Broadband Trailing Edge Noise Research Articles

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.

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.

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.

Nonflat-Plate Transition-Serrated Trailing Edge for Airfoil Self-Noise Reduction

Trailing-edge noise is the primary source of airfoil self-noise. Nonflat-plate serrated trailing edge can suppress the broadband trailing-edge noise at high frequencies, while the overall noise reduction is compromised by low-frequency narrowband bluntness-induced noise at the sawtooth root. In this paper, a nonflat-plate transition-serrated trailing edge is proposed to reduce the bluntness-induced noise. The configuration uses a wedge-shaped transition structure within the sawtooth gap to eliminate the bluntness noise. Firstly, the appropriate parameters of the transition structure are determined by numerical simulation. Then, optimal configuration is employed for test verification. Finally, the noise reduction mechanism of the configuration with the best noise reduction is analyzed. Simulation results indicate that the configuration can accelerate the fragmentation of large-scale spanwise vortices at sawtooth root, weakening the vortex shedding noise. Moreover, compared with the nonflat-plate serrated trailing edge, it forms counter-rotating oblique vortices at a position further forward within the sawtooth gap, promoting transverse flow, thereby reducing broadband noise over most frequencies. Experimental results show that the configuration with an appropriate transition structure can afford the highest reduction of 27.2 dB for the bluntness-induced noise and 5.7 dB for the trailing-edge noise.

Broadband trailing edge noise reduction through porous velvet-coated serrations

This study experimentally investigates the potential of using combinations of trailing edge serrations and thin porous membrane/velvet structures for turbulent boundary layer trailing edge noise reduction. The experiments were conducted in an anechoic wind tunnel, with a flat plate model as the baseline model. The chord-based Reynolds number ranged between 2×105 and 5×105, and the boundary layers were fully tripped near the leading edge. Two different installation methods were tested, where the serration structure was aligned/misaligned with the undisturbed wake flow. It was observed that the noise reduction capability of the conventional serrations deteriorates significantly when the serrations are misaligned with the flow, while the performances of the combined structures are only slightly affected by flow misalignment. A novel combined treatment is developed, in which the trailing edge serrations are surrounded by serrated porous velvet structures. This treatment is found to outperform the unmodified serrations and can achieve approximately 10 dB noise reduction in both flow-aligned and flow-misaligned conditions, within a wide frequency range corresponding to a boundary layer thickness-based Strouhal number Stδ between 0.3 to 1. A 30%–40% increase in the aerodynamic drag due to the velvet structures was observed. Further hotwire wake survey revealed the possible mechanisms for the additional noise reduction capability of the combined treatments.

Trailing-Edge Noise Prediction by Solving Helmholtz Equation with Stochastic Source Term

A numerical approach to predict broadband trailing-edge noise for low Mach number flows is presented. It is based on the combination of a Helmholtz solver to propagate sound waves with a sound source term computed stochastically. The sound propagation is performed in the frequency domain using a high-order finite element solver. The stochastic approach is based on a random particle-mesh method. The performance of this numerical approach is first examined for a Gaussian source. The numerical approach is then applied to a NACA0012 airfoil for flow Mach numbers of 0.11 and 0.16 and to a controlled-diffusion airfoil for a Mach number of 0.047. The predicted sound levels are compared with experimental data and acoustic results obtained from the acoustic perturbations equations. The mean flow does not significantly modify the acoustic propagation. Using a no-flow propagation model like the Helmholtz equation is therefore a valid approach for low Mach numbers. The reduced computational cost of a Helmholtz calculation, together with the speed of the random-particle mesh turbulence synthesis, allows for fast predictions. While this approach provides relative assessments between different configurations (in particular the spectrum shape), it often requires the inclusion of calibration factors determined from reference measurement or numerical data.

