Petiolate wings: effects on the leading-edge vortex in flapping flight

Abstract

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 petiolation P using particle image velocimetry flow field measurements on an array of rectangular wings of aspect ratio 3 and petiolation values of P = 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 amplitude Λ* (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 for P = 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.

On the lift-optimal aspect ratio of a revolving wing at low Reynolds number

“…In particular, C L is found to increase roughly linearly as Ro decreases within the range of Ro considered, with a slope that appears to be figure 11a. This is also in line with recent results on flapping and revolving wings (obtained by Phillips et al [31] and Smith et al [32], respectively) where increased LEV growth rate, quicker vortex shedding and lower lift were observed with increasing Ro. Second, it can be seen from figure 13 that LEV attachment is highly correlated with the development of spanwise flow.…”

The role of the leading edge vortex in lift augmentation of steadily revolving wings: a change in perspective

“…Later, Phillips et al [28] adopted the findings of Pitt Ford and Babinsky for unsteady translating wings, and calculated the lift based on the circulation contained only within the formed LEV for a series of flapping wings with different Rossby number (note that translating motion corresponds to infinite Rossby number while revolving motion corresponds to a finite Rossby number value equivalent to the ratio of the radius of gyration to the mean chord [19]). The LEV lift coefficient was found to increase as the Rossby number increases [28], a result which is inconsistent with previous experimental force measurements [19] and numerical simulation [29]. Thus, while unsteady effects undoubtedly play an effect in establishing the flow field, the evidence for this in the literature is incomplete.…”

The leading-edge vortex on a rotating wing changes markedly beyond a certain central body size

“…Recently, Phillips et al [ 31 ] have studied the three-dimensional (3D) LEV structures on the flapping petiolate wings by extending the root of a rectangular wing from its flapping axis. The petiolation has been calculated as the ratio of the wing-offset to the wing-chord ( P = b 0 / c ).…”