Numerical analysis of noise reduction mechanisms of serrated trailing edges under zero lift condition

Velden, W.C.P. van der; Avallone, Francesco; Ragni, Daniele

doi:10.2514/6.2017-4173

Cited by 11 publications

(6 citation statements)

References 37 publications

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“…The funneling effect is also evident from the contours of time averaged streamwise velocity component at the serration surface that show the flow approaching the serrations seeping into the serration gaps. This observation has later been verified by a numerical study in 15 , where simulations on a serrated NACA 0018 aerofoil under zero lift conditions have shown that the near-wall (z = 0.5 mm from the wall) spatial distribution of time averaged mean streamwise velocity component has shown an increase when moving away from the root towards the tip, indicating an acceleration of the flow.…”

Section: Introductionmentioning

confidence: 60%

On the spatial-temporal development of synthetic turbulent boundary layer on a serrated trailing edge

Juknevicius

Chong

2019

25th AIAA/CEAS Aeroacoustics Conference

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This paper presents results from an experimental study to investigate the flow mechanism that enables the add-on saw-tooth serrations to achieve the reduction of broadband trailing edge noise produced by a NACA 0008 aerofoil. The current work traces the temporal-spatial development of an artificially generated synthetic turbulent boundary layer (SBL) on a straight and serrated aerofoil trailing edge. The experiment was performed at Re = 0.96 × 10 5 based on the aerofoil chord. Temporal variations in the acoustic pressure radiated as a result of the flow interaction between the aerofoil trailing edge and the SBL were studied by conducting a wavelet analysis of the far-field noise measurement data. The results successfully capture a short duration variation in the radiated acoustic pressure associated with the passing of the SBL over the trailing edges. Comparison of the results obtained for the straight and serrated trailing edges identifies noise reduction over a short time domain that is exactly linked to a reduced level of wallnormal turbulence intensity close to the surface of trailing edge serrations. A distinct high momentum vortical structure is formed in the near wake of serrations at a later stage of the SBL. This secondary flow is responsible for the reduction of the fluctuations in the wall-normal velocity component at the serration, as well as the increase of the anisotropic level of the turbulence at the serration. As a result, turbulence scattering at the oblique edges is weakened, leading to the reduction of the radiated noise. The results presented in this paper provide a new outlook on the turbulence interaction with trailing edge serrations and the associated trailing edge broadband noise generation.

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Section: Introductionmentioning

confidence: 60%

On the spatial-temporal development of synthetic turbulent boundary layer on a serrated trailing edge

Juknevicius

Chong

2019

25th AIAA/CEAS Aeroacoustics Conference

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“…This effect is also evident from the contours of time averaged u (streamwise) velocity component at the serration surface that show the flow approaching the serrations seeping into the serration gaps. This observation has later been verified by a numerical study in [14], where in case of a serrated NACA0018 aerofoil under zero lift conditions, the near-wall (y=0.5mm from the wall) spatial distribution W of time averaged mean velocity u (streamwise) component has shown an increase moving away from the root towards the tip, indicating an acceleration of the flow.…”

Section: Introductionmentioning

confidence: 58%

On The Spatial–Temporal Developments Of Turbulent Spots On a Serrated Trailing Edge

Juknevicius

Chong

2018

2018 AIAA/CEAS Aeroacoustics Conference

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Aim of the paper is to investigate the flow in the close vicinity of flat-plate insert-type trailing edge serrations. It is now firmly established that aerofoil subjected to a serrated trailing edge is quiet. Recent advances in the analytical and numerical solutions have established several mechanisms that can help to explain the causes of the lower level of selfnoise radiation by a serrated aerofoil. Some high fidelity experimental works, such as the timeresolved Particle Image Velocimetry (PIV), have been used recently to track the evolution of a fully-developed turbulent boundary layer flow structures over a serrated trailing edge. However, due to the high cost and technical challenging, the time-resolved PIV measurements still have some limitations in the construction of a complete spatial and temporal evolution of these small-scale turbulent structures. The current work seeks to develop a simple technique by artificially generating the turbulent spots-the so called "building block" of the turbulent boundary layer, and to track in the space and time domains a complete evolution of these spots over the serrated trailing edge of an aerofoil using the ensemble-averaging analysis method for the hot-wire data. The test was conducted on a symmetrical NACA0008 aerofoil under zero-lift conditions and at a Reynolds number. ×. Aero-acoustic measurements at these conditions confirm that significant broadband self-noise reduction can be achieved by a serrated trailing edge. After gathering the relevant boundary layer data, a preliminary examination of the velocity perturbation and turbulence intensity distributions near the straight trailing edge would confirm the classical turbulent spot structure that can typically be found in the literature. However, in the case of a serrated trailing edge, it is observed that each passage of a turbulent spot, especially at its so called 'calm region', would draw high momentum flow (i.e. positive velocity perturbation) periodically from the sawtooth gap towards the sawtooth surface. It is believed that this periodic calming and stabilization effect of the boundary layer might represent the general mechanism for the reduction of the selfnoise generated at the trailing edge, although currently more data analysis is still underway to dissect the internal flow details of the turbulent spot at the serrated trailing edge.

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“…In addition, at all the Reynolds number, the noise levels in the upstream are found to be higher compared to the downstream for both the airfoils indicating that directivity of the acoustic sources is dominant towards the upstream. 36 Figure 16 shows the directivity pattern at Reynolds number of 3.0 Â 10 5 at angles of attack of 0 , 2 , and 4 . At this Reynolds number, the noise emissions from airfoil with holes are larger compared to the reference airfoil at angles of attack of 0 and 2 , and the noise dominance is observed at the upstream angle of around 130 .…”

Section: Directivity Studiesmentioning

confidence: 99%

Aerodynamic noise characteristics of a thin airfoil with line distribution of holes adjacent to the trailing edge

Sumesh

Jothi

2019

International Journal of Aeroacoustics

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This paper investigates the noise emissions from thin airfoils experimentally. An attempt has been made to study the effect of line distribution of 3 mm diameter holes adjacent to the trailing edge on noise emission characteristics. Thin airfoils having chord length and span of 150 and 240 mm, respectively, are considered for the investigation. The airfoil is subjected to the flow Reynolds number (defined based on the chord length of airfoil) in the range of 2.0 Â 10 5 to 5.0 Â 10 5 , and angles of attack of 0 , 2 , and 4. The spectral results indicate that the modified airfoil with holes is effective in minimizing the lower frequency noise (<3.5 kHz) compared to that of the reference airfoil. The reduction in the low-frequency sound pressure levels for the modified airfoil is found up to 5 dB. The modified airfoil with holes is ineffective in reducing the higher frequency noise components, and in fact, generated a broadband frequency noise at higher Reynolds number. From the spectral studies, a critical Strouhal number is estimated to be around 0.15 that defines the limiting value for the effectiveness of the modified airfoil. The overall sound pressure level studies depicted that the modified airfoil is effective at higher angles of attack with the reduction of around 1-2 dB. The noise generated from the thin airfoils is found to have the directivity towards the upstream.

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Numerical analysis of noise reduction mechanisms of serrated trailing edges under zero lift condition

Cited by 11 publications

References 37 publications

On the spatial-temporal development of synthetic turbulent boundary layer on a serrated trailing edge

On the spatial-temporal development of synthetic turbulent boundary layer on a serrated trailing edge

On The Spatial–Temporal Developments Of Turbulent Spots On a Serrated Trailing Edge

Aerodynamic noise characteristics of a thin airfoil with line distribution of holes adjacent to the trailing edge

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