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

Abstract

Unsteady wake characteristics of a butterfly shaped wing undergoing flapping and feathering motions are studied. Four bar quick return mechanism is used for generating the flapping motion. To achieve considerable feathering motion downstroke is made faster than upstroke with a flexible wing. The flapping frequency and wing size considered for this study covers a Reynolds number (Re = UtC/ν; Ut = average tip velocity, C is wing chord and v is kinematic viscosity), 6050 < Re < 10080 for zero advance ratio (J). 2-D and 3-D (stereoscopic) time resolved velocity field is measured to understand the dynamics of lift and thrust generation. Conservation of momentum principle has been used to estimation the thrust force. Mean velocity profiles in the wake shows frequency dependence ejection of the TEV, which is believed to be effectively used for thrust production and maneuvering by different size butterflies. Variation of circulation with time is obtained for the TEV. Thrust is compared with the analytical results of unsteady lifting line theory [1] and Theodorsen’s lift theory [2] for the same configurations.