Time domain broadband noise predictions for non-cavitating marine propellers with wall pressure spectrum models

The broadband noise can be dominant or important for total characteristics for marine propeller noise representing the minimum base of self-noise. Accurate prediction of such noise is crucial for survivability of underwater military vessels. While the FW-H Formulation 1B can be used to predict broadband trailing edge noise, the method required experiment measurements of surface pressure correlations, showing its limitations in generality. Therefore, in this study, the methods are developed to utilize wall pressure spectrum models to overcome those limitations. Chase model is adopted to represent surface pressure along with the developed formulations to reproduce pressure statistics. Newly developed method is validated with the experiments of airfoils at different velocities. Thereafter, with its feasibility and generality, the procedure incorporating computational fluid dynamics is established and performed for a propeller behind submarine hull. The results are compared with the experiments conducted at Large Cavitation Tunnel, thus showing its usability and robustness.

Influence of rotor solidity on trailing edge noise from wind turbine blades

Noise prediction from streamlined bodies such as wind turbine blades can be predicted accurately using CFD computations that use spatio-temporal turbulence models at the expense of high computational power. In this work, empirical methods proposed from BPM, Grosveld and Lowson are used to compute numerically to analyse the influence of rotor solidity factor on broadband trailing edge noise from a 2 MW horizontal axis wind turbine with a blade length of 37 m. Inputs to acoustic solver are the velocity vector field and boundary layer data which are obtained using blade element momentum and X-Foil. The outputs from acoustic solver are directivity and far field sound pressure on a receiver located at distance of 120 m from tower base. The results have shown that for a wind speed of 10 m/s measured at 10 m above ground, sound power level was found to increase between mid-bands to high frequencies for all three methods. Rotor solidity effect was illustrated at constant rotational speed of 17 RPM and receiver height of 0.5 m located in downwind position. A minimum difference of 1.5dBA was found at f ~ 100 Hz for Lowson method and maximum of ~ 2.8dBA at 1 kHz between two and three blade rotor. For BPM and Grosveld methods however, the sound levels were ~ 5dBA lower for two blade rotor than three blade rotor between f ~ 100 Hz and f ~ 1 kHz. The study also demonstrated that as number of blades increase by integral multiples, the effect on noise radiation from trailing edge of blades increase by 2-5dBA due to amplitude modulation.

Benefits of curved serrations on broadband trailing-edge noise reduction

Far-field noise and flow field over a novel curved trailing-edge serration (named as iron-shaped serration) are investigated. Spectra of the far-field broadband noise, directivity plots and the flow-field over the iron-shaped serration are obtained from numerical computations performed using a compressible Lattice-Boltzmann solver. The new design is compared to a conventional trailing-edge serration with a triangular geometry. Both serration geometries were retrofitted to a NACA 0018 airfoil at zero degree angle of attack. The iron-shaped geometry is found to reduce far-field broadband noise of approximately 2dB more than the conventional sawtooth serration for chord-based Strouhal numbers Stc<15. At higher frequencies, the far-field broadband noise for the two serration geometries has comparable intensity. Near-wall velocity distribution and surface pressure fluctuations show that their intensity and spectra are independent on the serration geometry, but a function of the streamwise location. It is found that the larger noise reduction achieved by the iron-shaped trailing-edge serration is due to the mitigation of the scattered noise at the root. This effect is obtained by mitigating the interaction between the two sides of the serration, by delaying toward the tip both the outward (i.e., the tendency of the flow to deviate from the centerline to the edge of the serration) and the downward (i.e., the tendency of the flow to merge between the upper and bottom side of the serration) flow motions present at the root of the sawtooth.

