Trailing Edge Noise Research Articles

A Feasibility Study of Riblet for Aeroacoustics Applications

Although the use of finlet, serration or porous surface has been shown a good level of success in reducing aerofoil trailing-edge noise, they are largely incompatible to the otherwise streamlined aerofoil body. This paper is a feasibility study to investigate the riblet, which has so far been quite successful as a drag-reducing device, for its potential to reduce the turbulent pressure sources that are important for aerofoil self-noise radiation. The completed results show that the riblet used in the current study can reduce the skin-friction coefficient, as well as the turbulence-intensity in the boundary-layer profiles. In addition, the turbulence structures in the convective field can be dissipated more rapidly when crossing the riblet surface. It is also found that (1) the riblet produces a slight reduction of the wall-pressure power spectral density level at the low and high frequency ranges, but experiences an increase at the mid frequency, (2) the riblet can reduce the lateral turbulence coherence length-scale across a large frequency range. The product of these two hydrodynamic sources represents an important mechanism for the radiation of the trailing-edge self-noise, whose low and high frequency ranges are found to be sensitive to the riblet effect where reduction can occur.

Experimental investigation of the effect of sectional airfoil profile deviation on propeller noise

The geometry of the sectional airfoil has a determinative impact on the aeroacoustic characteristics of propellers. However, there are always slight deviations in the practical profiles due to manufacturing tolerance, wear loss, and limitations of processing techniques, which can potentially introduce uncertainties to aeroacoustic measurements. To this end, a systematic investigation is conducted on a benchmark propeller with a diameter of 217.2 mm and several of its variants in an anechoic wind tunnel. The variants are redesigned by modifying the sectional airfoil shapes with varying finite trailing-edge thicknesses. High-accuracy computer numerical control machining is employed to ensure the subtle geometrical differences between the blades. Force measurements indicate that the aerodynamic performances are insensitive to the slight variations of the sectional geometry, as expected. As for the acoustic performance, both the tonal and broadband noise are slightly affected when the axial flow speed is lower than 5 m/s. By contrast, a discernible noise reduction above 3 dB can be achieved due to the finite trailing-edge thickness. The noise source features are also investigated using a wavelet-based beamforming method, confirming that the noise reduction is caused by the weakened trailing-edge noise around the tip. This study is beneficial for the quantification of uncertainties in propeller noise measurements. It also suggests that adjusting trailing-edge thickness might be an useful approach in reducing propeller noise in practical applications.

Reduction in Airfoil Trailing-Edge Noise Using a Pulsed Laser as an Actuator

Trailing-edge noise (TE noise) is an aeroacoustic sound radiated from an isolated airfoil in the specific ranges of low-speed flow. We used a pulsed laser as an actuator to reduce the TE noise without modifying the airfoil’s surface. The wind tunnel test was conducted to verify the capability of an Nd:YAG laser as the actuator. The laser beam was focused into the air just outside the velocity boundary layer on the lower side of an NACA0012 airfoil. The experimental result shows that the TE noise is suppressed for a certain period after beam irradiations. We then analyzed the physical mechanism of the noise reduction with the laser actuation by the implicit large eddy simulation (ILES), a high-fidelity numerical method for computational fluid dynamics (CFD). The numerical investigations indicate that the pulsed energy deposition changes the unstable velocity amplification mode of the boundary layer, the source of an acoustic feedback loop radiating the TE noise, to another mode that does not generate the TE noise. The sound wave attenuation is observed once the induced velocity fluctuations and consequently generated vortices sweep out the flow structure of the unstable mode. We also examined the effect of the laser irradiation zone’s shape by numerical simulations. The results show that the larger irradiation zone, which introduces the disturbances over a wider range in the span direction, is more effective in reducing the TE noise than the shorter focusing length with the same energies.

