Dragonfly flight: free-flight and tethered flow visualizations reveal a diverse array of unsteady lift-generating mechanisms, controlled primarilyviaangle of attack

Abstract

SUMMARY Here we show, by qualitative free- and tethered-flight flow visualization,that dragonflies fly by using unsteady aerodynamic mechanisms to generate high-lift, leading-edge vortices. In normal free flight, dragonflies use counterstroking kinematics, with a leading-edge vortex (LEV) on the forewing downstroke, attached flow on the forewing upstroke, and attached flow on the hindwing throughout. Accelerating dragonflies switch to in-phase wing-beats with highly separated downstroke flows, with a single LEV attached across both the fore- and hindwings. We use smoke visualizations to distinguish between the three simplest local analytical solutions of the Navier–Stokes equations yielding flow separation resulting in a LEV. The LEV is an open U-shaped separation, continuous across the thorax, running parallel to the wing leading edge and inflecting at the tips to form wingtip vortices. Air spirals in to a free-slip critical point over the centreline as the LEV grows. Spanwise flow is not a dominant feature of the flow field – spanwise flows sometimes run from wingtip to centreline, or vice versa –depending on the degree of sideslip. LEV formation always coincides with rapid increases in angle of attack, and the smoke visualizations clearly show the formation of LEVs whenever a rapid increase in angle of attack occurs. There is no discrete starting vortex. Instead, a shear layer forms behind the trailing edge whenever the wing is at a non-zero angle of attack, and rolls up, under Kelvin–Helmholtz instability, into a series of transverse vortices with circulation of opposite sign to the circulation around the wing and LEV. The flow fields produced by dragonflies differ qualitatively from those published for mechanical models of dragonflies, fruitflies and hawkmoths, which preclude natural wing interactions. However, controlled parametric experiments show that, provided the Strouhal number is appropriate and the natural interaction between left and right wings can occur, even a simple plunging plate can reproduce the detailed features of the flow seen in dragonflies. In our models, and in dragonflies, it appears that stability of the LEV is achieved by a general mechanism whereby flapping kinematics are configured so that a LEV would be expected to form naturally over the wing and remain attached for the duration of the stroke. However, the actual formation and shedding of the LEV is controlled by wing angle of attack, which dragonflies can vary through both extremes, from zero up to a range that leads to immediate flow separation at any time during a wing stroke.

Numerical Study on Hydrodynamic Performance of Bionic Caudal Fin

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Improvement of the aerodynamic performance by wing flexibility and elytra–hind wing interaction of a beetle during forward flight

“…How the leading edge becomes well aligned with the incoming flow during flight is dependent on wing flexibility. In an experimental study of a dragonfly, the mechanism involved in the foundation of the LEV was observed through a smoke wire visualization method [38]. The role of the AOA for the generation of the LEV was emphasized.…”

Bioinspired Corrugated Airfoil at Low Reynolds Numbers

“…The corrugated wing design does not appear to be very suitable for flight because it would have very poor aerodynamic performance (i.e., low lift and extremely high drag) according to traditional airfoil design principles. However, several studies on corrugated dragonfly wings in steady flow or gliding flight [4][5][6][7][8][9][10][11][12][13][14][15][16][17] have led to a surprising conclusion: a corrugated dragonfly wing could have comparable or even better aerodynamic performances (i.e., higher lift and bigger lift-to-drag ratio) than conventional streamlined airfoils in the low Reynolds number regime in which dragonflies usually fly.…”

“…Detailed studies were conducted more recently to try to elucidate the fundamental physics of the dragonfly flight aerodynamics [12][13][14][15][16][17]. A number of hypotheses have been suggested to explain the fundamental mechanism of the rather unexpected aerodynamic performance improvement of the corrugated dragonfly airfoils or wings over conventional smooth airfoils.…”