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Dragonfly flight: free-flight and tethered flow visualizations reveal a diverse array of unsteady lift-generating mechanisms, controlled primarily via angle of attack.
J Exp Biol. 2004 Nov; 207(Pt 24):4299-323.JE

Abstract

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.

Authors+Show Affiliations

Department of Zoology, Oxford University, South Parks Road, Oxford, OX1 3PS, UK. Adrian.thomas@zoo.ox.ac.ukNo affiliation info availableNo affiliation info availableNo affiliation info availableNo affiliation info available

Pub Type(s)

Comparative Study
Journal Article
Research Support, Non-U.S. Gov't

Language

eng

PubMed ID

15531651

Citation

Thomas, Adrian L R., et al. "Dragonfly Flight: Free-flight and Tethered Flow Visualizations Reveal a Diverse Array of Unsteady Lift-generating Mechanisms, Controlled Primarily Via Angle of Attack." The Journal of Experimental Biology, vol. 207, no. Pt 24, 2004, pp. 4299-323.
Thomas AL, Taylor GK, Srygley RB, et al. Dragonfly flight: free-flight and tethered flow visualizations reveal a diverse array of unsteady lift-generating mechanisms, controlled primarily via angle of attack. J Exp Biol. 2004;207(Pt 24):4299-323.
Thomas, A. L., Taylor, G. K., Srygley, R. B., Nudds, R. L., & Bomphrey, R. J. (2004). Dragonfly flight: free-flight and tethered flow visualizations reveal a diverse array of unsteady lift-generating mechanisms, controlled primarily via angle of attack. The Journal of Experimental Biology, 207(Pt 24), 4299-323.
Thomas AL, et al. Dragonfly Flight: Free-flight and Tethered Flow Visualizations Reveal a Diverse Array of Unsteady Lift-generating Mechanisms, Controlled Primarily Via Angle of Attack. J Exp Biol. 2004;207(Pt 24):4299-323. PubMed PMID: 15531651.
* Article titles in AMA citation format should be in sentence-case
TY - JOUR T1 - Dragonfly flight: free-flight and tethered flow visualizations reveal a diverse array of unsteady lift-generating mechanisms, controlled primarily via angle of attack. AU - Thomas,Adrian L R, AU - Taylor,Graham K, AU - Srygley,Robert B, AU - Nudds,Robert L, AU - Bomphrey,Richard J, PY - 2004/11/9/pubmed PY - 2005/3/9/medline PY - 2004/11/9/entrez SP - 4299 EP - 323 JF - The Journal of experimental biology JO - J. Exp. Biol. VL - 207 IS - Pt 24 N2 - 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. SN - 0022-0949 UR - https://www.unboundmedicine.com/medline/citation/15531651/Dragonfly_flight:_free_flight_and_tethered_flow_visualizations_reveal_a_diverse_array_of_unsteady_lift_generating_mechanisms_controlled_primarily_via_angle_of_attack_ L2 - http://jeb.biologists.org/cgi/pmidlookup?view=long&pmid=15531651 DB - PRIME DP - Unbound Medicine ER -