Cascade trailing-edge noise modeling using a mode-matching technique and the edge-dipole theory

An original analytical approach is proposed to model the broadband trailing-edge noise produced by high-solidity outlet guide vanes in an axial turbomachine. The model is formulated in the frequency domain and first in two dimensions for a preliminary assessment of the method. In a first step the trailing-edge noise sources of a single vane are shown to be equivalent to the onset of a so-called edge dipole, the direct field of which is expanded in a series of plane-wave modes. A criterion for the distance of the dipole to the trailing-edge and a scaling of its amplitude is defined to yield a robust model. In a second step the diffraction of each plane-wave mode is derived considering the cascade as an array of bifurcated waveguides and using a mode-matching technique. The cascade response is finally synthesized by summing the diffracted fields of all cut-on modes to yield upstream and downstream sound power spectral densities. The obtained spectral shapes are physically consistent and the present results show that upstream radiation is typically 3dB higher than downstream radiation, which has been experimentally observed previously. Even though the trailing-edge noise sources are not vane-to-vane correlated their radiation is strongly determined by a cascade effect that consequently must be accounted for. The interest of the approach is that it can be extended to a three-dimensional annular configuration without resorting to a strip theory approach. As such it is a promising and versatile alternative to previously published methods.

Aerodynamics and Aeroacoustics Study of the Flow Around an Automotive Fan Airfoil

Turbulent flows around airfoils are very challenging to simulate accurately because of the complex flow phenomena, including leading-edge boundary layer separation, shear layer transition to turbulent behavior, reattachment and trailing-edge vortex shedding. In this paper, the flow around a CD (Controlled-Diffusion) airfoil is studied at a Reynolds number of 160,000 using Large-Eddy Simulations (LES). The in-house hybrid spectral/finite element SFELES solver [1] is used to solve the flow. The aim of this research is to study the unsteady turbulent boundary layer and to predict the corresponding wall-pressure spectrum which will be used for the noise predictions, to implement and evaluate the new SGS model proposed recently by G. Ghorbaniasl et al. [2] and to study the influence of the LES sub grid-scale models on the results. Another goal is to calculate the radiated broadband trailing-edge noise by applying Amiet's aeroacoustic theory and its extensions [3, 4] using the wall-pressure spectrum for a station near the trailing edge. The sound pressure level and its directivity are computed for the three SGS models. The aerodynamics and aeroacoustics results are compared with experimental results obtained by Moreau and Roger [5] and Moreau et al. [6] as well as other numerical results obtained by Christophe [7] using OpenFoam and Wang et al. [8] using a hybrid finite differences/spectral solver. The high variability of the leading edge recirculation bubble size with the SGS model is found. The predicted noise spectra is found insensitive to the choice of the SGS model, the difference does not exceed 2 dB for frequencies of interest (100-2000 Hz). This is linked to the insensitivity of the wall-pressure spectra to the choice of the SGS model.

Effect of Wind Shear on Amplitude Modulation of Wind Turbine Noise

Wind turbines normally produce a swishing sound due to directivity of broadband trailing edge noise from the blades. The sound level variation of this ‘Normal Amplitude Modulation’ (NAM) is usually only a few decibels. However, in some cases periods of increased swish or thumping are reported. The source mechanism for this phenomenon, here denoted as ‘Other Amplitude Modulation’ (OAM), is presently not clear. In order to identify potential causes for OAM, a simulation study is performed into the effects of wind shear on the noise from a modern large wind turbine. The results show that, as long as the flow over the blades is attached, wind shear has practically no effect on amplitude modulation. However, strong wind shear can lead to local stall during the upper part of the revolution. This can yield noise characteristics which are very similar to those of OAM. Thus, it can be concluded that local stall is a plausible explanation for OAM. A sensitivity study indicates that the occurrence of wind shear induced amplitude modulation mainly depends on the size of the stall region and the stall noise behaviour of the airfoils. Apart from wind shear, other causes for a non-uniform inflow, such as yaw or topography, may also lead to local stall and OAM.

