Effect of spanwise folding on the aerodynamic performance of three dimensional flapping flat wing

Xinyu Lang, Bifeng Song, Wenqing Yang, Xiaojun Yang

Research output: Contribution to journalArticlepeer-review

21 Scopus citations

Abstract

Bird flight incorporates complex multi-degree-of-freedom motions, such as flapping, pitching, twisting, and folding. The aerodynamic mechanism of these complex movements, especially the coupled flapping-folding motion, still lacks in-depth understanding. In this study, the flapping-folding motion is numerically investigated based on a three-dimensional two-section flapping plate with various folding parameters. Different folding amplitudes, phase angles between flapping and folding, and mean folding angles are considered under a preset flapping motion. Kinematics of different flapping-folding motions is analyzed to establish the relationship between the motion and aerodynamic characteristics. The aerodynamic forces and flow structures are also compared. It is found that the folding motion affects the horizontal projection area of the wing, which primarily dominates the trend of lift variation. The folding motion mainly affects the development of the leading-edge vortex on the outer part of the wing by changing the local tangential velocity and the effective angle of attack, which in turn affects the instantaneous aerodynamic forces. A larger folding amplitude will cause an obvious phase shift on lift force and generate more thrust force. An increase in the phase angle between flapping and folding will reduce the lift amplitude, but a small phase angle is beneficial for thrust. A negative mean folding angle can enhance lift during downstroke and reduce negative lift in upstroke, thus significantly increasing the mean lift force, indicating a potential advantage of folding motion in lift generation. The research in this paper is helpful to understand the flight mechanism of birds and to design a micro-air vehicle with bio-inspired flapping kinematics.

Original languageEnglish
Article number021906
JournalPhysics of Fluids
Volume34
Issue number2
DOIs
StatePublished - 1 Feb 2022

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