| 1. |
- Coelho Leite Fava, Thales, et al.
(author)
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Influence of free-stream turbulence on the boundary layer stability of a wind turbine airfoil and near wake
- 2023
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In: Journal of Physics, Conference Series. - : IOP Publishing. - 1742-6588 .- 1742-6596. ; 2505:1, s. 012002-012002
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Journal article (peer-reviewed)abstract
- Free-stream turbulence (FST) alters the boundary layer of wind turbine blades, changing the hydrodynamic stability and near wake. Large-eddy simulations (LES) of a blade section with a laminar separation bubble for several turbulence intensities (TI) and a Reynolds number of 100,000 are performed. The effects of boundary-layer streaks generated by FST on Tollmien-Schlichting (TS) and Kelvin-Helmholtz (KH) instabilities are analyzed with a model based on the parabolized stability equations (PSE). Two competing effects on flow stability are identified. The spanwise-averaged mean-flow distortion stabilizes primary TS/KH modes for increasing TI. However, this contribution seems dominant only for TI ≥ 8.6%. For lower TI, the spanwise-oscillating distortion caused by streaks destabilizes the flow, and the growth rates of secondary modal instabilities increase with the streak amplitude. The destabilization occurs mainly at spanwise locations with negative streaks since the inflection point shifts away from the wall, enhancing inviscid instabilities. Inflection points in the spanwise direction formed by the streaks also contribute to the destabilization. The modal structures from PSE and LES agree. Finally, the trailing-edge near-wake coherent structures are more energetic for TI ≥ 8.6% due to the partial stabilization of modal instabilities, delaying the turbulent breakdown.
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| 2. |
- Coelho Leite Fava, Thales, et al.
(author)
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Numerical study of the hydrodynamic stability of a wind-turbine airfoil with a laminar separation bubble under free-stream turbulence
- 2023
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In: Physics of fluids. - : AIP Publishing. - 1070-6631 .- 1089-7666. ; 35:8
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Journal article (peer-reviewed)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 100,000 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.
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| 3. |
- Coelho Leite Fava, Thales, et al.
(author)
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On the Stability and Transition to Turbulence of the Flow over a Wind-Turbine Airfoil under Varying Free-Stream Turbulence Intensity
- 2022
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In: 12th International Symposium on Turbulence and Shear Flow Phenomena, TSFP 2022. - : International Symposium on Turbulence and Shear Flow Phenomena, TSFP.
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Conference paper (peer-reviewed)abstract
- The present work investigates the laminar-turbulent transition of the flow around a typical section of a wind turbine blade under different levels of isotropic inflow turbulence at a Reynolds number of Rec = 105. Wall-resolved large-eddy simulations are performed under different turbulence intensities (TI) up to TI = 2.8 %. The results are analyzed with tools based on the linear stability and optimal-perturbation theory. At TI = 0%, transition to turbulence via the breakdown of Kelvin-Helmholtz (KH) rolls formed over the laminar separation bubble (LSB) is seen. These modes start to grow upstream of the LSB as an inflectional instability. At TI = 1.4 % rolls are also seen; but, instabilities formed by the interaction between streaks and the LSB also contribute to transition. The streaks are estimated to have a spanwise wavenumber around β = 100 (βδ∗ = 0.19 at 20 % chord; δ∗ is the displacement thickness) and a frequency in the range f = 5-8 (f δ∗/ue = (1.4-2.3) × 10-2 at 40 % chord; ue is the boundary layer edge velocity), close to that of the shear-layer instability. In the TI = 2.8 % case, the flow is heavily influenced by the presence of streaks, which stabilize the flow concerning inflectional/KH instabilities, but, at the same time, undergoes varicose-type instabilities and breakdown to turbulence.
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| 4. |
- Kleinhans, David, 1979, et al.
(author)
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Synthetic turbulence models for wind turbine applications
- 2010
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In: Springer Proceedings in Physics. - Berlin, Heidelberg : Springer Berlin Heidelberg. - 0930-8989 .- 1867-4941. ; 131, s. 111-114
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Conference paper (peer-reviewed)abstract
- Wind energy converters such as wind turbines permanently work in the atmospheric boundary layer. For the modelling of the dynamics and for the optimisation of design and material of wind turbines synthetic models for atmospheric turbulence are applied already for a long time. The main purpose of these models is to provide fast and efficient methods for numerical simulation of random fields, that show some characteristic features of atmospheric turbulence. Typically they only have a partial connection to the fundamental equations of fluid dynamics. After a short overview summarizing widespread models by Veers and Mann, that are based on the simulation of random fields in the Fourier domain, advanced models for the simulation of velocity fields are discussed.
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