Trailing edge noise estimation by tomographic Particle Image Velocimetry

The feasibility of estimating broadband trailing edge noise with high-speed tomographic Particle Image Velocimetry (PIV) measurements is studied. A thin plate terminating in a sharp trailing edge provides a generic test case for turbulent boundary layer trailing edge interaction noise.Far-field noise is linked to the wavenumber–frequency spectrum of the surface pressure fluctuations in proximity of the trailing edge through diffraction theory. High-speed tomographic PIV measurements return volumetric and time-resolved information about all velocity components for the resolved spatio-temporal scales and can therefore provide the required statistical quantities. For the turbulent boundary layer interacting with the trailing edge, these statistics include the auto-spectral density, spanwise correlation length, and convection velocity of the unsteady surface pressure, which are thus estimated.Acoustic phased array measurements in an anechoic environment provide a reference for comparison. Over the resolved frequency band, PIV based noise estimation results compare favorably with the reference measurements. Especially at lower frequencies, where existing, empirical models for the unsteady surface pressure spectrum are not accurate, tomographic PIV can offer an alternative approach to complex and intrusive model instrumentation for assessing the relevant statistical quantities.

Frequency-domain prediction of broadband trailing edge noise from a blunt flat plate

The aim of this study is to develop an efficient methodology for frequency-domain prediction of broadband trailing edge noise from a blunt flat plate where non-zero pressure gradient may exist in its boundary layer. This is achieved in two ways: (i) by developing new models for point pressure spectra within the boundary layer over a flat plate, and (ii) by deriving a simple formula to approximate the effect of convective velocity on the radiated noise spectrum. Firstly, two types of point pressure spectra-required as input data to predict the trailing edge noise in the frequency domain-are used. One is determined using the semi-analytic (S-A) models based on the boundary-layer theory combined with existing empirical models. It is shown that the prediction using these models show good agreements with the measurements where zero-pressure gradient assumption is valid. However, the prediction show poor agreement with that obtained from large eddy simulation results where negative (favorable) pressure gradient is observed with the boundary layer. Based on boundary layer characteristics predicted using the large eddy simulations, new model for point wall pressure spectra is proposed to account for the effect of favorable pressure gradient over the blunt flat plate on the wall pressure spectra. Sound spectra that were predicted using these models are compared with measurements to validate the proposed prediction scheme. The advantage of the semi-analytic model is that it can be applied to problems at Reynolds numbers for which the empirical model is not available. In addition, it is expected that the current models can be applied to the cases where favorable pressure gradient exists in the boundary layer over a blunt flat plate. Secondly, in order to quantitatively analyze contributions of the pressure field within the turbulent boundary layer on the flat plate to trailing edge noise, total pressure over the surface of airfoil is decomposed into its two constituents: incident pressure generated in the boundary layer without a trailing edge and the pressure formed by the scattering of the incident pressure at the trailing edge. The predictions made using each of the incident and scattered pressures reveal that the convective velocity of turbulence in the boundary layer dominantly affects the radiated sound pressure spectrum, both in terms of the gross behavior of the overall acoustic pressure spectrum through the scattered pressure and in terms of the narrow band small fluctuations of the spectrum through the incident pressure. The interaction term between the incident and the scattered is defined and the incident is shown to contribute to the radiated acoustic pressure through the interaction term. Based on this finding, a simple model to effectively compute the effects of convection velocities of the turbulence on the radiated sound pressure spectrum is proposed. It is shown that the proposed method can effectively and accurately predict the broadband trailing edge noise from the plate with considering both the incident and the scattered contributions.

Addendum to the back-scattering correction of Amiet's trailing-edge noise model

A minor correction to a previous analytical formulation aimed at predicting broadband trailing-edge noise of subsonic airfoils is provided. An improved numerical implementation free of mathematical singularities is proposed, and a revised plot of the final results found in the reference paper is given.

Broadband trailing-edge noise prediction with a stochastic source model

A RANS-based numerical technique for the prediction of the airfoil trailing-edge broadband noise is presented. The definition of the sound sources from RANS turbulence statistics is based on a stochastic reconstruction of the turbulent velocity fluctuations. The method, named Eulerian Solenoidal Digital Filter, accounts for the convective transport and for the solenoidality of the velocity field. The acoustic near field is computed solving the Acoustic Perturbation Equations in the frequency domain. The integral formulation of Ffowcs Williams and Hawkings is used to evaluate the far-field radiation. The ESDF method is validated for the case of homogeneous isotropic turbulence and it is applied to the calculation of the broadband trailing-edge noise of a NACA 0012 airfoil. Near-field and far-field results are compared with analytical and experimental data.

