Spanwise Flow on an Impulsively-Started Rotating Wing at Low Reynolds Numbers

For a significant portion of the wingbeat cycle in insect-like flapping, the wing sweeps at more or less constant speed and angle of attack. The aerodynamics associated with this phase of insect-like wing kinematics are investigated by making 2D-2C particle image velocimetry measurements on a rotating wing immersed in a tank of seeded water. In an attempt to capture flow features pertaining to many insects and future flapping-wing micro air vehicles, the experiments are carried out at two Reynolds numbers—500 and 15000—and various techniques, including vortex identification schemes, are used to analyse the results. The behaviour of the leading-edge vortex, especially in terms of its spanwise propagation, is studied here. Results show that, despite its unsteady nature, the structure of the leading-edge vortex is essentially stable once fully developed. I