| The control of the flow around a harmonically oscillating NACA 0015 airfoil via a dynamically deflected simply-hinged trailing-edge flap was investigated experimentally at a Reynolds number of 2.46 x 105 by using a combination of techniques, including surface pressure measurements, hot-wire wake velocity surveys and particle image velocimetry flowfield measurements. The tests were conducted under deep-stall conditions, with special attention being focused on identifying the changes in the flow structures that led to the observed modified aerodynamic load characteristics, and on the evaluation of the effects of the prescheduled trapezoidal flap motion profile. In addition, light-stall and attached-flow oscillations were also considered, as were static flap deflections and higher harmonic flap motions. The results indicate that a trailing-edge flap imposed an effective camber in the trailing-edge region, and was highly effective in the control of the aerodynamic loads. This was achieved in large part by the manipulation of the lower flap surface pressure distribution via changes to the windward-side flow stream, and was unaffected by the state of the flow above the airfoil. The leading-edge vortex, the predominant flow structure over the airfoil, was only marginally affected in its strength and initiation. The results also revealed that both the flap angle and deflection rate contributed to the above observations, and that the active motion was crucial in preventing the flow separation observed over the lower flap surface for an equivalent static flap, which would have hindered its performance. Furthermore, control was limited to the duration of the flap motion, and, in general, no effect on the flow or aerodynamic loads was observed while the flap was withdrawn to its initial undeflected position. The detailed parametric study showed the characteristics of the flap motion profile to be highly influential on the degree of control. In the application of an optimum flap motion schedule to dynamic stall, the severe nose-down pitching moment decreased by 40%, the performance ratio improved by 30%, and the aerodynamic damping became positive and increased four-fold; this was, however, accompanied by a 20% reduction in the maximum lift. |
