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

Turner, Jacob; Kim, Jae Wook

doi:10.1063/5.0009664

Cited by 27 publications

(12 citation statements)

References 19 publications

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“…At higher angles, particularly in the stall regime, a larger span is required in order to fully capture the extent of spanwise structures in the separated flow. In this paper a span length of L z = L c is used for α = 10 • and 15 • , which has been verified to obtain converged results for far-field noise predictions in stall [19].…”

Section: Domain Geometry and Meshingmentioning

confidence: 70%

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: Domain Geometry and Meshingmentioning

confidence: 70%

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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“…This is considered to be sufficient for reliable simulations in stall conditions. A numerical validation on this has previously been undertaken by the authors (Turner & Kim 2020b).…”

Section: Computational Domain and Gridmentioning

confidence: 99%

“…The current mesh sizes remain either within or below these requirements over the full aerofoil surface. An extensive validation of the current numerical set-up has been carried out in Turner & Kim (2020 b ), including verification of the spanwise domain size and comparison to experiments for aerodynamic coefficients. In addition, the wall mesh sizes away from the wall have been compared in the previous paper to the local Kolmogorov micro-scale .…”

Section: Computational Setupmentioning

confidence: 99%

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Quadrupole noise generated from a low-speed aerofoil in near- and full-stall conditions

Turner

Kim

2022

J. Fluid Mech.

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In this paper, direct numerical simulations are performed for low-speed flows past a NACA0012 aerofoil at high incidence angles. The aim is to investigate the significance of quadrupole noise generated due to separated shear layers, in comparison to dipole noise emanating from the aerofoil surface. The two different noise components (dipole and quadrupole) are calculated by using the Ffowcs Williams & Hawkings method in two different approaches: one with a solid surface and another with a permeable surface. The quadrupole noise is then estimated approximately by taking the relative difference between the two. The current study provides detailed comparisons between the quadrupole and dipole noise components at various observer locations in a wide range of frequencies. The comparisons are also made in terms of Mach number scaling, which differs significantly from theoretical predictions and changes rapidly with frequency. Additionally, pre-, near- and full-stall conditions are cross-examined, which reveals significant differences in the quadrupole contributions, including changes in the major source locations and frequencies. It is found that the inclusion of the quadrupole sources gives rise to the predicted noise power level at all frequencies (varying between 2 and 10 dB for an observer above the aerofoil) compared to the dipole-only case. The quadrupole contribution is far from negligible even at the low subsonic speeds (Mach 0.3 and 0.4) when aerofoil stall occurs.

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“…Since the monopole and quadrupole contributions are irrelevant to the current study, the loading noise is solely considered and the following equation is used: where subscript designates the loading noise component; ‘’ is the time derivative; ‘ret’ stands for ‘retarded time’; is the Mach number; is the unit radiation vector between the source and the observer; is the source–observer distance; ; ; and is the surface of integration. A numerical validation of this implementation can be found in Turner & Kim (2020). A constant non-dimensionalised radius of 10 is taken for all the presented results and appropriate observer angles are specified later on.…”

Section: Computational Detailsmentioning

confidence: 99%