Abstract
A high-fidelity computational analysis carefully validated against concurrently obtained experimental results is employed to examine self-noise radiation of airfoils at transitional flow regimes, with a focus on elucidating the connection between the unsteady behavior of the laminar separation bubble (LSB) and the acoustic feedback-loop (AFL) resonant interactions observed in the airfoil boundary layers. The employed parametric study examines AFL sensitivity to the changes in the upstream flow conditions and the airfoil loading. Implicit Large-Eddy Simulations are performed for a NACA-0012 airfoil in selected transitional-flow regimes for which experimental measurements recorded characteristic multiple-tone acoustic spectra with a dual ladder-type frequency structure. The switch between the tone-producing and no-tone-producing regimes is traced to the LSB size and position as a function of the flow Reynolds number and the airfoil angle of attack, and further substantiated by the linear stability analysis. The results indicate a strong multi-tonal airfoil noise radiation associated with the AFL and attributed to the switch from the slowly-growing Tollmien–Schlichting to the fast-growing Kelvin–Helmholtz instabilities occurring in thin LSB regions when those are localized near the trailing-edge (TE) on either side of the airfoil. Such a process eventually results in the nonlinearly saturated flapping vortical modes (“rollers”) that scatter into acoustic waves at the TE.
Highlights
The noise generated by airfoils under specific low Reynolds number flow regimes have been historically documented by Paterson et al [1], Tam [2], and Arbey and Bataille [3]to be well over 30 dB higher relative to the optimally designed operating condition
Results from the full 3D Implicit Large-Eddy Simulations (ILES) reveal that the original hypothesis of the tonal noise suppression based on 2D simulations is, misleading
The actual 3D mechanism responsible for the acoustic feedback-loop (AFL) suppression at a high angle of attack (AoA) is the transition to interacting with the TE and scattering as acoustics) is, misleading
Summary
The noise generated by airfoils under specific low Reynolds number flow regimes have been historically documented by Paterson et al [1], Tam [2], and Arbey and Bataille [3]. To be well over 30 dB higher relative to the optimally designed operating condition At such levels, the radiated sound (typically a combination of airfoil self-noise and tonal noise) can become extremely discomforting for individuals living nearby wind turbines or those operating small gliders. Much has been accomplished in understanding the airfoil self-noise sources, the exact genesis of the tonal noise production and Ju [9], and Yakhina et al [10]) contributed towards elucidating the mechanisms re of 29 sponsible for airfoil trailing-edge (TE) noise generation. Thephenomena current work focuses on numerically investigating the underlying physical phephysical and their effects on the mechanisms responsible for the generation nomena effects on the mechanisms responsible for the generation tonal of airfoil and tonaltheir noise in transitional flow regimes. The validation of the 2D analysis through cussed in detail
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