Dickinson, Michael H. and Götz, Karl G. (1993) Unsteady Aerodynamic Performance of Model Wings at Low Reynolds Numbers. Journal of Experimental Biology, 174 (1). pp. 45-64. ISSN 0022-0949 http://resolver.caltech.edu/CaltechAUTHORS:20120307-134504204
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The synthesis of a comprehensive theory of force production in insect flight is hindered in part by the lack of precise knowledge of unsteady forces produced by wings. Data are especially sparse in the intermediate Reynolds number regime (10<Re<1000) appropriate for the flight of small insects. This paper attempts to fill this deficit by quantifying the time-dependence of aerodynamic forces for a simple yet important motion, rapid acceleration from rest to a constant velocity at a fixed angle of attack. The study couples the measurement of lift and drag on a two-dimensional model with simultaneous flow visualization. The results of these experiments are summarized below. 1. At angles of attack below 13.5°, there was virtually no evidence of a delay in the generation of lift, in contrast to similar studies made at higher Reynolds numbers. 2. At angles of attack above 13.5°, impulsive movement resulted in the production of a leading edge vortex that stayed attached to the wing for the first 2 chord lengths of travel, resulting in an 80 % increase in lift compared to the performance measured 5 chord lengths later. It is argued that this increase is due to the process of detached vortex lift, analogous to the method of force production in delta-wing aircraft. 3. As the initial leading edge vortex is shed from the wing, a second vortex of opposite vorticity develops from the trailing edge of the wing, correlating with a decrease in lift production. This pattern of alternating leading and trailing edge vortices generates a von Karman street, which is stable for at least 7.5 chord lengths of travel. 4. Throughout the first 7.5 chords of travel the model wing exhibits a broad lift plateau at angles of attack up to 54°, which is not significantly altered by the addition of wing camber or surface projections. 5. Taken together, these results indicate how the unsteady process of vortex generation at large angles of attack might contribute to the production of aerodynamic forces in insect flight. Because the fly wing typically moves only 2–4 chord lengths each half-stroke, the complex dynamic behavior of impulsively started wing profiles is more appropriate for models of insect flight than are steady-state approximations.
|Additional Information:||© 1993 The Company of Biologists Limited. Accepted 17 August 1992. We wish to thank Fritz Olaf-Lehman and Roland Strauss for intellectual and technical support during the execution of this work. The project was funded by the Max-Planck Gesellschaft.|
|Subject Keywords:||insect, flight, wings, aerodynamics, vortex lift.|
|Official Citation:||M. H. Dickinson and K. G. Gotz UNSTEADY AERODYNAMIC PERFORMANCE OF MODEL WINGS AT LOW REYNOLDS NUMBERS J Exp Biol 1993 174:45-64.|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Ruth Sustaita|
|Deposited On:||12 Mar 2012 22:00|
|Last Modified:||26 Dec 2012 14:55|
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