Aeroacoustic airfoil shape optimization enhanced by autoencoders

Aeroacoustic noise is a major concern in wind turbine design that can be minimized by optimizing the airfoils that shape the rotating blades. To this end, we present a framework for airfoil shape optimization to reduce the trailing edge noise for the design of wind turbine blades. Far-field noise is evaluated using Amiet’s theory coupled with the TNO-Blake model to calculate the wall pressure spectrum and fast turn-around XFOIL simulations to evaluate the boundary layer parameters. The computational framework is first validated using a NACA0012 airfoil at 0° angle of attack. Particle swarm optimization is used to find the optimized airfoil configuration. The multi-objective optimization minimizes the A-weighted overall sound pressure level at various angles of attack, while ensuring enough lift and minimum drag. Furthermore, traditional shape optimization techniques show difficulties to converge to the optimum, when too many shape parameters are included in the optimization. For this reason, we compare classic parameterization methods to define the airfoil geometry (i.e., CST) to a machine learning method (i.e., a variational autoencoder). We observe that variational autoencoders can represent a wide variety of geometries, with only four encoded variables, leading to efficient optimizations, which result in improved optimal shapes. When compared to the baseline geometry, a NACA0012, the autoencoder-based optimized airfoil reduces by 3% (1.75 dBA) the overall sound pressure level (with decreased noise across the entire frequency range), while maintaining favorable aerodynamic properties in terms of lift and drag.

Angle-of-attack and Mach number effects on the aeroacoustics of an SD7003 airfoil at Reynolds number 60,000

The aeroacoustics of an SD7003 airfoil at Reynolds number 60,000 is investigated using Large Eddy Simulation. Five simulations are performed to study the effects of angle-of-attack and Mach number at fixed Reynolds number. For the three cases with angle-of-attack equal to 0° (M = 0.1, 0.3 and 0.6), a pure tonal noise associated with a 2D organisation of the flow is obtained. This flow topology is due to the establishment of a well known aeroacoustic feedback loop between the separation point on the suction side of the airfoil and the trailing edge. The occurrence of this loop is corroborated by the presence of a standing wave pattern with characteristic mode number in accordance with Panda’s model. The main effect of the Mach number is to promote flow separation and hence increase separation length and mode number. In addition, the first harmonic and the sub-harmonic of the tone, observed in the far field acoustic spectrum, are found to be generated in the wake, presumably due to non-linear vortex interactions. For the two other angles-of-attack 4° and 8° at M = 0.1, the feedback loop does not establish and a Laminar Separation Bubble (LSB) is observed. When increasing the angle-of-attack, the LSB shrinks with earlier reattachment. For those two cases, far-field spectra are characterized by a low frequency associated with the breathing motion of the LSB and the reattachment point fluctuating in space. The frequency of this fluctuation depends on the curvature of the bubble. Far-field spectra are also characterized by a broadband trailing edge noise whose frequency range decreases with the angle-of-attack. Again, this evolution is found to depend on the curvature of the bubble which may promote a centrifugal instability in the separated shear layer.

Trailing edge noise reduction using bio-inspired finlets

Inspired by the silent flight of the owl, the current work focuses on the reduction of trailing edge noise using 2D finlets. Overset-Large Eddy Simulations are carried out of two configurations, (i) the baseline configuration consisting of a NACA0012 airfoil and (ii) an identical configuration, however, now with finlets installed near the trailing edge. The considered chord based Reynolds number Rec equals 4.2×105 for a non-zero angle of attack α of 4°. The resulting turbulence statistics in the boundary layer and the power spectral density (PSD) spectrum of the surface pressure fluctuations show good agreement with the experimental data. Detailed analysis of the unsteady flow data revealed various noise reduction mechanisms for the here considered case. Firstly, finlets provide a ‘lifting-up’ effect for the most energetic eddies in the boundary layer, which prolongs downstream of the finlets. As a result, the edge scattering is significantly weakened. Secondly, the finlets mounted on the airfoil provide an additional wetted area to the flow which in-turn has a dissipating effect on the surface pressure fluctuations, thereby again weakening the edge scattering phenomena. A reduction in the flow velocity is observed for the flow exiting the finlets channel and towards the trailing edge, which also has a favourable effect in reducing the trailing-edge noise. Finally, a marginal breakdown of the span-wise coherent length scale for frequencies between 2.5 kHz to 9 kHz is also observed.

