Numerical investigation of transition on a wind turbine blade under free-stream turbulence at ??? = 1,000,000

• Laminar-turbulent transition on a wind turbine blade section at ??? = 1,000,000 under anisotropic inflow turbulence is studied with wall-resolved large-eddy simulations and linear stability theory. The turbulence intensities (??) are 0, 0.6, 1.2, 2.4, 4.5 and 7%. The laminar separation bubble (LSB) for ?? = 0% develops two spanwise lobes upon reattachment due to the strong amplification of disturbances generated in the upstream attached boundary layer. Transition occurs via the breakdown of Kelvin-Helmholtz vortices in this case. Considering even modest turbulence levels (?? = 0.6%) leads to the suppression of the LSB due to the strong streak growth. Inner modes, which in the limit of zero FST tends to Tollmien-Schlichting (TS) waves, are present for ?? ≤ 2.4%, where Mack’s correlation agrees well with the simulations. Furthermore, linear receptivity occurs in this range of ??, with the initial streak growth well predicted by optimal perturbation analysis. The breakdown of streaks arises predominantly through an inner varicose instability for ?? ≤ 2.4%. For ?? ≥ 4.5%, non-linear receptivity is found, and narrower streaks dominate the flow. These streaks are not the most energetic, and SPOD fails to identify them. The streaks break down via an outer varicose instability in this range of ??, leading to bypass transition. The transition location displays a variation ∝ exp(??−1) for ?? ≤ 2.4% and ∝ ??−2 for 2.4% < ?? ≤ 7.0%, which can be explained by receptivity arguments. Finally, a model for a low-frequency cut-off for computing the effective ?? in atmospheric turbulence isderived considering TS waves.

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TEKNIK OCH TEKNOLOGIER  -- Maskinteknik -- Strömningsmekanik och akustik (hsv//swe)

ENGINEERING AND TECHNOLOGY  -- Mechanical Engineering -- Fluid Mechanics and Acoustics (hsv//eng)

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