Numerical investigation of transition on a wind turbine blade under free stream turbulence at $\boldsymbol {Re_c=10^6}$

Laminar–turbulent transition on the suction surface of the LM45.3p blade ( $20\,\%$ thickness) was investigated using wall-resolved large eddy simulation (LES) at a chord Reynolds number of $Re_c=10^6$ and angle of attack $4.6^\circ$ . The effects of anisotropic free stream turbulence (FST) with intensities $TI=0\,\%$ – $7\,\%$ were examined, with integral length scales scaled down from atmospheric measurements. At $TI=0\,\%$ , a laminar separation bubble (LSB) forms and transition is initiated by Kelvin–Helmholtz vortices. At low FST levels ( $0\,\%\lt TI \leqslant 2.4\,\%$ ), robust streak growth via the lift-up mechanism suppresses the LSB, while transition dynamics shifts from two-dimensional Tollmien–Schlichting (TS) waves ( $TI=0.6\,\%$ ) to predominantly varicose inner and outer instabilities ( $TI=1.2\,\%$ and $2.4\,\%$ ) induced by the wall-normal shear and inflectional velocity profiles. The critical disturbance kinetic energy scales with $TI^{-1.80\pm 0.11}$ , compared with $TI^{-2.40}$ from Mack’s correlation. For $TI\geqslant 4.5\,\%$ , bypass transition dominates, driven by high-frequency boundary layer perturbations and streak breakdown via outer sinuous modes induced by the spanwise shear and inflectional velocity profiles. The scaling of streak amplitudes with $TI$ becomes sub-linear and spanwise non-uniformity characterises the turbulent breakdown. The critical disturbance kinetic energy reduces to $TI^{-0.90\pm 0.16}$ , marking a transition regime distinct from modal mechanisms. The onset of bypass transition ( $TI\approx 2.4\,\%{-}4.5\,\%$ ) aligns with prior studies of separated and flat-plate flows. A proposed turbulence spectrum cutoff links atmospheric measurements to wind tunnel data and Mack’s correlation, offering a framework for effective $TI$ estimation in practical environments.