Forewings match the formation of leading-edge vortices and dominate aerodynamic force production in revolving insect wings

Abstract

In many flying insects, forewings and hindwings are coupled mechanically to achieve flapping flight synchronously while being driven by action of the forewings. How the forewings and hindwings as well as their morphologies contribute to aerodynamic force production and flight control remains unclear. Here we address the point that the forewings can produce most of the aerodynamic forces even with the hindwings removed through a computational fluid dynamic study of three revolving insect wing models, which are identical to the wing morphologies and Reynolds numbers of hawkmoth (Manduca sexta), bumblebee (Bombus ignitus) and fruitfly (Drosophila melanogaster). We find that the forewing morphologies match the formation of leading-edge vortices (LEV) and are responsible for generating sufficient lift forces at the mean angles of attack and the Reynolds numbers where the three representative insects fly. The LEV formation and pressure loading keep almost unchanged with the hindwing removed, and even lead to some improvement in power factor and aerodynamic efficiency. Moreover, our results indicate that the size and strength of the LEVs can be well quantified with introduction of a conical LEV angle, which varies remarkably with angles of attack and Reynolds numbers but within the forewing region while showing less sensitivity to the wing morphologies. This implies that the forewing morphology very likely plays a dominant role in achieving low-Reynolds number aerodynamic performance in natural flyers as well as in revolving and/or flapping micro air vehicles.

Aerodynamic performance of a bristled wing of a very small insect

“…The mean chord length is then equal to c mean = S ref ∕R = 0.52R , hence the aspect ratio is as small as R 2 ∕S ref = 1.9 (cf. Chen et al (2018):…”

Versatile reduced-order model of leading-edge vortices on rotary wings

“…7, panels (a), (b), and (c) correspond to fruit fly, bumblebee, and hawkmoth, respectively. The solid lines with markers show the circulation calculated using available CFD data [21]. Dashed lines show the respective theoretical estimates, which are remarkably close: the r.m.s.…”

“…Equations (13) and (14) can be used not only for rectangular wings, but also for any wing shape as long as the local chord is wider than the LEV. It was shown [21] that fruit fly, bumblebee, and hawkmoth wing shapes generated nominally the same shape of LEVs. The LEV cone angle mainly depends on Re and α.…”

Beneficial wake-capture effect for forward propulsion with a restrained wing-pitch motion of a butterfly

“…To analyse the effect of the wing-pitch motion among varied cases ( §4), we assumed the wings to be flat plates to separate the effect of the wing-pitch motion from the wing flexibility. Many authors presented satisfactory results using flat plates [34][35][36][37][38]; the use of a flat plate does not affect the natural trend of flight of a butterfly [5,39]. Table 1 shows a comparison of the geometry and features between the experimental butterflies and the butterfly model.…”