As wind turbine rotors grow larger, compressibility effects near the blade tip become an emerging concern. This research experimentally investigated compressibility effects on the flow field and aerodynamic performance of the FFA-W3-211 wind turbine airfoil at Mach numbers within 0.5–0.65 and angles of attack from -4° to -11°. An experimental campaign was conducted at the TST-27 wind tunnel at TU Delft using Particle Image Velocimetry (PIV) and Schlieren techniques. Non-intrusive pressure field reconstruction and load determination methods are used to infer the aerodynamic loads from the flow field data.

Results show that compressibility strongly influences the mean flow field and aerodynamic loads of the FFA-W3-211 airfoil. Trends toward transonic flow and unsteady shock waves were identified with increasing absolute angle of attack and freestream Mach number. The results at α = -6° display a 30% reduction in the negative lift coefficient from cl = -0.43 to -0.31 and a 190% increase in drag coefficient from cd = -0.026 to -0.075, with increasing Mach number from M∞ = 0.5 to 0.65. For the same Mach number range, the lift coefficient results at α = -10° display a plateau around cl ≈ -0.65 to -0.67, and a 100% increase in drag coefficient, from cd = -0.085 to -0.174. At moderate angles of attack, the increased mean drag was influenced by the growth of trailing edge separation. Beyond M∞ = 0.6, the emergence of shock waves plays a greater role in inducing earlier separation and less efficient pressure recovery. The growth of the separation region decreases the mean negative lift for α ≤ -6°. For α ≥ -8°, conversely, the emergence of supersonic flow and lower pressures over the trailing edge counteract the effects of increased separation. For the phase-averaged analysis of the transonic buffet cycle at α = -10° and M∞ = 0.65, the phase-averaged lift coefficient varies periodically in a range of approximately ~22% of the time-averaged lift coefficient, while the phase-averaged drag coefficient varies up to ~80% of the time-averaged drag coefficient.

This work is part of the foundation for understanding the effects of compressibility in wind turbine airfoils and wind turbines. It identifies the complex interplay of shock waves, separation, and shock wave–boundary layer interaction (SWBLI) in the transonic regime. The results highlight and challenge current assumptions of incompressible flow for the design and operation of modern and future large-scale wind turbines that rely on incompressible aerodynamic polars.