Reynolds Number Effects on Lift Enhancement Mechanisms of Dragonfly Wings: Their Effective Ranges and Determination by Local Reynolds Numbers

A corrugated structure, rather than a smooth surface, is a characteristic feature of insect wings (e.g., dragonfly wings), which enhances their aerodynamic performance at low Reynolds numbers ($Re\simeq O({10}^3)$). However, the mechanisms responsible for these improvements remain largely unexplored. Previous studies have shown that a secondary vortex forms on a flat wing, opposite in sign to the leading-edge vortex (LEV). At $Re=4000$, the lift enhancement in the corrugated wing is associated with vortex collapse and confinement within the V-shaped region, a part of corrugated structure. Conversely, when there was no lift improvement, the vortex remained intact and erupted without collapsing. In addition, the alternating vortices within the V-shaped region, comprising a negative vortex originating from the LEV and a positive vortex from the secondary vortex, induced a strong negative pressure, thereby further enhancing the lift. However, the working range of these mechanisms has yet to be investigated. In this study, lift enhancement was investigated over a broader Reynolds number range ($100\le Re\le 4000$), focusing on the effective ranges. No characteristic mechanism was observed for $100\le Re\le 500$. For $1000\le Re\le 4000$, the alternating vortices around the V-shaped region were correlated with the improved aerodynamic performance. Furthermore, for $2000\le Re\le 4000$, the secondary vortex collapse plays a major role in lift enhancement. These findings demonstrate that the lift enhancement mechanisms for corrugated wings operate within distinct working ranges depending on the Reynolds number, thereby providing insights into bioinspired aerodynamic designs.