Mosquitoes exhibit unusual wing kinematics; their long, slender wings flap at remarkably high frequencies for their size (>800 Hz)and with lower stroke amplitudes than any other insect group1. This shifts weight support away from the translation-dominated, aerodynamic mechanisms used by most insects2, as well as by helicopters and aeroplanes, towards poorly understood rotational mechanisms that occur when pitching at the end of each half-stroke. Here we report free-flight mosquito wing kinematics, solve the full Navier–Stokes equations using computational fluid dynamics with overset grids, and validate our results with in vivo flow measurements. We show that, although mosquitoes use familiar separated flow patterns, much of the aerodynamic force that supports their weight is generated in a manner unlike any previously described for a flying animal. There are three key features: leading-edge vortices (a well-known mechanism that appears to be almost ubiquitous in insect flight), trailing-edge vortices caused by a form of wake capture at stroke reversal, and rotational drag. The two new elements are largely independent of the wing velocity, instead relying on rapid changes in the pitch angle (wing rotation) at the end of each half-stroke, and they are therefore relatively immune to the shallow flapping amplitude. Moreover, these mechanisms are particularly well suited to high aspect ratio mosquito wings.
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The authors were supported by the EPSRC (EP/H004025/1 and EP/M003698/1), BBSRC (BB/J001244/1). R.J.B. was supported by an EPSRC Career Acceleration Fellowship. S.M.W. was supported by a Royal Society University Research Fellowship. The work reported in this paper was funded by the Autonomous Systems Underpinning Research (ASUR) programme under the auspices of the Defence Science and Technology Laboratory (Dstl), UK Ministry of Defence. The authors acknowledge useful discussions with I. Russell and G. Gibson, P. Simoes for rearing the mosquitoes, and F. Albert-Davie and M. Inglis for assistance during raw data collection. The authors thank G. Taylor for the loan of four high-speed cameras purchased on European Research Council (ERC) grant 204513, and H. Liu for the permission to use the simulator and surface pressure distribution of the fruit fly wing.
Extended data figures
Video showing the experimental apparatus, raw data, wing geometry routine, kinematics, vortex wake (using isosurfaces of the Q-criterion), and pressure distribution and instantaneous flow fields at key instants (t1-t5) throughout the wing stroke cycle.
About this article
A chordwise offset of the wing-pitch axis enhances rotational aerodynamic forces on insect wings: a numerical study
Journal of The Royal Society Interface (2019)