Propagation effects in the synthesis of wind turbine aerodynamic noise

The sound field radiated by a wind turbine changes significantly with propagation distance, depending on the meteorological conditions and on the type of ground. In this article, we present a wind turbine noise synthesis model which is based on theoretical source and propagation models. The source model is based on Amietâ’s theory for the prediction of the trailing edge noise and the turbulent inflow noise. The trailing edge noise uses the wall pressure spectrum calculated with Leeâ’s model for the suction side and Goodyâ’s model for the pressure side. The Kolmogorov spectrum is used for the prediction of the turbulent inflow noise. To account for the propagation effects associated with atmospheric refraction and ground reflection, a wide angle parabolic equation in inhomogeneous moving medium is considered. The scattering due to the turbulence in the atmosphere is accounted for using the Harmonoise model. The synthesis method is based on the moving monopole model to accurately predict the amplitude modulations at the receiver, and uses cross-fading between overlapping grains to obtain the time signals from the frequency-domain prediction model. Finally, audio signals are provided for a few test cases to emphasize various propagation phenomena associated with wind turbine noise.

Evaluation of the Audibility Boundaries of Multirotor Systems of Different Configurations

The paper proposes a method for calculating the limits of audibility of multirotor systems such as quad- and hexacopters. The components of the proposed methodology are data on the noise of the device and the ambient noise, as well as the criterion of the audibility of the device. The acoustic-vortex method is used to simulate the tonal noise of multirotor systems. A trailing edge noise model is used to calculate the broadband noise of propellers.

Numerical Study on Flow and Noise Characteristics of an NACA0018 Airfoil with a Porous Trailing Edge

An airfoil with a porous trailing edge has a low noise emission; thus, using a porous medium is a good technique for further reduction of wind turbine noise. In this paper, to reduce airfoil trailing edge noise while minimizing the negative influence of a porous medium on aerodynamic performance, a new filling method is proposed such that a porous medium is only used in the suction side half of the trailing edge, which is more sensitive to the noise generation. The large eddy simulation (LES) technique for flow and the Ffowcs Williams and Hawkings (FW-H) method for acoustics are used. At a Reynolds number of 2.63 × 105 and various angles of attack, an NACA0018 airfoil profile with a porous trailing edge covering 20% of the chord is studied under two porous configurations, namely a fully porous and a suction-side porous trailing edge type. The results show that the flow direction, velocity magnitude, and their distributions along the boundary layer of the two porous airfoils are significantly modified due to the presence of the porous medium. The fluctuation of the pressure coefficient and the increase in the boundary layer thickness are significant at low angles of attack. As compared to the solid airfoil counterpart, the noise radiation from the newly proposed suction-side porous airfoil achieves a noise reduction of 4.3 dB at an angle of attack α = 0°, and a noise reduction of 4.07 dB at an angle of attack α = 2°.

Aeroacoustic Benchmarking of Trailing-Edge Noise from NACA Airfoil with Trailing-Edge Serrations

Experimental results on trailing-edge (TE) noise from a NACA airfoil are presented for a chord-based Reynolds number range between and . Far-field TE noise from the baseline airfoil with a straight TE and TE serrations is measured with varying , angle of attack, and serration shape and flap angle. Additionally, aerodynamic coefficients and boundary-layer parameters at the TE are also reported. To cover such a broad range, two NACA airfoil models were tested in two different wind tunnels. The measurements include the emitted noise with natural and forced transition locations. For the straight TE, the forced transition location results in up to 5 dB increase of the far-field TE noise level, compared to the natural one. Scaling of the far-field noise spectra from the baseline TE shows that the Strouhal numbers at which the peak noise level is measured reduce as increases. TE noise spectra for the cases with the TE serrations are found to be dependent on the airfoil lift and . The present data are to be included in the framework of the Benchmark Problems for Airframe Noise Computations category I and are publicly available in a repository with the following digital object identifier (DOI): https://doi.org/10.4121/20940646.

