Aerodynamics of active and passive dispersal of miniature insects with bristled wings

Abstract

Tiny insects with body lengths under 2 mm, such as thrips, use fringed/bristled wings for active flapping at Reynolds number (Re) on the order of 10. Even at such tiny scales these insects were found to fly effectively owing to significant variations in wing kinematics and bristled wing morphology. Very few data is available on these variations at such small scales. Morphological investigation on forewing images of bristled wings revealed large diversity in their wing design. This includes variations in gaping or spacing between pair of bristles (G), bristle diameter (D), number of bristles (n) and wing span (S). In the present study, we quantified these design parameters from forewing images of 59 species of thrips and fairyfly species from previously published data. Physical scaled-up bristled wing models were then fabricated based on these parameters and tested for aerodynamic force generation using a robotic model. Results revealed that tiny insects may experience less biological pressure to optimize n or G/D for a given wingspan. Thrips have been observed to use wing-wing interaction via the clap and fling mechanism to augment lift generation. However, drag was also found to significantly increase. We found that tiny insects use large rotation angle to reduce this drag and proposed that circulatory lift alone cannot explain lift force generation and other lift generating mechanisms such as pressure distribution in the flow field were discussed. Actively flying at such tiny scales demands lot of power and these miniature insects were found to employ two additional strategies that helps in overcoming large power demand. We found that pausing between upstroke and downstroke decrease power required during a cycle with small compromise in lift. In addition to active flight, these insects can intermittently parachute by spreading their bristled wings at a particular inter-wing angle (O). We found that a dense bristled wing maintains aerodynamic loading relative to leakiness through the bristles for O>/=100 degrees. Also, we developed a scaled up robotic flapping device that could mimic any flapping flight in a horizontal stroke plane and proposed that pitch rate significantly alters the aerodynamic force generation compared to wing revolution.