Boundary-layer instability noise on aerofoils

Nash, Emma; Lowson, MV; McAlpine, Alan

doi:10.1017/s002211209800367x

Cited by 227 publications

(202 citation statements)

References 21 publications

(56 reference statements)

Supporting

Mentioning

161

Contrasting

Unclassified

Order By: Relevance

“…This incongruity suggests that the present inflectional instability near the trailing edge may not be associated directly with the frequency selection mechanism of the trailing-edge noise. This observation also contrasts with the result that T-S instabilities play an important role in the frequency selection mechanism observed by Nash et al 8) and others. To unravel the frequency-selection mechanism, we need further investigation of the interaction between the inflectional boundary layer near the trailing edge and the wake of the airfoil, where a street of Karman vortices is formed.…”

Section: )contrasting

confidence: 96%

“…Figure 3 also shows that there is a ladder-like structure with a constant slope on the general trend curve, which has been found by numerous researchers [1][2][3] in conventional closed test sections to be prone to sound reflections from hard walls, resulting in acoustic resonance. Contrarily, Nash et al 8) demonstrated that each step does fit the U 0:8 dependence in anechoic environments. Since the overall trend coincides with previous observations with and without acoustic resonance, we do not discuss this point further, and attempt to suppress generation of tonal noise.…”

Section: Resultsmentioning

confidence: 99%

See 1 more Smart Citation

Suppression of Trailing-Edge Noise Emitted by Two-Dimensional Airfoils

Takagi

Konishi²

2010

Trans. Japan Soc. Aero. S Sci.

View full text Add to dashboard Cite

Acoustic noise from the trailing edge of 2D airfoils is known to be discrete at certain moderate Reynolds numbers. The most widely accepted explanation for this phenomenon is the acoustic linkage between acoustic radiation and Tollmien-Schlichting (T-S) disturbances growing in the airfoil boundary layer. This paper has two objectives: (1) is to devise a technical method for suppressing or abating trailing-edge noise from a practical viewpoint; (2) is to obtain evidence whether or not T-S instabilities are associated with the frequency selection mechanism mentioned in the literature. From previous experiments (Atobe, T., et al., Trans. Jpn. Soc. Aeronaut. Space Sci., Vol. 52, pp. 74-80, 2009), generation of trailing-edge noise is dominated primarily by suddenly amplified unsteady disturbances in the presence of reverse flow on the pressure side of the airfoil rather than the suction side. Therefore, the boundary layer near the trailing edge on the pressure side is disturbed artificially with distributed roughness elements. As a result, the separation is swept away, and the trailing-edge noise is drastically suppressed. We reconfirmed that under the natural configuration with sound emission, most unsteady disturbances are not amplified by T-S instabilities, but rather by other inflectionaltype instabilities. The frequency selection mechanism in the tonal noise generation process remains unsolved.

show abstract

Section: )contrasting

confidence: 96%

Section: Resultsmentioning

confidence: 99%

Suppression of Trailing-Edge Noise Emitted by Two-Dimensional Airfoils

Takagi

Konishi²

2010

Trans. Japan Soc. Aero. S Sci.

View full text Add to dashboard Cite

show abstract

“…This receptivity mechanism prevails over the direct growth of instabilities on the suction surface as the total spatial growth along the pressure-surface is larger than the analogous growth on the suction-surface. This observation is supported by previous experiments (Nash et al, 1999) and numerical calculations (Desquesnes et al, 2007): the frequency of the dominant tone in the spectrum coincides with the most amplified frequency along the pressure surface based on local stability theory.…”

Section: Leading Modes: the Coupled Dynamics Of The Separation Bubblessupporting

confidence: 85%

“…This flow configuration was chosen to match typical mean flow characteristics described in the experimental work of Paterson et al (1973); Nash et al (1999) and the numerical simulations of Desquesnes et al (2007). The modal structures span a rather wide frequency range, but only a few modes (in the leading M-category) contain a significant acoustic component to account for the observed sound pressure levels as well as the observed frequencies.…”

