The leading-edge vortex on a rotating wing changes markedly beyond a certain central body size.

Shantanu S Bhat,Kerry Hourigan,Jisheng Zhao,Mark C Thompson,John Sheridan

doi:10.1098/rsos.172197

Abstract

Stable attachment of a leading-edge vortex (LEV) plays a key role in generating the high lift on rotating wings with a central body. The central body size can affect the LEV structure broadly in two ways. First, an overall change in the size changes the Reynolds number, which is known to have an influence on the LEV structure. Second, it may affect the Coriolis acceleration acting across the wing, depending on the wing-offset from the axis of rotation. To investigate this, the effects of Reynolds number and the wing-offset are independently studied for a rotating wing. The three-dimensional LEV structure is mapped using a scanning particle image velocimetry technique. The rapid acquisition of images and their correlation are carefully validated. The results presented in this paper show that the LEV structure changes mainly with the Reynolds number. The LEV-split is found to be only minimally affected by changing the central body radius in the range of small offsets, which interestingly includes the range for most insects. However, beyond this small offset range, the LEV-split is found to change dramatically.

Highlights

Further advances in the design of micro air vehicles (MAVs) may require detailed understanding of the aerodynamics of the flapping wings of insects, which outperform the lifting mechanisms used in standard MAVs
The wing, with an offset ratio b0 = 0.08, was rotated with a constant angular velocity corresponding to a Reynolds number of ReR = 900, and particle image velocimetry (PIV) images were recorded at different phases in steps of 45◦
The effect of the Reynolds number and the central body size on the spanwise position where the leading-edge vortex (LEV) splits into dual-LEVs, which is used as a proxy for overall LEV development, is studied experimentally for the flow over a rotating fruit fly wing model

Summary

Introduction

Further advances in the design of micro air vehicles (MAVs) may require detailed understanding of the aerodynamics of the flapping wings of insects, which outperform the lifting mechanisms used in standard MAVs. Researchers in the past have proposed different mechanisms in order to explain the higher lift observed for insects wings flapping at very high angles of attack. The stable attachment of the leading-edge vortex (LEV) at such high angles of attack (α ∼ 45◦), as observed by Maxworthy [1] and Ellington et al [2], is considered to be the principal mechanism.

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Conclusion