Inflow Noise Research Articles

Noise of a shrouded propeller due to ingestion of grid-generated turbulence

An experimental investigation of the noise produced by a shrouded propeller interacting with grid-generated turbulence in an anechoic wind tunnel is presented. Inflow turbulence statistics and the far-field noise directivity were measured. Measured inflow turbulence statistics and propeller operating parameters were used in a theoretical model to predict the turbulent inflow noise spectra and directivity. The theoretical model uses rapid distortion theory to calculate the distorted turbulent field at the propeller plane. The unsteady loading on the rotating blades is calculated using isolated blade response functions. The noise radiated to the far-field is then evaluated using a tailored Green's function approach which accounts for the acoustic scattering effect of the shroud. The theoretical model is able to capture the shape of the directivity pattern of the turbulent inflow noise produced by the shrouded propeller. The agreement between the measured and predicted spectra is reasonable, however the high frequency broadband levels tend to be underpredicted.

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.

Turbulent inflow noise produced by a shrouded propeller

A theoretical model is presented for predicting the noise produced by the interaction of turbulence with a shrouded propeller in an axisymmetric mean-flow. For cases where the free-stream velocity is low, the turbulent eddies well upstream of the propeller are significantly elongated as they pass through the induced streamtube contraction. These elongated turbulent eddies interact multiple times with the rotating propeller blades causing partially coherent loading on the blades. The resulting radiated acoustic pressure spectrum contains peaks which occur at harmonics of the blade passing frequency. The theory is developed in the frequency domain and incorporates rapid distortion theory to obtain the distorted turbulent field at the propeller face, accounting for the potential mean-flow effect of the propeller and the shroud. The unsteady loading on the rotating blades is calculated using isolated blade response functions. The noise radiated to the far-field is then evaluated using a tailored Green's function approach which accounts for the scattering effect of the shroud and is solved using a boundary integral approach. Results are presented for shrouded and unshrouded propellers operating in flows with a range of different free-stream velocities. It is shown that as the free-stream velocity increases, the peak in the noise spectrum at the blade passing frequency reduces in level and the ‘haystack’ surrounding this peak widens. Results showing the effect of the shroud on the radiated noise levels at different frequencies and observer locations are also presented.

Influence of atmospheric conditions on measured infrasound from wind turbines

Scientific interest towards low frequencies and infrasound from wind turbines has renewed over the last decade as the size of wind turbines and the number of wind farms have continuously increased. However, few comprehensive studies are found assessing such noise emitted by wind turbines in response to meteorological parameters, including turbulent characteristics of the atmospheric boundary layer. The current work presents a detailed long-term measurement campaign combining infrasound recordings close to wind turbines and high-resolution meteorological parameters at three heights. An artificial neural network (ANN) model, developed based on the measurement data, was able to adequately predict both blade passing frequency levels and broadband infrasonic noise. The model allowed discriminating between the broadband inflow noise emitted by the wind turbines and the remaining microphone-induced wind noise under the wind-shielding dome for a wide range of atmospheric conditions. At the same time, the ANN allowed deriving physically plausible relationships between meteorological and turbulence parameters and wind turbine emitted infrasound components. Dominant predictors for the infrasound production were mean wind speed, turbulence intensity and turbulent vertical heat flux.

Dynamics analysis of high-speed water entry of axisymmetric body using fluid-structure-acoustic coupling method

The high-speed water entry problem of a rotating body has instantaneous and strong nonlinear characteristics. In order to study the unsteady evolution of the multiphase flow field entering the water and the coupling effect of multiple media and analyze the influence of the noise of different structures, the fluid–structure-acoustic coupling method is used to analyze the numerical simulations of high-speed vertical water entry with different structures are carried out. In this article, we use different materials and structural forms of rigid cylinder, elastic cylinder and concave-head cylinder to analyze the unsteady evolution of the flow field during the water entering process, the dynamic response of the structure and the water flow noise. Through the comparison and analysis of different materials and structural forms, it can be found that the impact load of the rigid cylinder to the water has increased significantly and appeared earlier, the diameter of the cavity contour of the rigid cylinder is small, and the cavity and contraction of the tail of the concave-head cylinder faster. At the same time, the stress distribution of the elastic cylinder and the bottom of the concave-head cylinder is annular, and expands outward along the bottom center, and the stress transmission of the side surface of the concave-head cylinder is faster. Finally, the overall sound pressure level (OSPL) of the three different structures inflow noise at different distances in the four azimuths has basically the same change, and shows a ”saddle-shaped” trend. The research results in this paper can provide a certain reference for the study of multiphase flow field evolution, structural dynamic response characteristics and noise characteristics of high-speed entering water.

Synthesis of wind turbine trailing edge noise in free field.

The proposed sound synthesis tool converts a physics-based frequency-domain model of wind turbine trailing edge noise to a time-domain signal while accounting for the appropriate time shift due to the propagation between the moving blades and the fixed observer. A window function that implements cross-fading between consecutive signal grains is proposed and a method to objectively estimate the influence of the synthesis parameters is described. As the synthesis tool is independent of the aerodynamic noise model, it can be readily adapted to auralize other noise sources such as turbulent inflow noise or stall noise.