Broadband trailing edge noise from a sharp-edged strut

This paper presents experimental data concerning the flow and noise generated by a sharp-edged flat plate at low-to-moderate Reynolds number (Reynolds number based on chord of 2.0 × 10(5) to 5.0 × 10(5)). The data are used to evaluate a variety of semi-empirical trailing edge noise prediction methods. All were found to under-predict noise at lower frequencies. Examination of the velocity spectra in the near wake reveals that there are energetic velocity fluctuations at low frequency about the trailing edge. A semi-empirical model of the surface pressure spectrum is derived for predicting the trailing edge noise at low-to-moderate Reynolds number.

Validity of Amiet's Model for Propeller Trailing-Edge Noise

This paper presents a fundamental investigation into the validity of the classical model by Amiet for predicting the broadband trailing-edge noise due to rotating blades. This approximate model is compared analytically against a model in which the effects of rotation are treated exactly. Low- and high-frequency limits are identified, within which Amiet's model is in excellent agreement with the exact solution. Estimates of the error of Amiet's model outside of these limits are also provided. The method is illustrated by application to an open propeller, a model cooling fan, and a wind turbine. The use of Amiet's model for treating the effects of rotation can provide substantial reductions in computation time when the unsteady blade response is computed from computationally demanding methods, such as computational fluid dynamics.

Location and quantification of noise sources on a wind turbine

Acoustic field measurements were carried out on a three-bladed wind turbine with a rotor diameter of 58 m, in order to characterize the noise sources and to verify whether trailing edge noise from the blades was dominant. To assess the effect of blade roughness, one blade was cleaned, one blade was tripped, and one blade remained untreated. A large horizontal microphone array, positioned about one rotor diameter upwind from the turbine, was used to measure the distribution of the noise sources in the rotor plane and on the individual blades. The operation parameters of the turbine were recorded in parallel to the acoustic tests. In total more than 100 measurements were performed at wind speeds between 6 and 10 m/s. The array results reveal that besides a minor source at the rotor hub, practically all noise (emitted to the ground) is produced during the downward movement of the blades. This strongly asymmetric source pattern can be explained by convective amplification and trailing edge noise directivity. The blade noise is produced at the outer part of the blades (but not at the very tip), and the level scales with the fifth power of the local flow speed. Comparison of the noise from the individual blades shows that the tripped blade is significantly noisier than the other two. Narrowband analysis of the de-dopplerized blade noise spectra indicates that trailing edge bluntness noise is not important. All in all, the test results convincingly show that broadband trailing edge noise is the dominant noise source for this wind turbine.

Back-scattering correction and further extensions of Amiet's trailing-edge noise model. Part 1: theory

A previously published analytical formulation aimed at predicting broadband trailing-edge noise of subsonic airfoils is extended here to account for all the effects due to a limited chord length, and to infer the far-field radiation off the mid-span plane. Three-dimensional gusts are used to simulate the incident aerodynamic wall pressure that is scattered as acoustic waves. A leading-edge back-scattering correction is derived, based on the solution of an equivalent Schwarzschild problem, and added to the original formula. The full solution is found to agree very well with other analytical results based on a vanishing Mach number Green's function tailored to a finite-chord flat plate and sources close to the trailing edge. Furthermore, it is valid for any subsonic ambient mean flow velocity. The back-scattering correction is shown to have a significant effect at lower reduced frequencies, for which the airfoil chord is acoustically compact, and at the transition between supercritical and subcritical gusts. It may be important for small-size airfoils, such as automotive fan blades and similar technologies. The final far-field noise formula can be used to predict trailing-edge noise in an arbitrary configuration, provided that a minimum statistical description of the aerodynamic pressure fluctuations on the airfoil surface close to the trailing edge is available.