Aerodynamics of a flapping wing as a function of altitude: New insights into the flight strategy of migratory birds

Abstract

Migratory birds have developed remarkable physiological and biomechanical adaptive capacities in order to fly at very high altitudes and benefit from favorable wind. Numerous studies suggest that increased frequency would be an adaptive mechanism of flapping flight in hypodense and hypobaric air. We sought to assess this hypothesis using a numerical model of the wing flapping kinematics of a migratory bird and an evaluation of the cyclic variation of aerodynamic forces as a function of altitude (100 to 4000 m). In an attempt to reproduce the variations in the thermophysical parameters of air vs altitude, subroutines have been implemented in a finite-volume-based code. Numerical results indicate a strong correlation between the intensity of forces exerted on the wing and the flight altitude. For instance, it has been shown that mean lift ranges from 2.63 N at 100 m to 0.76 N at 4000 m. In addition, the results suggest that increasing the flapping frequency to 5 Hz would induce a value of (C L / C D)max of 31.8, corresponding to a 158% increase compared to a flapping frequency of 4 Hz. Finally, this study aims at broadening current knowledge of the biomechanical mechanisms used by migratory birds to maintain a favorable balance between flight performance at high altitude and the energetic cost of the migration.