Decoupling the bifurcated effects of Reynolds and Mach numbers on the dynamic stall of a wind turbine airfoil

Modern ultra-large wind turbines can expose their outboard blade sections to Reynolds numbers above 1 × 107 and Mach numbers above 0.3. In conventional fixed-geometry tests, both parameters vary with inflow velocity. Their individual aerodynamic effects are, therefore, difficult to distinguish. This study presents a variable-chord sectional framework to examine the Reynolds- and Mach-number effects separately. Transitional unsteady Reynolds-averaged Navier–Stokes simulations are conducted for the FFA-W3-211 airfoil. The results reveal a regime-dependent competition between viscous scaling and compressibility. Increasing the isolated Reynolds number promotes earlier transition, strengthens boundary-layer momentum exchange, and accelerates flow reattachment. The lift-hysteresis intensity decreases by 67.2%. Increasing the isolated Mach number enhances lift in attached flow. During deep stall, however, local supersonic flow and shock–boundary-layer interaction promote earlier separation and delay pressure recovery. The downstroke aerodynamic efficiency decreases by 68.1% at the selected post-stall state. These findings indicate that extrapolating traditional uncorrected dynamic stall models to modern large-scale blades may substantially mispredict stall margins. The results suggest that incorporating distinct, decoupled time constants for viscous scaling and compressibility-induced structural persistence may improve predictions of unsteady sectional loads. Their quantitative implications for complete rotors remain to be established through three-dimensional rotating aeroelastic simulations.