Influence of inflow turbulence intensity on static and dynamic stall of NACA 0021 at Re= 5.5 × 10⁶ using DDES method

Abstract. Airfoil aerodynamics at high Reynolds numbers (on the order of 10⁶) are of significant interest due to the increasing dimensions of wind turbine rotors. Recent studies indicate that the stall mechanisms at high Reynolds numbers differ from those at lower values, raising questions regarding the validity of conventional low-order prediction tools for simulating aerodynamic performance under these conditions. In this study, static and dynamic stall simulations employing Unsteady Reynolds-Averaged Navier–Stokes (URANS) and Delayed Detached Eddy Simulation (DDES) methods are validated against experimental data for a NACA 0021 airfoil at Re=5.5 x 10⁶. Firstly, two spanwise extensions of a Q3D model are simulated using URANS. Results demonstrate that an aspect ratio (AR) of 0.25 is insufficient for accurately capturing large-scale stall cells, resulting in overpredicted lift values. Subsequent simulations with AR=2.5 confirm that the assumption of fully turbulent boundary layers is valid at such high Reynolds numbers. The DDES method—a hybrid approach combining RANS and LES—shows promise in predicting both the abrupt onset of stall and turbulent flow separation with minimal additional computational cost. Further simulations using DDES with varying inflow turbulence intensities reveal a stall-delay effect as freestream turbulence increases. While the sensitivity of freestream turbulence on dynamic stall during airfoil pitching is negligible, it significantly influences post-stall behaviour. Spectral analysis indicates that the post-stall regime is dominated by the shedding of dynamic stall vortices at a Strouhal number of 0.5.