High-Incidence Airfoil Aerodynamics Improvement by Leading-Edge Oscillating Flap

Abstract

Introduction F LOW separation has been an important issue of study in  uid mechanics that often occurs in aircraft engineering applications. One of the classical topics involved is the stalled airfoil performance at high angle of attack (a). For an airfoil at a high a, the strong interaction between the separated  ow and the freestream creates a complex  ow phenomenon, and many techniques, such as acoustic excitation, boundary-layer blowing or suction, and oscillating- ap excitation, have been tested to control the separated  ow for improving the airfoil’s aerodynamic performance. In the past few decades acoustic excitation in controlling  ow separation has been broadly investigated. However, Chang et al. found that the in uence of the internal velocity perturbations on the  ow structure is more important than that of the pressure  uctuations. As a result the effectiveness is limited in some cases. When the a of the airfoil greatly exceeds the stalled angle, no signiŽ cant aerodynamic improvement is achieved by the acoustic excitation technique. For the oscillating- ap excitation technique, Francis et al. mounted a vertical oscillating strip on the surface of the NACA 0012 airfoil and found that the vortex structure generated is very similar to that of dynamic stall over the airfoil. Their results show that the  ow structure after the oscillating-strip excitation becomes an energetic rotating  uid. Miau and Chen also studied a vertically oscillating-strip excitation on the turbulent boundary layer over a wall and found that the vortices shed from the oscillating strip enhance the momentum transfer between the freestream and the boundary layer, which makes the reattachment of vortices occur more upstream. Hsiao et al. applied an oscillating  ap on the leading edge of a NACA 633018 airfoil and showed that the lift coefŽ cient increases when the excitation frequency corresponds to the vortex-shedding frequency. However, the lift-to-drag coefŽ cient does not always increase. When the leading-edge oscillating  ap perturbs the shear layer in the leading-edge portion a better result can be obtained. Although it has been proven that the sectional lift coefŽ cient of a high-a airfoil is increased by  ap excitation at certain excitation frequencies and positions of the  ap, no conclusive result was obtained as regards the oscillating mode shapes of the  ap motion. In this study, six different oscillating modes of the  ap motion are employed to study the effectiveness of the aerodynamic improvement of a high-a airfoil. Phase averaging of pressure  uctuations on the airfoil’s surface is calculated for investigating the dynamic leading-edge vortex evolution.