Prediction of broadband noise generated from low-pressure fan based on experimental pressure power spectral density

Low-pressure fans are widely used in industrial machinery as well as in home-use electrical apparatus. Consequently, their improved aerodynamic performance and reduced fan noise are important technical issues. This study developed a methodology for the experimental determination of the pressure power spectral density (PSD) of a flat plate based on a wind tunnel experiment and the broadband noise generated from a low-pressure fan predicted based on its pressure PSD. In the proposed methodology, the broadband noise of the fan was evaluated by superimposing the predicted broadband noise of the divided blade elements in the impeller. Using this methodology, the broadband noise of the objective fan could be predicted via the wind tunnel experiment and the representative internal flow properties of the fan such as relative velocity and deviation angle. Moreover, the relationship between the broadband noise in the design and off-design conditions and the flow regime were analyzed by referring to the parameters for the prediction. In the off-design low flow rate condition, the flow around the impeller became a swirling flow owing to the increasing back pressure, and the fan noise in the off-design condition increased, especially in the low-frequency domain. The broadband noise was generated by leakage flow in the vicinity of the leading edge at the blade tip which interfered with the adjacent blade. Therefore, the mechanism of broadband noise generation in the off-design condition distinguished the mechanism of trailing edge noise. As indicated by the results of the predicted broadband noise comparison, the suggested experimental methodology best represented the measured noise spectra in the design condition. The experimental pressure PSD showed that the broadband noise in the design condition was affected by the trailing edge noise generated at the blade tip.

Airfoil trailing-edge noise and drag reduction at a moderate Reynolds number using wavy geometries

This study utilizes a hybrid aeroacoustic model to investigate how airfoils with spanwise wavy geometries can be used to reduce trailing-edge noise alongside improving the aerodynamic performance. A smooth airfoil is compared to four variants, which have spanwise surface waves of different wavelengths, at a Reynolds number of Re = 64 000 and an angle-of-attack of 1°. The first three variants have a geometry modified by a single wavelength, whereas the fourth has a surface composed of two wavelengths, which creates a more irregular surface variation. The results show that modest noise reductions of around 4 dB are achieved for the first three variants, but a much larger reduction of 17.7 dB is achieved for the fourth variant. The mechanisms behind the noise reduction are explored, and it is shown that the geometry reduces the spanwise correlation of the pressure fluctuations and also modifies the boundary layer dynamics, which contributes to the large reduction. It is further shown that a wavy geometry can reduce the drag force by reducing the shear stress over parts of the airfoil surface and by limiting the flow separation on the suction side. The fourth variant is also assessed across a wider range of angles (0°≤α≤4°) and is shown to produce less noise than the smooth wing across the entire range as well as a drag reduction for α≥1°.

Experimental investigation of the laminar boundary layer vortex-shedding noise by an airfoil within a closed-vein wind tunnel

This study concerns the experimental characterization of trailing edge noise, the understanding of which is crucial for mitigating acoustic pollution across major industries. An aeroacoustic experiment is carried out using a closed-vein wind tunnel to investigate the laminar boundary layer vortex-shedding (LBL-VS) noise of a symmetric NACA0021 airfoil in low Reynolds number flows (Re ≤ 163,500). Steady aerodynamic and acoustic measurements are performed, with numerous conditions covered (flow velocity from 10 m/s to 24.5 m/s, airfoil incidence from −10° to 10°). The aerodynamic results reveal that, in the pre-stall regime, the airfoil’ suction side exhibits both a laminar separation bubble (LSB) and a trailing edge detached flow – which both make LBL-VS noise likely to occur. The acoustic results reveal that, when at low speed and moderate incidence, the airfoil emits one to two tones, which can be both attributed to LBL-VS noise. In particular, their respective frequency is seen to scale as the 0.8th power of the flow velocity, whereas varying linearly with the incidence. At higher speeds, these two tones vanish to the profit of other, more intense tonal emissions, whose frequency does not scale with the velocity nor the incidence. These tones are attributed to resonance effects coming from a retroaction of the reverberant environment onto the LBL-VS noise emission, which then locks-on to some of the duct resonant frequencies via an acoustic feedback loop. Revealing indirectly the presence of the pre-existing LBL-VS noise, these resonant tones emerge only when the flow velocity and incidence obey specific conditions, namely a roughly linear relationship.

