Highlights d Hummingbirds counter a challenging, continuous perturbation in their first attempt d Roll control uses bilaterally different muscle activation, posture, and tail fanning d Differential wing rotation and elevation generate oval versus figure 8 tip trajectories d Computational simulations show wing rotation to be critical to mitigate perturbation
We present a quantitative characterization of the unsteady aerodynamic features of a live, free-flying dragonfly under a well-established flight condition. In particular, our investigations cover the span-wise features of vortex interactions between the fore- and hind-pairs of wings that could be a distinctive feature of a high aspect ratio tandem flapping wing pair. Flapping kinematics and dynamic wing-shape deformation of a dragonfly were measured by tracking painted landmarks on the wings. Using it as the input, computational fluid dynamics analyses were conducted, complemented with time-resolved particle image velocimetry flow measurements to better understand the aerodynamics associated with a dragonfly. The results show that the flow structures around hindwing’s inner region are influenced by forewing’s leading edge vortex, while those around hindwing’s outer region are more influenced by forewing’s shed trailing edge vortex. Using a span-resolved approach, we found that the forewing–hindwing interactions affect the horizontal force (thrust) generation of the hindwing most prominently and the modulation of the force generation is distributed evenly around the midspan. Compared to operating in isolation, the thrust of the hindwing is largely increased during upstroke, albeit the drag is also slightly increased during the downstroke. The vertical force generation is moderately affected by the forewing–hindwing interactions and the modulation takes place in the outer 40% of the hindwing span during the downstroke and in the inner 60% of the span during the upstroke.
Insect wings change its shape dynamically through the interactions of the structure, and the aerodynamic and inertial forces when flapping, which can greatly affect its aerodynamic performances. While the detailed change of the wing shape has been extensively measured with high-speed photogrammetry, its implications on the flapping wing aerodynamics are poorly understood. In order to clarify the linking between the wing deformation and the flapping wing aerodynamics, the aerodynamic effect of the wing deformation in terms of the twist, the camber and the spanwise bending have been systematically investigated by means of the computational fluid dynamic analyses of a hovering hawkmoth with artificially deformed flapping wings. With the appropriate magnitude and phase, the twist and the camber are found to enhance the aerodynamic efficiency of flapping wing by redirecting the aerodynamic force vector on the wing so as to reduce the drag or increase the lift. The spanwise bending can increase the aerodynamic force without the redundant increase in aerodynamic power by appropriately adjusting the speed of the wing. We specified the magnitude and the phase of deformation that give the highest efficiency in the range of the study, and pointed out that, while the twist and the camber can enhance the efficiency, the deformation beyond the optima can reduce the aerodynamic efficiency drastically. The results in this study revealed the aerodynamic contributions of each kind of wing deformation, and will be of great implications for the design of bio-inspired micro air vehicles.
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