Vortex trapping recaptures energy in flying fruit flies

Abstract

Flapping flight is one of the most costly forms of locomotion in animals. To limit energetic expenditures, flying insects thus developed multiple strategies. An effective mechanism to reduce flight power expenditures is the harvesting of kinetic energy from motion of the surrounding air. We here show an unusual mechanism of energy harvesting in an insect that recaptures the rotational energy of air vortices. The mechanism requires pronounced chordwise wing bending during which the wing surface momentary traps the vortex and transfers its kinetic energy to the wing within less than a millisecond. Numerical and robotic controls show that the decrease in vortex strength is minimal without the nearby wing surface. The measured energy recycling might slightly reduce the power requirements needed for body weight support in flight, lowering the flight costs in animals flying at elevated power demands. An increase in flight efficiency improves flight during aversive manoeuvring in response to predation and long-distance migration, and thus factors that determine the worldwide abundance and distribution of insect populations.

Flight efficiency is a key to diverse wing morphologies in small insects

“…As mentioned above, it has been suggested that bristles might be an adaption to clap-and-fling kinematics because bristles lower the forces needed to pull the wing surfaces apart during fling motion (Hymenoptera: Aphelinidae [ 34 ] and Mymaridae [ 37 ]; Thysanoptera: Phlaeothripidae [ 25 ]; Diptera: Drosophilidae [ 47 ]). We thus repeated the simulations on two wings with Δ b = 0, 0.033 and 0.081 at Re = 14, using a modified lift-based wingbeat kinematics (flapping angle amplitude = 180°) with near-clap conditions (§4, electronic supplementary material).…”

Flow development and leading edge vorticity in bristled insect wings

“…Although the flow during wing flapping is three-dimensional, fluid velocities in the spanwise direction of the wing (axial flow) were thought to be negligible because of low Reynolds number. This assumption is supported by detailed experimental and numerical analyses on the loss of enstrophy of the LEV in Drosophila model wings (see supplemental material in Lehmann et al 2021). The latter study found that axial flow and vortex stretching are minimal during wing flapping, compared to other flow components.…”

“…Figure 5a-l shows that low Reynolds number impedes vortex shedding, keeping LEV and TEV close to the wing during the stroke reversals. This is different from wing rotation in larger insects such as the fruit fly (Re ~ 130, Lehmann et al 2021) and wing models flapping at similar Reynolds number (Lehmann et al 2005). A previously published study has shown that in fruit flies, the wing bends chordwise during the dorsal stroke reversal, trapping the upstroke LEV after it is shed during wing rotation.…”

“…This behaviour is different in hovering flight at higher Reynolds numbers and smaller viscous forces (e.g. Dickinson et al 1999;Lehmann et al 2021;Sun and Tang 2002;Sane 2003). The behaviour of LEV and TEV at the reversals are described in more detail in one of the following paragraphs.…”

“…This means that LEV, TEV and bound circulation must be shed from the wing into the wake at the end of each half stroke. It has been shown that part of this circulation may be recycled by recapturing kinetic energy and enstrophy from the surrounding fluid (Dickinson et al 1999;Lehmann et al 2021). This even works in animals such as birds when flying in flocks, in which shed tip vortices from leading animals are re-used by animals flying behind (Weimerskirch et al 2001;May 1979;Bajec and Heppner 2009).…”