Rapid manoeuvring with spanwise-flexible wings

In this study, it is hypothesized that spanwise-profile bending contributes towards limiting leading-edge vortex (LEV) growth and increasing LEV stability in natural swimming and flight, due to the spanwise flow produced by profile bending. Specifically, as a propulsor undulates and subsequently bends, the profile tip can have a phase lag relative to the root, producing both a spanwise flow and an angle-of-attack gradient, transporting vorticity and thus circulation along its span. This relative phase of the profile tip versus the root is investigated experimentally using a combined pitching-and-flapping motion on a nominally two-dimensional NACA0012 profile, utilizing direct measurements of vorticity transport to estimate the circulation budget. In order to measure vorticity transport the entire velocity gradient tensor must be resolved, and therefore 4D-PTV, a high-density, time-resolved volumetric technique, was used to measure the flow around the profile. Tip-leading kinematics were found to increase LEV size and strength due to an unbalanced circulation budget: vorticity was not transported along the span, but instead accumulated to increase circulation. Meanwhile for tip-lagging kinematics, that mimics the bending found in nature, both reduced LEV size and circulation were observed, as vorticity transport acted to balance the circulation budget instead.