Determining the relative stability of leading-edge vortices on nominally two-dimensional flapping profiles

It is hypothesized that the relative stability of leading-edge vortices (LEVs) on flapping profiles can be improved by moderating LEV growth through spanwise vorticity convection and vortex stretching. Moreover, it is hypothesized that the reduced frequency k and profile sweep Lambda are critical in predicting relative LEV stability as determined by the aforementioned effects. These hypotheses are then confirmed experimentally with phase-averaged particle image velocimetry (PIV) and three-dimensional particle tracking velocimetry. In particular, more stable LEVs are observed at higher reduced frequencies, which is argued to represent the ratio between the limiting vortex size and the rate of vorticity feeding. The introduction of profile sweep increased both relative LEV stability and spanwise vorticity transport. It is thought that spanwise vorticity transport improved LEV stability by acting as a sink for vorticity generated in the leading-edge shear layer.