Lift Enhancement of Flapping Airfoils by Generalized Pitching Motion

The pitching and plunging motions of airfoils have received a lot of attention recently, due to the increased interest in the design of micro air vehicles. The use of combined pitch plunge motion with phase difference between them has often been used for the generation of thrust and lift. These aerodynamic forces could be significantly enhanced under similar operating conditions by using generalized pitch motion with variable center of wing rotation. The current study investigated the flowfield and aerodynamic forces for this generalized pitching motion. Two-dimensional rigid airfoils were taken as prototypes of micro air vehicle wings. First, the computational results were compared with the available measurements for an SD 7003 airfoil in pure-pitch and pure-plunge motions at Re = 10,000. Good agreement was observed between the numerical computations and the experimental results in terms of streamwise velocity, location of the vorticity contours, and wake profiles. Next, the pure-pitch case was considered with the stationary centers of rotation located at different positions along the chord of the airfoil. It was found that the maximum value of the computed average coefficient of lift was obtained when the pitching axis was positioned at either the leading edge or the trailing edge. The generalized pitching motion computations were performed next. It was observed that a phase difference of 90 deg between the pitching motion and the motion of the axis caused a twofold increase of the mean coefficient of lift compared to the pitching about leading edge and combined pitch plunge motion with a 90 deg phase difference. The stability of the leading-edge vortex was found to be responsible for the enhancement of lift by reduction in pressure at the upper surface of the airfoil. However, thrust force was not generated by applying the generalized pitching alone, whereas it was generated by the combined pitch plunge motion. Finally, a generalized pitching motion combined with a superimposed plunging motion was studied. It was found that for this motion, thrust was generated and the generated lift was higher than that for the generalized pitching motion. This result may help in the use of superposition of kinematic motions of wings to produce the desired amount of lift and thrust.