Laminar–Turbulent Transition on a DU89-134/14 Airfoil at Low Reynolds Number

Force measurements, XFOIL computations, and Reynolds-averaged Navier–Stokes (RANS) simulations using the transition shear stress transport (SST) turbulence model were used to evaluate the performance of a DU89-134/14 airfoil at chord-based Reynolds numbers Rec = 2.5 × 10⁵ and Rec = 5 × 10⁵. In addition, oil flow visualizations, infrared thermography, pressure measurements, and hot-wire anemometry were employed to gain insight into local flow features influencing airfoil performance, including flow separation, transition, reattachment, and the formation of a laminar separation bubble (LSB). These findings were compared to predictions from XFOIL and RANS, given their wide use in engineering applications. Changes in airfoil performance were observed depending on the Reynolds number and were attributed to the increased length and height of the LSB. In several cases, the numerical simulations exhibit discrepancies in predicting lift and drag coefficients, as well as in local flow features. These inconsistencies are investigated in detail. Flow visualizations for Rec = 5 × 10⁵ and α = 5 deg revealed wave-like patterns and low-temperature streaks, indicating streamwise vortical structures over the convex wall within the LSB. RANS simulations further suggested the presence of Görtler-type vortices, likely due to centrifugal instability, which was evaluated using the Rayleigh discriminant.