Meeting Abstract

P3.52  Jan. 6  Rotational accelerations stabilize leading edge vortices on revolving wings LENTINK, D*; DICKINSON, M.H.; Wageningen University, Wageningen, The Netherlands; Caltech, Pasadena, USA david.lentink@wur.nl

The remarkable flight performance of insects is largely due to a stably attached leading edge vortex (LEV) on top of their wings. As they flap back and forth, insect wings revolve around the hinge at their base, as propellers do around their shaft. Although both insect wings and propellers can generate stable LEVs and corresponding high lift under certain conditions, LEVs are unstable when generated by two-dimensional translating wings. The mechanism that accounts for the stability of LEVs on revolving wings under some conditions and their shedding on translating two-dimensional wings is not fully understood. Here we show that LEVs on revolving wings are stabilized by centripetal and Coriolis acceleration provided that the Rossby number, a dimensionless number that measures the relative importance of these rotational accelerations, is of order one. This implies that the key kinematic feature necessary for LEV-based aerodynamic force in insect flight is not the back and forth flapping of the wing but rather its propeller-like revolution. Our experiments show that this mechanism operates within a Reynolds number range of at least 110 to 14,000, and that even LEVs that have undergone spiral bursting remain coherent and continue to generate elevated aerodynamic force. These findings are analogous to observations on propellers at Re= 280.000 and wind turbines at even higher Re, which suggests that this mechanism extends across an even larger range of scales, well beyond the realm of flying insects. It is thus physically feasible that flying and swimming organisms employ LEV-based force augmentation to a much greater extent than has been previously appreciated.