Takeshi Ohnuki - ohnuki@kawachi.jst-c.go.jp
Masako Sakayanagi
Keiji Kawachi
Millibioflight Project,
ERATO, JRDC
Park Bldg. 3F, 4-7-6, Komaba, Meguro, Tokyo, 153, Japan
Popular Version of Paper 5aEA11
Presented Friday morning, December 6, 1996
3rd Joint ASA/ASJ Meeting,Honolulu, Hawaii
Embargoed until December 6, 1996
It is well understood how an airplane flies. The sophisticated airfoil shape with a round leading edge and a sharp trailing edge produces a lift force effectively and a 350-ton Jumbo Jet can fly in the air. On the contrary, insect wings are very thin and rough and have to move very quickly to get appropriate aerodynamic forces. Though the basic equations governing the aerodynamics of flying insects and airplanes are identical, there still remains room for explanation of how an insect flies.
Insect wings produce more lift than predicted by the conventional steady or quasi-steady aerodynamic analysis. The reciprocating kinematics of the wings makes the aerodynamics strongly unsteady and this makes it difficult to understand. There have been proposed several mechanisms such as fling, peel, or flex mechanisms[1] trying to explain the unsteady aerodynamic forces generated from the beating wings. It is, however, difficult to give aerodynamic validity to those mechanisms. We are taking an aeroacoustics approach to explain the unsteady aerodynamics.
We calculated the flow field around a hovering insect by means of a Computational Fluid Dynamics (CFD) method coupled with measured wing kinematics of a live insect[2]. Simulating the three-dimensional unsteady flow around the moving wing, we captured a leading-edge vortex on the lower surface of the wing during up-stroke as well as pronation and on the upper surface during down-stroke, and captured a trailing-edge vortex during supination. With this information, sound field around the insect was calculated using Ffowcs-Williams and Hawking's equation[3]. This calculation gives us evaluation of sound components. Sound generated by a moving wing can be physically understood as composition of wing thickness noise, loading noise, and other elements such as vortex noise. Because the velocity of the wing tip is much smaller than the velocity of sound in insect flight, the compressibility of the air can be neglected. In this case, it becomes easy to calculate the beating noise from given aerodynamic load on the wing surfaces.
We selected a bumblebee as a sample of this study. A bumblebee (Bombus terrestris) is suitable for this research purpose because phenomenon around the flying bumblebee is completely unsteady and the sound pressure level is big enough for measurement due to its high wing loading. Tethering a bumblebee, we measured the beating sounds from various direction in a sound proof chamber.
References
[1]Ellington, C. P. "Unsteady Aerodynamics of Insect Flight", Biological Fluid Dynamics, The Company of Biologists Limited Press, pp. 109-129, 1995
[2]Zeng, L., Liu, H., and Kawachi, K. "Measurement and Flow Visualization of a Beating Bumblebee Wing", Submitted to The 1st Pacific Symposium on Flow Visualization and Image Processing, 1997
[3]Ffowcs Williams, J. E. and Hawkings, D. L. "Sound Generation by Turbulence and Surfaces in Arbitrary Motion", Philosophical Transactions of the Royal Society of London, A264, pp. 321-342, 1969