Abstract
Particle image velocimetry has been the preferred experimental technique with which to study the aerodynamics of animal flight for over a decade. In that time, hardware has become more accessible and the software has progressed from the acquisition of planes through the flow field to the reconstruction of small volumetric measurements. Until now, it has not been possible to capture large volumes that incorporate the full wavelength of the aerodynamic track left behind during a complete wingbeat cycle. Here, we use a unique apparatus to acquire the first instantaneous wake volume of a flying animal's entire wingbeat. We confirm the presence of wake deformation behind desert locusts and quantify the effect of that deformation on estimates of aerodynamic force and the efficiency of lift generation. We present previously undescribed vortex wake phenomena, including entrainment around the wing-tip vortices of a set of secondary vortices borne of Kelvin–Helmholtz instability in the shear layer behind the flapping wings.
Highlights
In order to understand the mechanics by which animals fly, it is important to understand their interaction with the air
The utility of animal flight investigations using a quantitative flow imaging technique known as particle image velocimetry (PIV) has increased in recent times, mirroring advances in the technology available to researchers
The aerodynamics of locusts have been studied in progressively greater detail over the years using smoke visualization [18], longitudinal PIV [19], transverse PIV [5] and time-resolved, small-volume, tomo-PIV [8], all of which provide a solid framework upon which the results from this large-volume, tomo-PIV experiment can build
Summary
In order to understand the mechanics by which animals fly, it is important to understand their interaction with the air This interaction, mediated by the flapping wings, leaves behind a wake or aerodynamic footprint that can be captured, quantified and subsequently assessed within the framework of aerodynamic theory. The step to time-resolved data, where sampling rate is in excess of the wingbeat frequency, came some time later [3,4,5]. These studies marked the introduction to animal flight experiments of stereo-PIV, where three-component vectors could be arranged on the illuminated measurement plane through the wake. In studies of animal wakes, it has become commonplace to reconstruct wakes into a pseudothree-dimensional arrangement by assembling, or stacking, stereo planes [3,4,5]
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