MILLER, L A; PESKIN, C S; Courant Institute of Mathematical Sciences, New York University; Courant Institute of Mathematical Sciences, New York University: Tiny insects, sticking vortices, and clap and fling

Although a large number of studies have investigated insect flight at Reynolds numbers of 100 and higher, few studies have considered the aerodynamics of flight at lower Reynolds numbers. Insects such as thrips and the tiny wasp Encarsia formosa fly at Reynolds numbers in the range of 10-20 and appear to push the lower limit of flight. It is well known that many of these tiny insects use the so called �clap and fling� mechanism to augment lift forces. What is not well known, however, is how increasing viscous effects change the aerodynamics of flight. We have used computational fluid dynamics to study changes in flight aerodynamics for Reynolds numbers between 8 and 128. More specifically, the immersed boundary method was used to model a two dimensional wing through one stroke cycle. Our results show that as the Reynolds number is lowered, viscous effects do not alter the flight aerodynamics in a continuous way. In fact, an aerodynamic transition is observed between the Reynolds numbers of 32 and 64. For Reynolds numbers of 64 and higher, lift coefficients of about 3 are generated during the pure translational phase of the upstroke when the leading edge vortex is attached to the wing and the trailing edge vortex is shed. For Reynolds numbers of 32 and lower, lift coefficients are significantly smaller during the upstroke, averaging about 1 � 1.5. In this case, both the leading and trailing edge vortices remain attached to the wing. Lower lift forces are generated in part because the trailing edge vortex is not shed. We suggest that �clap and fling� could add additional lift during translational phases of the stroke because the trailing edge vortex is not initially formed with such flight kinematics. This work was supported by the National Science Foundation under Grant Number DMS-9980069.