MILLER, L. A.; University of Utah: Clap and peel in insect flight with flexible and bristled wings

In ‘typical’ insect flight, lift is produced when a leading edge vortex (LEV) is formed and remains attached to the wing and a trailing edge vortex (TEV) is formed and separates from the wing during each stroke. The wings translate along a horizontal plane, and lift forces are produced by this ‘vortical asymmetry’ (attached LEV and shed TEV). For Reynolds numbers below 40, numerical simulations and physical models have shown that lift is reduced when the TEV is no longer shed from the wing, resulting in vortical ‘near-symmetry.’ Previous work also suggests that very small insects have developed a trick known as ‘clap and fling’ to augment lift production. The clap and fling motion introduces vortical asymmetry when the wings fling apart at the beginning of the downstroke. Although the lift enhancing effects of clap and fling increase at lower Reynolds numbers, the drag forces increase considerably, and the ratios of lift to drag forces decrease. Moreover, the relative forces required to clap the wings together and to fling the wings apart grow drastically as the Reynolds number is lowered. A porous media version of the immersed boundary method was used to simulate flexible, bristled wings performing a clap and fling. The results of this study suggest that wing flexibility can lower the drag forces required to fling the wings apart while actually increasing the lift force generated. Augmented lift can be attributed to a flexible peel motion rather than a rigid fling motion. Furthermore, wing ciliation (fringing) might also reduce the drag force required to fling the wings. During fling, the wing surface might act like a permeable ‘rake,’ but during translation the wing surface might act like a solid plate.