On the controlled evolution for wingtip vortices of a flapping wing model at bird scale

Abstract

The reduction of wingtip vortices is a critical and challenging task for aircraft wing-design. Inspired by the wingtip morphology of soaring birds, Whitcomb created winglets to reduce the drag of commercial airplanes significantly. As a critical feature of bird flight, the dynamic flapping wing motion could potentially affect the evolution of tip vortices, which has been rarely investigated. To explore the control mechanisms of wingtip vortices by flapping, a flapping wing model with two-jointed arms was designed to mimic geese's flapping motion. The evolution of tip vortices was recorded in a wind tunnel using particle image velocimetry (PIV) system, and physical features were analyzed. It is shown that wingtip vortices change the vorticity concentration and vortex radius as wings flap in a cycle. The continuous coherent streamwise structure could pinch off at the upstroke end. As flapping frequency increases, the vorticity-concentrated tip vortices become dispersed. Circulation change in a cycle is dramatic, and the tip vortices present a directly up- and downward movement, which indicates a wave-like trajectory. Flapping induces the wingtip vortices to wander in a large amplitude and a specific direction instead of randomly in a limited region. Proper orthogonal decomposition (POD) results show that flapping can not only affect the vorticity concentration and radius of tip vortices as well as energy contribution of the first mode, but also increase the energy contribution and induce the coherent field of the second mode into a time-dependent vortex pair. The relative energy contribution and direction of the vortex pair vary with flapping motion, which affects the wandering amplitude and direction. It is suggested that flapping can break up the vorticity-concentrated wingtip vortices, and give them ordered evolution trajectory and directional wandering motion, which could shed light on the effective vortex control and drag reduction for aircraft wing-design.