Numerical study of the hydrodynamic stability of a wind-turbine airfoil with a laminar separation bubble under free-stream turbulence

T. C.L. Fava, B. A. Lobo, P. A.S. Nogueira, A. P. Schaffarczyk, M. Breuer, D. S. Henningson, A. Hanifi

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The interaction of several instabilities and the influence of free-stream turbulence on laminar-turbulent transition on a 20% thick wind-turbine blade section with a laminar separation bubble (LSB) are investigated with wall-resolved large-eddy simulations (LES). Turbulence intensities (TI) of 0%, 2.2%, 4.5%, 8.6%, and 15.6% at chord Reynolds number 105 are considered. Linear receptivity occurs for the most energetic disturbances; high-frequency perturbations are excited via non-linear mechanisms for T I ≥ 8.6 % . Unstable Tollmien-Schlichting (TS) waves appear in the inflectional flow region for T I ≤ 4.5 % , shifting to inviscid Kelvin-Helmholtz (KH) modes upon separation and forming spanwise rolls. Sub-harmonic secondary instability occurs for T I = 0 % , with rolls intertwining before transition. Streaks spanwise modulate the rolls and increase their growth rates with TI for T I ≤ 4.5 % , reducing separation and shifting transition upstream. The T I = 4.5 % case presents the highest perturbations, leading to the smallest LSB and most upstream transition. Earlier inception of TS/KH modes occurs on low-speed streaks, inducing premature transition. However, for T I = 8.6 % , the effect of the streaks is to stabilize the attached mean flow and front part of the LSB. This occurs due to the near-wall momentum deficit alleviation, leading to the transition delay and larger LSB than T I = 4.5 % . This also suppresses separation and completely stabilizes TS/KH modes for T I = 15.6 % . Linear stability theory predicts well the modal evolution for T I ≤ 8.6 % . Optimal perturbation analysis accurately computes the streak development upstream of the inflectional flow region but indicates higher amplification than LES downstream due to the capture of low-frequency, oblique modal instabilities from the LSB. Only low-amplitude [ O ( 1 % ) ] streaks displayed exponential growth in the LES since non-linearity precludes the appearance of these modes.

Original languageEnglish
Article number084104
Number of pages25
JournalPhysics of Fluids
Issue number8
Publication statusPublished - 1 Aug 2023

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