Lattice-Boltzmann calculations of rotor aeroacoustics in transitional boundary layer regime

The goal of this paper is to demonstrate the predictive capabilities of the Lattice-Boltzmann/Very-Large-Eddy-Simulation CFD solver SIMULIA PowerFLOW® when a transitional flow past a small drone rotor blade is simulated, without using any numerical or physical turbulence triggering system. A recently developed variant of PowerFLOW VLES model is validated against measurements performed at Delft University of Technology. A 2-bladed propeller of 0.3m diameter is considered, which is operated at 4000 and 5000 RPM and two different advance ratios, 0.0 and 0.6, for each angular velocity. In such conditions, corresponding to a Reynolds number based on the chord at 75% of the tip radius ranging between 7⋅104 and 9⋅104, the presence of a laminar separation bubble was experimentally observed and related to the occurrence of a high-frequency broadband noise hump in the far-field noise spectra. The numerical results are in a good agreement with the loading, noise and PIV experimental measurements, demonstrating the capability of the new PowerFLOW VLES model to predict boundary layer flow transitional phenomena such as laminar separation, reattachment and transition to turbulence, as well as the corresponding broadband trailing-edge noise radiation.

Effects of trailing-edge serration shape on airfoil noise reduction with zero incidence angle

When controlling the trailing-edge (TE) interference noise of airfoil, the design of the TE serration shape is still an open issue. To this end, the flow and noise generation for different TE serration shapes are explored by the wall-resolved implicit large-eddy simulation and acoustic analogy. The feather-like serrations are found to achieve the most prominent noise reduction among the four types of curved serrations, especially in the low-frequency range. With the aid of acoustic analogy, the coherence analysis of far-field noise produced by the dipole sources on the airfoil surface is performed. The results show that destructive interference is still the critical mechanism responsible for noise reduction. Considering only the dipole sources, we find that the feather-like serrated TE shape can obtain the best noise reduction performance among all the serrated cases. Furthermore, since flow structures are reorganized near the TE serrations, we investigated the flow noise sources quantitatively in the near field. In these cases, the noise source due to flow structures is suppressed to a greater extent in the feather-like serrated case near the TE serration roots. Consequently, the above findings indicate that the feather-like serration favors suppressing dipole and flow noise sources in the near field, which makes it an efficient configuration for reducing airfoil noise.

Mitigation of turbulent noise sources by riblets

A feasibility study is presented on the application of riblets to suppress turbulent pressure sources leading to a reduction in the generation of aerofoil self-noise. It is shown that riblets can reduce skin friction as well as the turbulence intensity inside the boundary layer. In addition, near-wall turbulence structures were found to be dissipated quite rapidly when crossing the riblets surface. It is found that riblets (1) slightly reduce the wall fluctuating pressure power spectral density level at the low and high frequency ranges, but cause an increase at the mid frequency range, and (2) reduce the lateral turbulence coherence length scale across a large frequency range. As a result, riblets have the potential to reduce trailing edge noise at the low and high frequency regions. The fundamental mechanism by which riblets reduce the turbulent intensity level inside the boundary layer is investigated by studying the spatio-temporal evolution of turbulent spots, which are commonly regarded as the building blocks of a turbulent boundary layer. In this paper two mechanisms are proposed. First, the enhanced momentum achieved at the becalmed region of each turbulent spot will lead to a reduction in the overall turbulence intensity obtained when turbulent spots merge downstream. Second, the internal turbulence level at the rear part of a turbulent spot can be reduced directly by an enhanced re-laminarisation effect.

Flow and Noise Characteristics of Blunt-Trailing-Edge Flat Plate with an Upstream Cylinder

This paper experimentally studies the flow interaction and the associated noise between an upstream cylinder and a downstream finite-chord-length flat plate with a blunt trailing edge in an acoustic wind tunnel. The configuration of this cylinder–plate model varies by changing the cylinder diameter and the vertical gap between the cylinder and the flat-plate surface. Aerodynamic noise was measured using far-/near-field microphone arrays. The results show that, as compared to the single flat plate, the trailing-edge noise associated with the vortex shedding from the flat plate is significantly suppressed when the gap is : that is, when the cylinder is vertically close to the flat-plate surface. In addition, the corresponding vortex shedding frequency decreases visibly. The leading-edge interaction noise due to the cylinder wake impingement gradually loses its dominance with the increase of the vertical gap. Compared to the single cylinder, the vortex shedding frequency from the upstream cylinder in the cylinder–plate model is reduced slightly due to the acoustic feedback from the plate leading edge. Flow characteristics were measured using surface microphones and particle image velocimetry (PIV) techniques. The pressure fluctuations along the plate surface present the dominant frequencies at the leading edge and the trailing edge, respectively, and verify the changes in the peak frequency and its noise level. The PIV technique clearly shows the variation of the vortex–body interaction at the leading edge and the vortex shedding at the trailing edge, revealing the underlying flow mechanism responsible for the observed noise changes and frequency shifts.

