Petiolate wings: effects on the leading-edge vortex in flapping flight.
The wings of many insect species including crane flies and damselflies are petiolate (on stalks), with the wing planform beginning some distance away from the wing hinge, rather than at the hinge. The aerodynamic impact of flapping petiolate wings is relatively unknown, particularly on the formation of the lift-augmenting leading-edge vortex (LEV): a key flow structure exploited by many insects, birds and bats to enhance their lift coefficient. We investigated the aerodynamic implications of petiolationPusing particle image velocimetry flow field measurements on an array of rectangular wings of aspect ratio 3 and petiolation values ofP= 1-3. The wings were driven using a mechanical device, the 'Flapperatus', to produce highly repeatable insect-like kinematics. The wings maintained a constant Reynolds number of 1400 and dimensionless stroke amplitudeΛ* (number of chords traversed by the wingtip) of 6.5 across all test cases. Our results showed that for more petiolate wings the LEV is generally larger, stronger in circulation, and covers a greater area of the wing surface, particularly at the mid-span and inboard locations early in the wing stroke cycle. In each case, the LEV was initially arch-like in form with its outboard end terminating in a focus-sink on the wing surface, before transitioning to become continuous with the tip vortex thereafter. In the second half of the wing stroke, more petiolate wings exhibit a more detached LEV, with detachment initiating at approximately 70% and 50% span forP= 1 and 3, respectively. As a consequence, lift coefficients based on the LEV are higher in the first half of the wing stroke for petiolate wings, but more comparable in the second half. Time-averaged LEV lift coefficients show a general rise with petiolation over the range tested.
Structure and Motion Laboratory, Royal Veterinary College , University of London , Hatfield AL9 7TA , UK.
Nathan
0000-0003-1193-6747
N
Phillips
Aeromechanical Systems Group, Centre for Defence Engineering , Cranfield University , Defence Academy of the United Kingdom, Shrivenham SN6 8LA , UK.
Kevin
0000-0002-9627-9186
K
Knowles
Structure and Motion Laboratory, Royal Veterinary College , University of London , Hatfield AL9 7TA , UK.
Richard J
0000-0002-4748-0510
RJ
Bomphrey
flapping wing; insect flight; leading-edge vortex; micro air vehicle; petiolation