Title: Numerical study of the hydrodynamic stability of a wind-turbine airfoil with a laminar separation bubble under free-stream turbulence
Authors: Fava, Thales C. L.
Lobo, Brandon A.
Nogueira, P.A.S
Schaffarczyk, Alois P.
Breuer, Michael 
Henningson, Dan S.
Hanifi, Ardeshir
Language: eng
Subject (DDC): 000 Informatik, Information & Wissen, allgemeine Werke
500 Naturwissenschaften
600 Technik
Issue Date: Aug-2023
Publisher: AIP Publishing
Document Type: Article
Source: Phys. Fluids 35, 084104 (2023); doi: 10.1063/5.0159783
Journal / Series / Working Paper (HSU): Physics of Fluids
Volume: 35
Issue: 8
Page Start: 084104-1
Page End: 084104-25
Publisher Place: Maryland
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.
Organization Units (connected with the publication): Strömungsmechanik 
Publisher DOI: 10.1063/5.0159783
Appears in Collections:3 - Reported Publications

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