Vortex dynamics in the wake of bio-inspired flexible, slotted winglets

Flexible wingtip extensions matched to the Cauchy and Reynolds numbers of a peregrine falcon’s primary feather in flight have been tested in differing configurations and compared to rigid ones for reference. The wingtip configurations were attached to the end of a symmetric (NACA 0012) aerofoil and were tested at 5° and 10° angles of attack and Reynolds numbers of $70k$ and $90k$. Time resolved particle image velocimetry (TR-PIV) was used to study the dynamics of the individual vortices. The results show that, at increased angle of attack the configuration with C-type variation of the free length of the winglets is spreading the vorticity into spanwise and vertical directions, generating a circular multi-core vortex arrangement. In contrast, for the case of winglets of the same length (I-type configuration) a continuous vortex sheet is formed which rolls up into a single core, dislocated outboards and upwards from the original tip-vortex location. This remains the case even for larger angle of attack. It is concluded that – besides the known reduction of induced drag – the former configuration is also beneficial for a more rapid disintegration of the tip-vortex in the wake, while the latter shows less instability. This let us speculate that the latter could be relevant for reducing the tip-noise at higher angle of attack such as for Owls, who hunt during night and have adapted to fly silently.