Tytell, E.D.; Ellington, C.P.: The vortex wake of the hawkmoth, Manduca sexta: Simulation and Reynolds number effects

The vortex wake structure of the hawkmoth, Manduca sexta, was investigated using a vortex ring generator. Based on existing kinematic and morphological data, a piston and tube apparatus was constructed to produce circular vortex rings with the same size and disc loading as a hovering hawkmoth. The artificial rings were shown to be qualitatively similar to the vortex wake of actual hawkmoths. They were initially laminar, but developed an azimuthal wave instability and became turbulent. The initial impulse and circulation was accurately estimated for laminar rings using particle imaging velocimetry (PIV); after the transition to turbulence, though, initial circulation was generally underestimated. The underestimate for turbulent rings can be corrected if transition time and velocity profile are accurately known, but this correction will not be feasible for experiments on real animals. It is therefore crucial that the circulation and impulse are estimated while the wake vortices are still laminar. Based on the scaling of ring Reynolds number, hawkmoths and animals with similar wingspan and wing beat frequency are probably the largest for which the laminar period lasts long enough during hovering. Thus, at low advance ratios, they may be largest animals for which wake circulation and impulse can be accurately measured.