Reynolds-number scaling analysis on lift generation of a flapping and passive rotating wing with an inhomogeneous mass distribution

Abstract

Insects usually fly by passively rotating wings, which has been applied to the design of flapping-wing Micro-Air Vehicles (MAVs) to reduce mechanical complexity. In this paper, a robotic passive rotating-wing model is designed to investigate wing kinematics and lift generation, which are measured by a high-speed camera and a force transducer, respectively. In addition, flow fields are measured using the Particle Image Velocimetry (PIV). Experimental results demonstrate that passive rotating motion has a coordinative relationship with actively stroking motion. As the stroke amplitude or frequency increases, the rotating amplitude is enlarged. To characterize the active stroking motion, a driving Reynolds number Redriving is defined, which varies from 68 to 366 in this study. Moving the gravity center of the wing towards trailing edge induces the increase of additional torque M, which decreases the wing rotating amplitude and promotes the advance of wing rotation. We find that the timing of wing rotation is gradually delayed and the mean lift coefficient CL¯ monotonously decreases as Redriving increases. By increasing the additional torque M, CL¯ is slightly improved and approaches to the lift coefficient of a real fruit fly at Redriving≈230. The instantaneous lifts combined with the vortical structures further demonstrate that the lift generation associated with wing rotation is mainly attributed to the growth of the Leading-Edge Vortex (LEV) and the passive wake capture mechanism. Passive wake capture is influenced by LEV, reversal stroke motion and wing additional torque together, which can only maintain the lift at a high level for a considerable period. The high-lift generation mechanisms of flapping and passive rotating flight could shed light on the simplified design of MAVs and the improvement of their aerodynamic performance.