Unsteady Wake Characteristics of a Flapping Wing through 3D TR-PIV

Abstract

The unsteady wake characteristics of a butterfly-shaped wing undergoing flapping and feathering motions are studied. A 4-bar quick return mechanism is used to generate the flapping motion. To achieve considerable feathering motion, the leading edge of the wing is fixed to the flapping arm of the model, while the trailing edge of the wing is kept free to bend and deform out of weight and aerodynamic loading. The flapping frequency and wing size considered for this study cover rotational Reynolds numbers of 6,040–10,080 for a zero advance ratio. The three-dimensional (3D) (stereoscopic) time-resolved (TR) velocity field is measured to understand the dynamics of the lift and thrust generation. The conservation of momentum principle has been used to estimate the thrust force from the particle image velocimetry (PIV) results. The mean velocity profiles in the wake show the frequency dependence ejection of the trailing edge vortex, which is believed to be effectively used for thrust production and maneuvering with various sized butterflies. The unsteady lift generation mechanism is discussed with the help of the measured variation of circulation with time. The thrust estimated from the PIV indicates positive generation throughout the entire cycle of flapping. An analytical approach for lift and thrust estimation has been considered for better understanding of the expected nature and variation of these aerodynamic forces.