Fluid–structure resonance in spanwise-flexible flapping wings

We report direct numerical simulations of the flow around a spanwise-flexible wing in forward flight. The were performed at $Re=1000$ for wings aspect ratio 2 and 4 undergoing heaving pitching motion Strouhal number $St_c\approx 0.5$ . have varied effective stiffness $\varPi _1$ while keeping inertia constant, _0=0.1$ It has been found that there is an optimal aerodynamic performance linked to damped resonance phenomenon, occurs when imposed frequency oscillation approaches first natural structure fluid, $\omega _{n,f}/\omega \approx 1$ In situation, time-averaged thrust maximum, increasing by factor with respect rigid case increase propulsive efficiency approximately 15 %. This enhanced results from combination larger angles attack outboard sections delayed development leading edge vortex. With flexibility beyond resonant frequency, drops significantly, terms both production efficiency. cause this drop lies phase lag between deflection heaving/pitching motion, which weaker vortices, negative wing, drag generation half stroke. Our also show flexible same _{n,f}/\omega$ but different performance, emphasizing importance structural properties its performance.