Leaky Flow through Simplified Physical Models of Bristled Wings of Tiny Insects during Clap and Fling

Abstract

In contrast to larger flight-capable insects such as hawk moths and fruit flies, miniature flying insects such as thrips show the obligatory use of wing-wing interaction via "clap and fling" during the end of upstroke and start of downstroke. Although fling can augment lift generated during flapping flight at chord-based Reynolds number (Re) of 10 or lower, large drag forces are necessary to clap and fling the wings. In this context, bristles observed in the wings of most tiny insects have been shown to lower drag force generated in clap and fling. However, the fluid dynamic mechanism underlying drag reduction by bristled wings and the impact of bristles on lift generated via clap and fling remain unclear. We used a dynamically scaled robotic model to examine the forces and flow structures generated during clap and fling of: three bristled wing pairs with varying inter-bristle spacing, and a geometrically equivalent solid wing pair. In contrast to the solid wing pair, reverse flow through the gaps between the bristles was observed throughout clap and fling, resulting in: (a) drag reduction; and (b) weaker and diffuse leading edge vortices that lowered lift. Shear layers were formed around the bristles when interacting bristled wing pairs underwent clap and fling motion. These shear layers lowered leakiness of flow through the bristles and minimized loss of lift in bristled wings. Compared to the solid wing, peak drag coefficients were reduced by 50-90% in bristled wings. In contrast, peak lift coefficients of bristled wings were only reduced by 35-60% from those of the solid wing. Our results suggest that the bristled wings can provide unique aerodynamic benefits via increasing lift to drag ratio during clap and fling for Re between 5 and 15.

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

“…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.…”

“…Despite their agricultural and ecological importance in acting as biological vectors of plant viruses and as invasive pests of commercially important plants (Ullman et al, 2002; Jones, 2005), our understanding of the flight biomechanics of tiny insects is far from complete. Due to the difficulty in acquiring free-flight recordings of tiny insects, several studies have used physical and computational modeling to examine the functional significance of wing bristles (Santhanakrishnan et al, 2014; Jones et al, 2016; Lee and Kim, 2017; Kasoju et al, 2018). However, little is known about the extent of variation in bristled wing morphology among different species of tiny insects.…”

“…Although a number of studies have examined the flow structures and aerodynamic forces generated by bristled wings in comparison with solid wings (Sunada et al, 2002; Santhanakrishnan et al, 2014; Jones et al, 2016; Lee and Kim, 2017; Lee et al, 2018; Kasoju et al, 2018), morphological variation of bristled wing design in tiny flying insects is far less documented. Jones et al (2016) examined the inter-bristle gap ( G ), bristle diameter ( D ), and the wing area covered by bristles in the forewings of 23 species of fairyflies (Mymaridae).…”