Aerodynamic performance of a self-propelled airfoil with a non-zero angle of attack

Abstract

In the natural world, numerous flying creatures generate both thrust and lift by flapping their wings. Aerodynamic mechanisms of forward flight with flapping wings have received much attention from researchers. However, the majority of previous studies have simplified the forward-flight motion of flapping wings to be uniform, and there has been no detailed evaluation of the validity of this simplification. Motivated by this, aerodynamic characteristics of a self-propelled flapping wing with a non-zero angle of attack were investigated. The results showed that the asymmetric leading-edge vortex produced in the wing's upstroke and downstroke leads to transient thrust, driving the self-propelled wing to move with variable forward velocities. Compared to the uniform forward-flight cases, significant losses in lift and severe changes in the flow field were observed in self-propelled flapping wings. In addition, the changes in the aerodynamic performance - including the forward propulsion speed, lift, and power efficiency - of the self-propelled flapping wing with changes in various dimensionless parameters were also investigated. The heaving amplitude was shown to have significant effects on lift and propulsion speed of the self-propelled flapping wing, while the effects of ratio between the airfoil density and fluid density as well as the Reynolds number, were relatively small. In most conditions, when the Strouhal number was in the range 0.2-0.4, the self-propelled flapping wing performed well in terms of both lift generation and propulsive efficiency. These research results suggest that it is necessary to consider the fluctuating forward speed in aerodynamic modeling of propulsive flapping wings.