Aerodynamics of autorotating samaras in <i>Ventilago leiocarpa</i>

The samaras exhibit diverse autorotational descent behaviors that enhance their likelihood of colonizing new and more favorable environments. Among them, Ventilago leiocarpa represents a large dual-axis autorotating species, whose aerodynamic force mechanisms remain poorly understood. In this study, experimental measurements and three-dimensional numerical simulations were conducted to investigate the kinematic characteristics and aerodynamic force mechanisms of a Ventilago leiocarpa seed. Results indicate that the descent velocity Vd, the pitch angle φ, the yaw angular velocity θ̇, and the roll angular velocity ψ̇ all exhibit small-amplitude, nearly sinusoidal variations during stable autorotational descent. The horizontal force coefficients and all moment coefficients remain close to zero. The vertical force coefficient CZ exhibits a nearly sinusoidal variation, with a time-averaged value of C¯Z=1.23, indicating that the time-averaged vertical force closely balances the seed's weight. This study reveals that the wakes shed in the previous cycle exert only a minimal influence on the aerodynamic force and flow field near the wing. The leading-edge vortex and the trailing-edge vortex near the wing play dominant roles in the aerodynamic force generation. The rate of change in the first moment of vorticity, or the vertical force coefficient, is primarily driven by the growth rate of the leading-edge vortex and the trailing-edge vortex. These findings provide important insights into the understanding of biological dispersal strategies and offer practical implications for the design of miniature aerial vehicles.