Aerodynamic enhancement of spanwise-flexible flapping wings via fluid-structure resonance
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In this study, we investigate the interaction between wing flexibility and flapping motion during forward flight. Specifically, we seek to understand how the wing's natural frequency and effective stiffness affect its propulsive performance. To do this, we perform direct numerical simulations of a spanwise-flexible wing undergoing heaving and pitching at the root, at a Reynolds number Re=1000, and for rectangular wings of aspect ratio 2 and 4. Our results reveal that the wing's propulsive performance is optimized when the imposed flapping frequency approaches the first natural frequency of the structure in the fluid. This optimal performance is linked to a damped resonance of the non-linear oscillator, where both the maximal amplitude of the wing tips and the phase lag of the tip's motion contribute to optimize the development of the leading edge vortex and consequently, maximizing the generation of thrust.