Effect of rotational phase on the flow topology of a flapping flat-plate wing

The flight performance of insects can be attributed in part to the generation and main­tenance of a stable leading edge vortex, which is achieved by manipulating the wing kine­matics. Along with the prolonged attachment of the leading edge vortex during translation of the wing, the rotational motion at the end of the stroke is also critical as it helps the wing to maintain a positive angle of attack. The unsteady flow fields generated in the rotation stage of the motion are integral to maintaining force production. Phase-averaged data from particle image velocimetry are used to study the effects of advancement or delay in the rotational phase of a flapping wing on the flow characteristics. A topological study is conducted using Lagrangian vortex detection techniques in order to characterize the shear layer formation, vortex interactions and flow separation. The Lagrangian analysis includes the calculation of the finite time Lyapunov exponent based on particle trajectory behaviour. The timing between the rotational phase of the wing and the time-varying yaw velocity is a primary causes for distinct vortex dynamics observed in the cases of sym­metric, advanced, and delayed rotation. The leading edge vortex remains attached until the end of each half-stroke in the advanced rotation, whereas in symmetric rotation, the vortex gradually detaches from the the leading edge before the end of the half-stroke. The kinematics of the delayed rotation case allow the build up of a leading edge vortex twice in each stroke.