openHSU – Research Showcase

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°. The effects of anisotropic free stream turbulence (FST) with intensities T I = 0%–7% were examined, with integral length scales scaled down from atmospheric measurements. At T I = 0%, a laminar separation bubble (LSB) forms and transition is initiated by Kelvin–Helmholtz vortices. At low FST levels (0% < T I < 2.4%), robust streak growth via the lift-up mechanism suppresses the LSB, while transition dynamics shifts from two-dimensional Tollmien–Schlichting (TS) waves (T I = 0.6%) to predominantly varicose inner and outer instabilities (T I = 1.2% and 2.4%) induced by the wall-normal shear and inflectional velocity profiles. The critical disturbance kinetic energy scales with T I ^−1.80±0.11, compared with T I ^−2.40 from Mack’s correlation. For T I > 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 T I becomes sub-linear and spanwise non-uniformity characterises the turbulent breakdown. The critical disturbance kinetic energy reduces to T I ^−0.90±0.16, marking a transition regime distinct from modal mechanisms. The onset of bypass transition (T I ≈ 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 T I estimation in practical environments.

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