control approaches have been investigated to reduce separationand improve efŽ ciency at lowReynolds numbers. Continuous blowing and sucking have long been shown to havepronouncedeffects.More recently, intermittentblowing and sucking in the form of synthetic jets have shown their effectiveness and suggest the presence of optimum values in the range of frequency inputs, which can translate to other oscillatory inputs.3i5 Mechanical momentum transfer and acoustic excitation have also been explored. The approach presented herein employs an adaptive wing.6 Naturally, all practical wings are adaptive in the sense that they use actuators to alter lift coefŽ cient by changing effectiveproŽ le with a subsequentloss in efŽ ciency.A truly adaptivewing, however, refers to an airfoil, which can change its proŽ le to adapt optimally to  ow conditions. Similar concepts have been explored in the past, such as the snap-through airfoil, which changes local airfoil camber by moving a  exible portion of the pressure surface or the Defense AdvancedResearch Projects Agency “smart wing,” which uses torsional elements to twist the wing. Modern smart materials such as piezoelectricactuators offer great promise in the area of future stall