Aerodynamic Force Generation in Hovering Flight in a Tiny Insect

Abstract

Aerodynamic force generation in hovering flight in a tiny insect, Encarsia formosa, has been studied. The Reynolds number of the flapping wings (based on the mean chord length and the mean flapping velocity) is around 15. The flapping motion of the insect is unique in that the wing pair claps together near the end of an upstroke and flings open at the beginning of the subsequent downstroke. The method of solving the Navier-Stokes equations over moving overset grids is used. The fling produces a large lift peak at the beginning of the downstroke, the mechanism of which is the generation of a vortex ring containing a downward jet in a short period; the clap produces a large lift peak near the end of the subsequent upstroke by a similar mechanism. Because the vorticity generated during the clap and fling diffuses rapidly, the clap and fling has little influence on the flows in the rest part of the stroke cycle. The mean lift is enough to support the weight of the insect. The lift peaks due to the clap and fling result in more than 30% increase in mean lift coefficient compared to the case of flapping without clap and fling.