Flow transition and vortex evolution of a symmetric airfoil at low Reynolds number and high angle of attack

The flow around airfoil is a significant topic due to the complexities associated with separated structures and fluctuating forces. This study investigates flow transition and vortex evolution for a symmetric airfoil within the range of Reynolds number Re=800–2000 and angle of attack AoA=18∘–26∘. A series of numerical simulations are performed using a well-validated level-set based immersed boundary method. The effects of increasing Re and AoA on the near-profile flow and vortex structures are examined comprehensively through various physical results from the time-averaged, time-history and time-resolved perspectives, including lift and drag coefficients, Strouhal number, pressure and friction distributions, portrait phase map, and spectrum of lift coefficient and instantaneous vorticity. Within the adopted parameter space (Re=800–2000 and AoA=18∘–26∘), three distinct flow patterns, namely, single-period & single-amplitude, double-period & double-amplitude, and multi-period & multi-amplitude, are identified based on the time-history and spectrum of the lift coefficient. Comparative analyses reveal that the flow pattern is strongly influenced by both the Reynolds number and the angle of attack. A detailed analysis of these flow patterns is presented through the period time-evolution of the lift coefficient and vortex structures. The results indicate that the mechanisms of flow patterns are closely related to the formation, merging, splitting, and shedding of various vortices.