Kinematics and aerodynamic analysis in the turning flights of butterflies

Abstract

This study investigates the free-turning flight of butterflies (Idea leuconoe) and analyze the influence of body posture and asymmetric motions of left and right wings on aerodynamics and the asymmetric flow field structure. Three high-speed cameras were used in the biological experiment to observe the turning flight motions, and varied motion angles were calculated. The results showed that the body started to tilt to the right at 0.3 cycles and the flapping amplitude of the inner wing increased by 20.31% relative to the outer wing during a cycle. The outer wing showed a forward-then-backward deviation, whereas the inner wing exhibited the opposite trend. A three-dimensional numerical model with six degrees of freedom and prescribed motion functions was constructed to simulate the flight of butterflies. The results revealed that the roll angle was the primary factor influencing the direction of aerodynamic forces and had a similar mechanism as the banked turn of a fixed wing. During the downstroke, the outer wing provided the normal force while the inner wing contributed to the vertical force, and both wings generated horizontal thrust during the upstroke. The asymmetric wing motions and the lateral inflow velocity were the two major factors affecting the flow field structure. The difference in flapping amplitudes caused the inner wing to generate greater vertical-normal resultant force first. The asymmetric forewing-deviation angle and the lateral flow influenced the direction of the spanwise flow to enhance the strength of the leading-edge vortex and stabilize the attached flow for the outer wing.