Section: Discussionmentioning

confidence: 99%

On the receptivity of aerofoil tonal noise: an adjoint analysis

Pando

Schmid

Sipp³

2017

J. Fluid Mech.

View full text Add to dashboard Cite

(Received ?; revised ?; accepted ?. -To be entered by editorial office)For moderate-to-high Reynolds numbers, aerofoils are known to produce substantial levels of acoustic radiation, known as tonal noise, which arises from a complex interplay between laminar boundary-layer instabilities, trailing-edge acoustic scattering and upstream receptivity of the boundary layers on both aerofoil surfaces. The resulting acoustic spectrum is commonly characterised by distinct equally-spaced peaks encompassing the frequency range of convectively-amplified instability waves in the pressuresurface boundary-layer. In this work, we assess the receptivity and sensitivity of the flow by means of global stability theory and adjoint methods which are discussed in light of the spatial structure of the adjoint global modes as well as the wave-maker region. It is found that for the frequency range corresponding to acoustic tones the direct global modes capture the growth of instability waves on the suction surface and the near wake together with acoustic radiation into the far field. Conversely, it is shown that the corresponding adjoint global modes, which capture the most receptive region in the flow to external perturbations, have compact spatial support in the pressure surface boundarylayer, upstream of the separated flow region. Furthermore, we find that the relative spatial amplitude of the adjoint modes is higher for those modes whose real frequencies correspond to the acoustic peaks. Finally, the analysis of the wave-maker region points at the pressure surface near 30% of chord as the preferred zone for the placement of actuators for flow control of tonal noise.

show abstract

“…Considerable work on a NACA-0012 airfoil has shown that the pre-requisite condition for a broadband hump and/or tones to occur is the existence of a separation region near the trailing edge on the pressure surface [4,9,10]. It was concluded that the incoming T-S waves must be amplified by the separating shear layer before tonal noise can be radiated effectively.…”

Section: Introductionmentioning

confidence: 99%

An experimental study of airfoil instability tonal noise with trailing edge serrations

Chong¹,

Joseph²

2013

Journal of Sound and Vibration

102

View full text Add to dashboard Cite

This paper presents an experimental study of the effect of trailing edge serrations on airfoil instability noise. Detailed aeroacoustic measurements are presented of the noise radiated by an NACA-0012 airfoil with trailing edge serrations in a low to moderate speed flow under acoustical free field conditions. The existence of a separated boundary layer near the trailing edge of the airfoil at an angle of attack of 4.2 degree has been experimentally identified by a surface mounted hot-film arrays technique. Hot-wire results have shown that the saw-tooth surface can trigger a bypass transition and prevent the boundary layer from becoming separated. Without the separated boundary layer to act as an amplifier for the incoming Tollmien-Schlichting waves, the intensity and spectral characteristic of the radiated tonal noise can be affected depending upon the serration geometry. Particle Imaging Velocimetry (PIV) measurements of the airfoil wakes for a straight and serrated trailing edge are also reported in this paper. These measurements show that localized normal-component velocity fluctuations that are present in a small region of the wake from the laminar airfoil become weakened once serrations are introduced. Owing to the above unique characteristics of the serrated trailing edges, we are able to further investigate the mechanisms of airfoil instability tonal noise with special emphasis on the assessment of the wake and non-wake based aeroacoustic feedback model. It has been shown that the instability tonal noise generated at an angle of attack below approximately one degree could involve several complex mechanisms. On the other hand, the non-wake based aeroacoustic feedback mechanism alone is sufficient to predict all discrete tone frequencies accurately when the airfoil is at a moderate angle of attack.

show abstract

Boundary-layer instability noise on aerofoils

Cited by 227 publications

References 21 publications

Suppression of Trailing-Edge Noise Emitted by Two-Dimensional Airfoils

Suppression of Trailing-Edge Noise Emitted by Two-Dimensional Airfoils

On the receptivity of aerofoil tonal noise: an adjoint analysis

An experimental study of airfoil instability tonal noise with trailing edge serrations

Contact Info

Product

Resources

About