On the investigation of near-wake flow structures and the aerodynamics of flapping wing with sinusoidal leading edge tubercles in forward flight

This study investigates the near-wake flow dynamics and their influence on the aerodynamic thrust characteristics of a two-dimensional flapping wing with leading edge tubercles, comparing it to a smooth leading edge (baseline) counterpart under forward flight conditions. Using the numerical simulations, validated with in-house experiments, the aerodynamic performance is analyzed across a range of non-dimensional flapping frequencies (Strouhal numbers, St = 0.2–0.6) with a Reynolds number of 5000 and a maximum effective angle of attack of 15°. The tubercle geometry, characterized by a sinusoidal amplitude of 0.125c and a wavelength of 0.5c (c: chord length), consistently underperforms the baseline wing in both time-averaged and transient thrust generation. This disparity arises from distinct wake dynamics. While the baseline wing sustains thrust through the formation of coherent leading edge vortices, the tubercle wing generates counter-rotating vortex pairs (CRVPs) that lift away from the surface. The CRVP liftoff leads to the formation of secondary structures, including residual CRVPs and hairpin vortices, which impede thrust recovery. Moreover, the residual CRVPs form complex loops that interact with the wing surface, creating an unfavorable flow environment that exacerbates thrust loss. At higher flapping frequencies, these interactions intensify, revealing loop-like vortex formations along the wingspan, further reducing pressure differentials essential for thrust generation. A critical decline in thrust performance is observed for both wings beyond St = 0.5. These findings provide detailed insights into the wake dynamics, highlighting the inherent aerodynamic limitations imposed by the tubercle geometry in forward flight applications.