Clap and fling mechanism with interacting porous wings in tiny insect flight

Abstract

The aerodynamics of flapping flight for the smallest insects such as thrips is often characterized by a 'clap and fling' of the wings at the end of the upstroke and the beginning of the downstroke. These insects fly at Reynolds numbers (Re) of the order of 10 or less where viscous effects are significant. Although this wing motion is known to augment the lift generated during flight, the drag required to fling the wings apart at this scale is an order of magnitude larger than the corresponding force acting on a single wing. As the opposing forces acting normal to each wing nearly cancel during the fling, these large forces do not have a clear aerodynamic benefit. If flight efficiency is defined as the ratio of lift to drag, the clap and fling motion dramatically reduces efficiency relative to the case of wings that do not aerodynamically interact. In this paper, the effect of a bristled wing characteristic of many of these insects was investigated using computational fluid dynamics. We performed 2D numerical simulations using a porous version of the immersed boundary method. Given the computational complexity involved in modeling flow through exact descriptions of bristled wings, the wing was modeled as a homogeneous porous layer as a first approximation. High-speed video recordings of free-flying thrips in take-off flight were captured in the laboratory, and an analysis of the wing kinematics was performed. This information was used for the estimation of input parameters for the simulations. Compared with a solid wing (without bristles), the results of the study show that the porous nature of the wings contributes largely to drag reduction across the Re range explored. The aerodynamic efficiency, calculated as the ratio of lift to drag coefficients, was larger for some porosities when compared with solid wings.

Bristles reduce the force required to ‘fling’ wings apart in the smallest insects

“…4D) over a range of Re b and angles of attack. A previous study of porous wings in single wing translation also showed only a small improvement in the lift to drag ratio for a biologically relevant range (Santhanakrishnan et al, 2014).…”

“…Regarding the last point, it is important to note that the horizontal force required to fling the wings apart can be an order of magnitude greater than the force required to translate a single wing (Miller and Peskin, 2009;Santhanakrishnan et al, 2014). As the wings move in opposite directions at the beginning of the fling, these forces cancel.…”

“…During the fling, there could be some flow between the wings' bristles, enhanced by the presence of the other wing, which would reduce the force required to fling the wings apart. This idea is supported by a previous numerical study that showed that porous wings, compared with solid wings, reduce the drag required to fling two wings apart (Santhanakrishnan et al, 2014). However, it is unclear whether a bristled wing can be treated as a homogenized porous layer and, if so, what the biologically reasonable permeability should be.…”

“…6A), the wings started at a distance of 0.1c, where c is the chord length of the wing, based on observations of clap and fling in thrips (Santhanakrishnan et al, 2014). They accelerated over a distance of 0.05c, then translated at a steady-state velocity until each wing had traveled 0.8c, so that the two wings were 2c apart at the end of the simulation.…”

“…Because insect wing bristles are not well studied, their physiological or mechanical importance remains a mystery. Bristles on insect wings could serve many different functions: reducing the weight of the insect (Sunada et al, 2002), providing an aerodynamic benefit (Davidi and Weihs, 2012;Santhanakrishnan et al, 2014), enhancing electrostatic charges to help in dispersal, similar to what has been suggested for spiders (Gorham, 2013 preprint), helping to fold and unfold the wings (Ellington, 1980), or acting as mechanosensory structures, similar to those in Drosophila (Valmalette et al, 2015). The last two roles, however, are not universal as most parasitoid wasps do not have hinges for folding and unfolding the bristles and it is not clear that all bristles are innervated.…”