Reynolds number and aspect ratio effects on the leading-edge vortex for rotating insect wing planforms

TLDR

Previous studies on insect‑like wings have reported differing aerodynamic force trends with aspect ratio, but lacked detailed flow observations. The study investigates how aspect ratio and Reynolds number influence flow structures on insect‑like wings and proposes a wing‑span‑based scaling to decouple these effects. A numerical simulation of a rotating altered fruit fly wing at constant angular velocity was used to assess vortex structures and aerodynamic forces across varying aspect ratios and span‑based Reynolds numbers. The study found that Reynolds number and aspect ratio effects on leading‑edge vortex formation are linked, but a span‑based Reynolds number scaling decouples them, revealing universal LEV behavior across parameters and proving effective when strong spanwise velocity exists on the leeward side.

Abstract

Abstract Previous studies investigating the effect of aspect ratio ( $\mathit{AR}$ ) for insect-like regimes have reported seemingly different trends in aerodynamic forces, however no detailed flow observations have been made. In this study, the effect of $\mathit{AR}$ and Reynolds number on the flow structures over insect-like wings is explored using a numerical model of an altered fruit fly wing revolving at a constant angular velocity. Increasing the Reynolds number for an $\mathit{AR}$ of 2.91 resulted in the development of a dual leading-edge vortex (LEV) structure, however increasing $\mathit{AR}$ at a fixed Reynolds number generated the same flow structures. This result shows that the effects of Reynolds number and $\mathit{AR}$ are linked. We present an alternative scaling using wing span as the characteristic length to decouple the effects of Reynolds number from those of $\mathit{AR}$ . This results in a span-based Reynolds number, which can be used to independently describe the development of the LEV. Indeed, universal behaviour was found for various parameters using this scaling. The effect of $\mathit{AR}$ on the vortex structures and aerodynamic forces was then assessed at different span-based Reynolds numbers. Scaling the flow using the wing span was found to apply when a strong spanwise velocity is present on the leeward side of the wing and therefore may prove to be useful for similar studies involving flapping or rotating wings at high angles of attack.