Numerical study of the hydrodynamic stability of a wind-turbine airfoil with a laminar separation bubble under free-stream turbulence
Publication date
2023-08
Document type
Research article
Author
Fava, Thales C. L.
Lobo, Brandon A.
Nogueira, P.A.S
Schaffarczyk, Alois P.
Henningson, Dan S.
Hanifi, Ardeshir
Organisational unit
Series or journal
Physics of Fluids
Periodical volume
35
Periodical issue
8
First page
084104-1
Last page
084104-25
Peer-reviewed
✅
Part of the university bibliography
✅
Abstract
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 TI >= 8:6%. Unstable Tollmien–Schlichting (TS) waves appear in the inflectional flow region for TI <= 4:5%, shifting to inviscid Kelvin–Helmholtz (KH) modes upon separation and forming spanwise rolls. Sub-harmonic secondary instability occurs for TI = 0%, with rolls intertwining before transition. Streaks spanwise modulate the rolls and increase their growth rates with TI for TI <= 4:5%, reducing separation and shifting transition upstream. The TI = 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 TI = 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 TI = 4:5%. This also suppresses separation and completely stabilizes TS/KH modes for TI = 15:6%. Linear stability theory predicts well the modal evolution for TI <= 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.
Cite as
Phys. Fluids 35, 084104 (2023); doi: 10.1063/5.0159783
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