Meeting Abstract
The smallest free-flying insects (body lengths between 0.2 to 1 mm) show a marked preference for wings consisting of a thin solid membrane with long bristles at the margins. Flapping flight of these insects occurs under highly viscous conditions where Reynolds number (Re) is on the order of 10. Obligate use of clap and fling kinematics is also seen in tiny insects such as thrips. Recent evidence has shown that bristled wings can provide drag reduction benefits during clap and fling. However, the biological range of variation in bristled wing geometry in tiny insects remains unclear. In this study, we examined the aerodynamic effects of varying the ratio of solid membrane area (MA) to total area (TA) of the wing. Forewing images of thrips were analyzed from published data and MA/TA was found to range from 14-27%. A dynamically scaled robotic platform was used to test 6 different wing pairs with MA/TA varying from 15% to 100%. Experiments were performed with each wing pair at Re=10, and also at Re=120 that is relevant to larger insects such as the fruit fly. Dimensionless lift and drag forces were measured using strain gauges attached to the wing root. Time-resolved particle image velocimetry was performed at 3 different chordwise planes along the span in order to visualize the leading and trailing edge vortices. With decreasing MA/TA, peak lift and drag coefficients decreased, but the lift to drag ratio increased. The magnitude of circulation about the leading and trailing edges of the wings also decreased with decreasing MA/TA. Leading and trailing edge vortices remained attached throughout the stroke at Re=10, while vortex shedding was observed at Re=120 in wing models with high MA/TA. Our results suggest that clap and fling with bristled wings is more advantageous for Re=10 as compared to Re=120.
