Insect Flight
Description of Research
Fore and Hind Wing Interactions of Dragonfly Flight -
(DR, ZJW, 2003 [PDF]; ZJW, DR 2007 [PDF])
A distinctive feature of dragonflies is their use of two pairs of wings instead of one pair. This reflects their ancient origin. As such, understanding the coupling between their fore and hind wings might shed light on the evolution of flight based on four wings to that based on two. We are tackling this problem on two fronts. The first is to develop a computational tool to simulate flows around multiple bodies in relative motion. The second involves an experiment on tethered dragonflies in which we measure the 3D wing kinematics and vertical forces. The kinematics is an input to our computation and we are analyzing the forces due to wing coupling .
Passive Motions in Insect Flight -
(AJB, SX, ZJW, 2007 [PDF])
Wing pitch reversal is the rapid change of angle of attack near stroke transition. This motion represents one of the most significant differences between hovering with flapping wings and with a continuously rotating blade (e.g. helicopter flight). We found the surprising fact that, eventhough insects have the musculature to control this motion, aerodynamic and wing inertia forces are sufficient to pitch the wing without the aid of the muscles.
Erratic Trajectory of Butterfly Flight
Some butterflies appear to fly crooked, lurching here and there. Their close relatives, moths, however appear to fly relatively straight. Why? We have filmed free butterfly flight under different conditions and extracted their 3D trajectories. Currently we are analyzing the data in the hopes of figuring out a correlation between their flight behavior and wing geometry.
Forward Flight -
(ZJW, 2000 [PDF])
Frequency variation among different species is one of the distinctive features of insect flight. However, classical quasi-steady analysis does not predict a preferred frequency.Computations of a minimal model of forward flapping motion shows an unsteady mechanism for selecting a preferred range of flapping frequency for a given size of wing. The frequency range is dictated by the time scales associated with the growth of the trailing and leading edge vortices. The predicted inverse scaling between the frequency and the length of the wing is consistent with the zoological data available for birds and insects over almost three orders of magnitude in wing length.
Hovering Flight -
(ZJW, 2000 [PDF])
Hovering is an extreme mode of flight where the forward velocity is zero. To do this, insects must draw clean air from the ambient flow and get rid of the `messy vortices' they have created to obtain a large periodic lift. Solutions to the two dimensional Navier Stokes equation around generic hovering wing undergoing motions similar to dragonfly flight revealed a process by which a downward jet of pairs of counter-rotating vortices is created. The momentum carried down by the dipoles provides enough lift to support a typical insect's weight. This two dimensional mechanism of lift generation has a natural extension in three dimensions, where the pairs of vortices are replaced by vortex rings.
Using Drag to Hover and Lessons About Hovering Efficiency -
(ZJW, 2004 [PDF])
Airplanes and helicopters are airborne via aerodynamic lift, not drag. However, it is not
a priori clear that insects use only lift to fly. A dragonfly uses mostly drag to hover by employing asymmetric up and down strokes along an inclined stroke plane. But is it efficient to use drag? Our computations of a family of strokes, which use various combination of lift and drag to produce the net vertical force, show that using drag can be as efficient as using lift. This finding further lead us to construct a simple example in which hovering efficiency can be improved two-fold when using both lift and drag, compared to using lift alone.
Comparing Against Experiments -
(ZJW, JB, MD, 2004 [PDF])
We compare two dimensional computations against the robotic fruitfly experiments. In particular, we investigate unsteady effects and the degree of agreement between two dimensional computations and three dimensional experiments in several qualitatively different flows. Analysis of the success and failure of a two dimensional model in capturing the forces and flows in three dimensional experiments provides new insight about the role of three dimensional effects in flapping flight, for example, the relevance of the axial flow in dynamic stall.
Unsteady Forces on an Accelerating Plate -
(ZJW, 2000 [PDF])
The vortical flow seen in simulation of hovering flight suggested a need for better understanding of separated flow behind a plate, but there have been limited theoretical tools. To fill this gap, we study the aerodynamic forces on a flat plate accelerating from rest at fixed incidence in two-dimensional power-law flow analytically and numerically. An inviscid approximation is made in which separation at the two plate edges is modeled by growing spiral vortex sheets, whose evolution is determined by the Birkhoff-Rott equation, and solved with a similarity expansion. This gives a mechanism for dynamic stall based on a combination of unsteady vortex lift and pure added mass; the incidence angle for maximum vortex lift is arccos[sqrt(3/8)] or approximately 52.2° independent of the acceleration profile. Circulation on the flat plate makes no direct contribution. Both lift and drag force predictions from the unsteady inviscid theory are compared with those obtained from numerical solutions of the two-dimensional unsteady Navier Stokes equations for an ellipse of high aspect ratio, and with predictions of Wagner's classical theory. Estimates for the shed circulation and the size of the start-up vortices are also obtained.
Publications
- Z. Jane Wang and David Russell, Effect of Forewing and Hindwing Interactions on Aerodynamic Forces and Power in Hovering Dragonfly Flight, Phys. Rev. Lett. 99.14, 148101 (2007) [PDF]
- A. J. Bergou, S. Xu, and Z. J. Wang, Passive wing pitch reversal in insect flight, J. Fluid Mech. 591, 321-337 (2007) [PDF]
- G. Berman, and Z. J. Wang, Energy-minimizing kinematics in hovering insect flight, J. Fluid Mech. 582, 153-168 (2007) [PDF]
- Z. Jane Wang, Dissecting Insect Flight, Annu. Rev. Fluid Mech. 2005.37, 183-210 (2005) [PDF]
- David B. Russell, Numerical and Experimental Investigations into the Aerodynamics of Dragonfly Flight (2004)
- Z. Jane Wang, The role of drag in insect hovering, J. Expr. Bio. 207,4147-4155 (2004) [PDF]
- Z. Jane Wang, James M. Birch, and Michael H. Dickinson, Unsteady forces and flows in low Reynolds number hovering flight: two-dimensional computations vs robotic wing experiments, J. Expr. Bio. 207,449-460 (2004) [PDF]
- D.I. Pullin and Z. Jane Wang, Unsteady forces on an accelerating plate and application to hovering insect flight, J. Fluid Mech 509, 1-21 (2004) [PDF]
- Z. Jane Wang, Unsteady Aerodynamics of Insect Flight, Computational Modeling in Biological Fluid Dynamics, IMA Volumes in Mathematics and its Applications, Springer 2001
- Z. Jane Wang, Computations of Insect Hovering, Mathematical Methods in the Applied Sciences, Wiley, vol. 209 (2001)
- Z. Jane Wang, Two Dimensional Mechanism for Insect Hovering, Phys. Rev. Lett. 85.10, 2216-2219 (2000) [PDF]
- Z. Jane Wang, Vortex shedding and frequency selection in flapping flight, J. Fluid Mech. 410, 323-341 (2000) [PDF]
