Numerical and experimental study of the fluid dynamics of a flapping wing with low order flexibility

Abstract

A simple canonical problem for understanding the role of flexibility in flapping wing flight is investigated numerically and experimentally. The problem consists of a two-dimensional two-component wing structure connected by a single hinge with a damped torsion spring. One component of the wing is driven with hovering flapping wing kinematics, while the other component responds passively to the fluid dynamic and inertial/elastic forces. Numerical simulations are carried out with the viscous vortex particle method with strongly coupled body dynamics. The experiments are conducted in a water tank with suspended particles for flow visualization. The system is analyzed in several different kinematic test cases that are designed to span a broad parametric range of flapping. Hinge deflection is used as the primary metric for comparison; the agreement between computation and experiment is very good in all cases. The trajectories of shed vortices are also compared, again with favorable agreement. Fluid forces and moments are computed in the numerical simulation at two different Reynolds numbers. It is found that the rate and timing of wing rotation primarily controls the generation of lift; in contrast, the translational acceleration has little effect. Likewise, kinematics with rotation transition well separated from translation transition are captured utilizing rotation-only kinematics. Reynolds number has little effect on the wing deflection but does influence the mean lift generated by the wing.