Fluid dynamics of the flapping wings of the smallest insects

In contrast to larger species, little is known about the flight of the smallest flying insects, such as thrips and fairyflies. These tiny animals range from 300 to 1000 microns in length and fly at Re ranging from about 4 to 60. Previous work with numerical and physical models has shown that the aerodynamics of these diminutive insects is significantly different from that of larger animals, but most of these studies have relied on two-dimensional approximations. We performed a systematic study of the forces and flow structures around a three-dimensional revolving elliptical wing. We used both a dynamically scaled physical model and a three-dimensional computational model. The results showed that dimensionless drag, aerodynamic efficiency, and spanwise flow all decrease with decreasing Re. In addition, both the leading and trailing edge vortices remain attached to the wing over the scales relevant to the smallest flying insects. The numerical portion of the work is then extended to a pair of three-dimensional wings performing a clap and fling. Lift is slightly enhanced, but drag is also significantly increased. Overall, these observations suggest that there are drastic differences in the aerodynamics of flight at the scale of the smallest flying animals.