Mechanisms of airfoil noise near stall conditions

Lacagnina, Giovanni; Paruchuri, Chaitanya C.; Berk, Tim; Kim, Jung-Hoon; Joseph, Phillip; Ganapathisubramani, Bharathram; Hasheminejad, Seyed Mohammad; Chong, Tze Pei; Stalnov, Oksana; Choi, Kwing-So; Shahab, Muhammad Farrukh; Omidyeganeh, Mohammad; Pinelli, Alfredo

doi:10.1103/physrevfluids.4.123902

Cited by 29 publications

(33 citation statements)

References 34 publications

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“…Once the vortex reaches the TE a dipole pulse is emitted, seen clearly on the aerofoil pressure side by the expansion wave travelling upstream in frames (ii)-(vi). These results confirm the experimental observations made by Lacagnina et al [8], who suggested that a possible stall noise mechanism could be convection and scattering of wall pressure at the TE generated by nearby shear layer instabilities. The shear layer vortices are strongest from around the mid-chord location to the TE.…”

Section: Noise Source Mechanismssupporting

confidence: 92%

“…The most prominent observation in the literature is that the stall is characterised by a high level of low frequency sound [2][3][4][5][6][7][8][9]. More specifically, in the light stall regime increased low frequency broadband levels are observed, while in deep stall low frequency tones are also exhibited [5].…”

Section: Introductionmentioning

confidence: 99%

“…These results also indicate how the same noise reduction techniques for example TE serrations may be less effective for stall noise than TBL-TE where the sources are highly concentrated at the TE. A recent study by Lacagnina et al [8] calculated the coherence between hot-wire velocity measurements in the separated shear layer and the pressure at the aerofoil TE. Modal analysis of particle image velocimetry data was carried out at frequencies with high coherence in order to identify the flow structures responsible for the TE pressure changes.…”

Section: Introductionmentioning

confidence: 99%

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Aerofoil dipole noise due to flow separation and stall at a low Reynolds number

Turner

Kim

2020

International Journal of Heat and Fluid Flow

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Aerofoil self-noise produced by flow separation and stall is relatively little understood regarding the underlying generation mechanisms. The focus of this work is to provide an improved level of understanding particularly with regard to the dipole noise sources utilising a high-fidelity direct numerical simulation. A NACA0012 aerofoil is considered under three different flow conditions at a Reynolds number Re ∞ = 50, 000 and a Mach number M ∞ = 0.4. These include: a pre-stall condition with a laminar separation bubble (α = 5 •), near-stall (α = 10 •), and fully stalled (α = 15 •). The noise radiation in the far-field is significantly increased at low frequencies for the full-stall case for all observer directions which is consistent with previous experimental observations. The dominant source regions for each configuration are identified for low, medium and high frequencies, separately. A number of key findings are made concerning the source characteristics in full-stall case which differ considerably from the lower angle of attack cases. It is found that the location of the dominant sources changes more significantly with frequency for the full-stall case. Additionally, for medium to high frequencies the maximum acoustic source amplitude is weaker for the full-stall case, despite comparable levels observed in the far-field. This seemingly contradictory observation highlights the importance of phase variations in the wall pressure fluctuations. For the frequencies considered in this paper it is shown that the full-stall case usually produces a relatively more in-phase source distribution, resulting in a more efficient radiation despite the lower amplitude levels. The important flow structures which are responsible for the dipole sources are also identified through analysis of the pressure field at isolated frequencies. It is found that for low frequencies coherent structures in the shear layer are responsible for the scattering of the wall pressure fluctuations at the TE, which agrees with previous findings in the literature. However, at medium and high frequencies the shear layer structures are found to be relatively weak in the proximity of the TE. This indicates that the noise may be generated through other means, for example scattering of fluctuating pressure induced by vortices shed from the TE.

