How do flapping avian wings exhibit superior aerodynamic performance?

Abstract

This work investigates the unsteady aerodynamic performance and vortex dynamics of avian-inspired flapping wings using our in-house sharp-interface immersed-boundary solver, VorteXdyn. A falcon-inspired body-wing model based on NACA 4312 profile is employed to examine aerodynamic force generation and vortex evolution during steady forward flapping flight at Reynolds numbers of 2,500, 5000, and 10000 and Strouhal numbers of 0.18, 0.225, and 0.27. The influence of physiologies is examined using three wing configurations: a simplified wing without distinct feather structures, a wing incorporating feather-like structures (serrations) along the trailing edge, and a geometrically detailed wing incorporating multiple feather layers consisting of primary, secondary, and median feathers over its span. The aerodynamic performance of these configurations is quantified using the temporal profiles and time-averaged characteristics of the lift and drag coefficients and the lift-to-drag ratio. The associated vortex dynamics are characterized through the formation and evolution of multiple leading-edge vortices (LEVs), their spanwise coherence, circulation, characteristic size, and persistence over the wings' surfaces. Particular emphasis is placed on the spanwise development of the LEVs from the root to the wingtip, their interactions with the tip vortices, and the resulting wake evolution during the downstroke. Our results demonstrate that increasing geometric fidelity modifies aerodynamic force production, the formation and evolution of multiple LEV structures, vortex-vortex and votex-wing interactions, and wake topology. These findings provide insight into the aerodynamic role of feather morphology in three-dimensional flapping flight for the aerodynamic design of efficient bio-inspired flapping-wing micro air vehicles.