Aerodynamic characteristics of unsteady gap flow in a bristled wing

Abstract

A micro-scale flying insect has a unique wing configuration consisting of a central frame and several bristles. For the low-Reynolds-number regime in which the insect lives, the bristled wing utilizes a virtual fluid barrier inside gaps produced by strong viscous diffusion of shear layers to overcome its morphological limitations. Considering the unsteady flapping motion of such a wing, the aerodynamic characteristics of gap flow formation are investigated numerically using a two-dimensional bristled wing model for a wide range of Reynolds numbers. Inside a gap between bristles, the development of a stopping vortex during the deceleration phase and its effect on the extinction of an existing vortex generated at the same edge are dependent on the Reynolds number, which leads to a significant change in vorticity distribution at stroke reversal even with just a small change in the Reynolds number. As the Reynolds number decreases, the gap flow responds more rapidly to wing motion, and its pattern does not deviate significantly from the kinematics of the wing. A noticeable difference is also observed in the behavior of the aerodynamic force acting on each bristle at low and high Reynolds numbers. With regard to aerodynamic force generation by the bristles, each bristle behaves independently and produces similar force because of strong gap flows relative to the wing at high Reynolds numbers. Meanwhile, at low Reynolds numbers, each bristle experiences a different force depending on its relative position, which indicates the existence of collective interaction of bristles through a virtual fluid barrier.

Optimal configuration of a two-dimensional bristled wing

“…We introduce simplified 2-D bristled-wing models that capture the features of a biological bristled wing as used by Jones et al (2016) and Lee et al (2018). As a preliminary exploration of the effects of the Reynolds number and the gap size, we begin by considering a simple five-bristle model, which consists of five equally spaced cylinders (bristles) of identical diameter D, aligned in a straight line (figure 1a).…”

“…However, microscopic flying insects such as the fairyfly (Mymaridae) and thrips (Thysanoptera), which inhabit the low-Reynolds-number flow regime, have evolved bristled wings that comprise a main frame and several cylindrical bristles that extend from the main frame with gaps between them. For images of bristled wings, see George & Huber (2011), Huber & Noyes (2013) and Lee, Lahooti & Kim (2018).…”

“…Barta (2011) examined theoretically a row of oscillating slender bodies in Stokes flow and presented parametric results with respect to the oscillation frequency, the number of bristles, the slenderness ratio and the space between bristles. Recently, Lee et al (2018) investigated the vortices generated within the gaps of a flapping bristled wing, with particular emphasis on the phase of stroke reversal; they showed that the flow in the gaps responds faster to wing motion as the Reynolds number decreases. Santhanakrishnan et al (2014) modelled a bristled wing as a porous membrane wing and showed that in wing-wing interaction through the clap-and-fling mechanism, flow leakage through the wing surfaces reduces the drag required to fling the wings apart; a pair of porous wings can therefore achieve a large lift-to-drag ratio over a flapping cycle.…”

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

“…It has been reported that thrips (Kuethe, 1975) and Encarsia Formosa (Ellington, 1975) operate at Re b = 10 -2 and 10 -1 , respectively and at Re c ~10. With the exception of Jones et al (2016), the majority of modeling studies of bristled wing aerodynamics (Sunada et al, 2002; Santhanakrishnan et al, 2014; Lee and Kim 2017; Lee et al, 2018; Kasoju et al, 2018; Ford et al, 2019) only matched Re c ~10 without matching Re b to be relevant to tiny insects. Matching Re b ensures that the flow through bristles of a model (and hence Le ) would be similar to those of real 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).…”