Aerodynamic effects of varying solid surface area of bristled wings performing clap and fling

Abstract

The smallest flying insects with body lengths under 2 mm show a marked preference for wings consisting of a thin membrane with long bristles, and the use of clap and fling kinematics to augment lift at Reynolds numbers (Re) of approximately 10. Bristled wings have been shown to reduce drag forces in clap and fling, but the aerodynamic roles of several bristled wing geometric variables remain unclear. This study examines the effects of varying the ratio of membrane area (A M ) to total wing area (A T ) on aerodynamic forces and flow structures generated during clap and fling at Re on the order of 10. We also examine the aerodynamic consequences of scaling bristled wings to Re = 120, relevant to flight of fruit flies. We analyzed published forewing images of 25 species of thrips (Thysanoptera) and found that A M /A T ranged from 14% to 27%, as compared to 11% to 88% previously reported for smaller-sized fairyflies (Hymenoptera). These data were used to develop physical bristled wing models with A M /A T ranging from 15% to 100%, which were tested in a dynamically scaled robotic clap and fling model. At all Re, bristled wings produced slightly lower lift coefficients (C L ) when compared to solid wings, but provided significant drag reduction. At Re = 10, largest values of peak lift over peak drag ratios were generated by wing models with A M /A T similar to thrips forewings (15% to 30%). Circulation of the leading edge vortex and trailing edge vortex decreased with decreasing A M /A T during clap and fling at Re = 10. Decreased chordwise circulation near the wing tip, vortex shedding, and interaction between flow structures from clap with those from fling resulted in lowering C L generated via clap and fling at Re = 120 as compared to Re = 10. Clap and fling becomes less beneficial at Re = 120, regardless of the drag reduction provided by bristled wings.

Inter-species variation in number of bristles on forewings of tiny insects does not impact clap-and-fling aerodynamics

“…1A. Average wing chord ( C ave ) was calculated by measuring A T using the same procedure as in Jones et al (2016) and Ford et al (2019) and dividing A T by S max . As the forewing images obtained from the various sources were aligned in different orientations, we rotated the wings before measurements such that they were always oriented horizontally.…”

“…Due to the lack of adequately resolved free-flight recordings for characterizing instantaneous wing kinematics of tiny insects, we used a modified version of 2D clap-and-fling kinematics developed by Miller and Peskin (2005). Similar or modified forms of these kinematics have been used in several other studies (Miller and Peskin, 2009; Santhanakrishnan et al, 2014; Arora et al, 2014; Jones et al, 2016; Kasoju et al, 2018; Ford et al, 2019). The simplified kinematics used here do not capture: (a) 3D flapping translation during downstroke and upstroke, and (b) wing rotation at the end of downstroke (‘supination’).…”

“…Similar to Kasoju et al (2018) and Ford et al (2019), force measurements were performed using L-brackets with strain gauges mounted in half-bridge configuration (drag bracket shown in Fig. 3A).…”

“…However, the concomitant generation of large drag force at the start of fling undermines the lucrativeness of clap-and-fling at Re c relevant to tiny insect flight (Miller and Peskin, 2005; Arora et al, 2014). Previous studies (Santhanakrishnan et al, 2014; Jones et al, 2016; Kasoju et al, 2018; Ford et al, 2019) have shown that bristled wings can reduce the force required to fling the wings apart.…”

“…Ford et al (2019) found that the ratio of solid membrane area ( A M ) to total wing area ( A T ) in the forewings of 25 species of thrips (Thysanoptera) ranged from 14% to 27%, as compared to the A M / A T range of 11% to 88% in smaller-sized fairyflies examined by Jones et al (2016). Using physical models that were prescribed to execute clap-and-fling kinematics, Ford et al (2019) found that lift to drag ratios were largest for bristled wing models with A M / A T similar to thrips forewings. Inter-species variation of G, D , wing span ( S ) and number of bristles ( n ), as well as their concomitant effects on clap-and-fling aerodynamics, are currently unknown.…”