The control mechanism of the soft trailing fringe on the flow characteristics over an airfoil

Inspired by the owl’s silent flight, we experimentally investigated the flow control mechanism of the soft trailing fringes (STFs) on the wake of the S833 airfoil at the Reynolds number of Re = 2 × 104. A high-speed Particle Image Velocimetry (PIV) system is employed to visualize and analyze the flow structures in the wake of the airfoil at different angles of attack (AOA). Furthermore, spectral proper orthogonal decomposition and bispectral mode decomposition are carried out to identify the coherent flow structures and reveal the control mechanism from the perspective of simplified models. PIV measurements’ results demonstrate that the STFs evidently suppress the turbulent quantities including turbulent kinetic energy and Reynolds shear stress in the airfoil wake. On the one hand, the STFs at low AOAs prevent the interaction between the upper and lower shear layers, and the leading- and trailing-edge vortices (TEVs) are significantly suppressed, thus destructing the von Karman vortex streets. On the other hand, the STFs at high AOAs divide the lower shear layer into two parts, markedly attenuating the TEVs and modifying the vortical structures in the wake. Besides, the quadrant analysis reveals that the STFs can mitigate the high-amplitude wall-pressure peaks, indicating that the STFs may manipulate the trailing-edge noise. However, the control effect is limited at median AOAs because the region with high triadic interactions moves upward in the interaction maps, which limits the impact of the STFs.

Reconstruction of the deterministic turbulent boundary layer for the study of aerofoil self-noise mechanisms

In the experimental aeroacoustics, it is always a challenge to study the far-field radiation and near field hydrodynamics simultaneously, and be able to firmly establish the causality between them. The main objective of this paper is to present an experimental technique that can exploit the deterministic turbulent boundary layer generated under a base flow of either mildly separated or laminar boundary layer to either disrupt an existing acoustic scattering mechanism, or reconstruct a new acoustic scattering scenario to enable the ensemble-averaging and wavelet analysis to study the aerofoil trailing edge noise source mechanisms in the spatial, temporal and frequency domains. One of the main attractions of this technique is that the experimental tool does not need to be extremely high fidelity as a priori in order to fully capture the pseudo time-resolved boundary layer instability or turbulent structures. In one of the case studies presented here, roll-up vortices of the order of ∼\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim$$\\end{document} kHz can be captured accurately by a 15-Hz PIV. A single hot-wire probe is also demonstrated to be capable of reconstructing the turbulent/coherent structures in a spatio−temporal domain. The proposed experimental technique can be extended to other self-noise scenarios when the aerofoil trailing edge is subjected to different flow control treatments, such as the porous structure, surface texture, or finlet, whose mechanisms are largely not understood very well at present.Graphical abstract

Prediction of broadband noise from rotating blade elements with serrated trailing edges

This paper conducts a theoretical investigation into the prediction of broadband trailing-edge noise for rotating serrated blades. Lyu's semi-analytical noise prediction model for isolated flat plates is extended to rotating blades using Schlinker and Amiet's approach and applied to three test applications including a wind turbine, a cooling fan, and an open propeller. The model is validated by comparing the straight edge results with that presented in the work of Sinayoko et al., which shows an excellent agreement. The noise spectra obtained using different-order approximations show that the second-order solution yields a converged result. It is found that trailing-edge serrations can lead to noise reduction in the intermediate- and high-frequency ranges at an observer angle of 45° at low Mach numbers but may lead to noise increase in the intermediate-frequency range at high Mach numbers. The results show that the directivity patterns change due to the use of trailing-edge serrations and the directivity peaks are observed at high frequencies. A detailed analysis on the effects of rotation shows that for low-Mach number applications, the Doppler effect is weak and the peaky directivity pattern is mainly affected by the nonuniform directivity of an isolated flat plate at high frequencies. However, for high-Mach number applications, the Doppler effect is significant and also contributes to the final directivity pattern of rotating blades.