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Section: Noise Source Mechanismssupporting

confidence: 92%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Aerofoil dipole noise due to flow separation and stall at a low Reynolds number

Turner

Kim

2020

International Journal of Heat and Fluid Flow

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“…TE serrations) developed for TE noise may be less effective under separated flow conditions. Three possible mechanisms for the noise generation have been suggested in a recent paper by Lacagnina et al 6 for near stall conditions. They theorize that shear layer instabilities, flapping, and shear layer coherent shedding could be a source of unsteady pressure on the wall, which convects towards, and is scattered by the TE.…”

Section: Introductionmentioning

confidence: 96%

“…Stall noise has been documented by experimental studies [1][2][3][4][5][6][7] , where the key emerging feature is a high level of low frequency sound. Various airfoil profiles have been tested by Moreau et al 3 at Reynolds number Re ∞ = 150, 000 and for Mach numbers M ∞ < 0.1.…”

Section: Introductionmentioning

confidence: 99%

Effect of spanwise domain size on direct numerical simulations of airfoil noise during flow separation and stall

Turner

Kim

2020

Physics of Fluids

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It is well established that a large spanwise domain size is required for accurate numerical simulations of flow past an airfoil in stall. A number of numerical experiments support this conclusion with regard to aerodynamic and turbulence statistics. However, very little has been reported concerning the effect of the span length on aeroacoustic results. In this paper, a detailed investigation is carried out into the influence of spanwise domain length on the prediction of airfoil stall noise when spanwise periodic boundary conditions are applied. This study is based on direct numerical simulations of an NACA0012 airfoil at Re∞ = 50 000 and M∞ = 0.4 at near- and full-stall conditions. There are three main findings in this paper. First, the far-field acoustics are found to be highly sensitive to the choice of spanwise domain length. In the full-stall case, a span length equal to 20% of the airfoil chord over-predicts the radiated noise by more than 10 dB at low-to-medium frequencies relative to a case with one chord length in span. Discrepancies are found to occur for acoustic wavelengths shorter than the spanwise domain size. Under near-stall conditions, the changes caused by the small spanwise domain are noticeably milder. Second, the lower noise predictions from the large span simulation at low frequencies are attributed to the spanwise breakup of large scale flow structures and reduced spanwise coherence near the trailing edge. Third, a more destructive source phase relationship is observed with a large span for medium frequencies, which was inhibited by the periodic forcing in the small span case.

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Data-driven determination of low-frequency dipole noise mechanisms in stalled airfoils

Carter

Ganapathisubramani

2023

Exp Fluids

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An aeroacoustic investigation of planar time-resolved particle image velocimetry (PIV) measurements in the streamwise surface-normal plane of a NACA 0012 airfoil in static stall is presented at chord-based Reynolds number $$\textrm{Re}_c=7.1\times 10^4$$ Re c = 7.1 × 10 4 . Instantaneous planar pressure reconstructions are obtained using a Poisson solver and the dipole noise emanating from the surface is extrapolated via Curle’s acoustic analogy. To correlate structure in the velocity field to the generation of noise, a data-driven framework utilising the proper orthogonal decomposition (POD) and the spectral Linear Stochastic Estimation (sLSE) is employed. The flow structures responsible for noise are found to concentrate in proximity to the trailing edge. In addition, a conditional analysis for the extreme noise events reveals that downwash and upwash events in proximity to the trailing edge, coupled with slow and fast-moving fluid at the incipient shear layer, are correlated to local maxima and minima in the acoustic fluctuations, respectively. Graphical abstract

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Mechanisms of airfoil noise near stall conditions

Cited by 29 publications

References 34 publications

Aerofoil dipole noise due to flow separation and stall at a low Reynolds number

Aerofoil dipole noise due to flow separation and stall at a low Reynolds number

Effect of spanwise domain size on direct numerical simulations of airfoil noise during flow separation and stall

Data-driven determination of low-frequency dipole noise mechanisms in stalled airfoils

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