Evolving vortex dynamics around a pitching wing for dynamic stall at a transitional Reynolds number

This study investigates the dynamic-stall process over a pitching NACA 0018 wing at a Reynolds number of 160,000 using the detached-eddy simulation (DES) technique and proper orthogonal decomposition (POD). The aim is to employ POD as an energy-ranked framework for interpreting DES-resolved coherent structures in a representative unsteady flow around an oscillating wing. Our results show that the pre-stall flow is characterized by an organized leading-edge flow state and relatively regular trailing-edge vortex activity. As the angle-of-attack increases, separated-flow unsteadiness intensifies and the upstream shear layer loses its compact attached-bubble character. Near stall onset, the leading-edge separated structure enlarges and transitions into the dynamic stall vortex (DSV), generating a broad suction footprint over the suction surface and a rapid deterioration in aerodynamic loading. The POD results indicate that the first few modes contain the dominant large-scale coherent structures associated with the leading-edge and trailing-edge dynamics, whereas higher-order modes reflect progressively weaker, more wake-dominated small-scale unsteadiness. These findings clarify the sequence linking shear-layer development, large-scale vortex evolution, and modal energy distribution during transitional dynamic stall. The study also highlights the limitations associated with the present quasi-three-dimensional spanwise extent and the use of a single pitching amplitude and reduced frequency.