Low-Noise Synthetic Turbulence Tailored to Lateral Periodic Boundary Conditions

The present work is dedicated to turbulence synthesis tailored to lateral periodic boundary conditions for direct noise computations through compressible large eddy simulations. Synthetic turbulence can be essential for aeroacoustic applications when computing airfoil turbulent inflow noise or for accurately capturing the behavior of boundary layers. This behavior determines both trailing edge noise and complex flow structures such as laminar separation bubbles. For airfoil simulation purposes, spanwise periodic boundary conditions are usually considered. If synthetic perturbations are injected without observing the periodicity rule, strong spurious pressure waves are emitted and pollute the entire computational domain. In this work, the random Fourier modes method for turbulence generation is adapted in order to respect the spanwise periodicity constraint right at the computational domain inlet. This approach does not affect the turbulence properties such as the spectral shape and the turbulent kinetic energy decay. Since the emphasis is put on the generation and convection of the turbulence, only the turbulence convection region between the inlet and the airfoil is considered in this paper, without the airfoil. Two geometrical configurations are tested: the first one is a simple box with a constant mesh size, and the second one concentrates the fine cells on the area in front of the airfoil. In the second configuration, the computational cost is reduced by up to 25%, but more spurious noise is present because of interpolation areas between different grids using the Chimera method. Finally, the results’ reproducibility is assessed using different turbulence realizations.

Frequency-domain prediction of broadband inflow noise radiating from a finite-thickness airfoil

The purpose of this study is to develop a semi-analytic model for the prediction of broadband inflow noise of an airfoil in the frequency-domain, which can account for the effects of real airfoil geometry on the radiated sound pressure field in both phases of propagation and generation of aerodynamic sound. First, an acoustic transfer function is analytically derived to account for airfoil geometry’s effects in the propagation phase. The algebraic attenuation model is then combined to consider the effects of incident turbulence distortion in the generation phase. Predictions using the semi-analytic model provide much closer agreements with the measurements than the analytic model in a broader range of frequency and mean flow speed. Besides, the model is shown to reproduce a more realistic directivity of broadband inflow noise by including a drag-type loading source due to the finite thickness of an airfoil as well as a lift-type one.

A rapid distortion theory-based airfoil turbulent inflow noise prediction method

Turbulent inflow (TI) noise is reported as an important source of wind turbine (WT) broadband aeroacoustic noise, coexistent with many other sources (e.g., the airfoil self-noise). This manuscript intends to elucidate the discussion on TI noise by providing an alternative prediction method based on the rapidly distorted anisotropic turbulent velocity energy spectrum. A review on turbulent inflow noise prediction is presented, followed by the discussion on turbulence modeling and the derivation of a modified TI noise prediction expression, which allows a more detailed assessment of wind turbine noise sources. This can contribute for WT blade optimization, once it is implemented in design codes that consider noise as a constraint. Criteria for the turbulent velocity spectrum to be characterized by the rapid distortion theory (RDT) are established, and the validation methodology is carried through comparisons between the novel semi-empirical prediction method and data gathered from recent literature for a flat-plate airfoil with $$3\%$$ relative thickness and a NACA 0008 airfoil. The RDT-modified turbulent inflow noise prediction method shows better agreement with the measured data, when compared to predictions that adopt an isotropic model for the turbulent velocity energy spectrum, a condition only achieved in the absence of the airfoil in the test section of aeroacoustic facilities.

Numerical Simulation of Axial Inflow Characteristics and Aerodynamic Noise in a Large-Scale Adjustable-Blade Fan

Numerical simulation are conducted to explore the characteristics of the axial inflow and related aerodynamic noise for a large-scale adjustable fan with the installation angle changing from −12° to 12°. In such a range the maximum static (gauge) pressure at the inlet changes from −2280 Pa to 382 Pa, and the minimum static pressure decreases from −3389 Pa to −8000 Pa. As for the axial intermediate flow surface, one low pressure zone is located at the junction of the suction surface and the hub, another is located at the suction surface close to the casing position. At the outlet boundary, the low pressure is negative and decreases from −1716 Pa to −4589 Pa. The sound pressure level of the inlet and outlet noise tends to increase monotonously by 11.6 dB and 7.3 dB, respectively. The acoustic energy of discrete noise is always higher than that of broadband noise regardless of whether the inlet or outlet flow surfaces are considered. The acoustic energy ratio of discrete noise at the inlet tends to increase from 0.78 to 0.93, while at the outlet it first decreases from 0.79 to 0.73 and then increases to 0.84.

Wind turbine noise and its mitigation techniques: A review

This paper discusses various noise generation mechanisms in wind turbines and potential noise reduction techniques. Special emphasis has been laid on reviewing aerodynamic noise sources and recent advances in mitigation of aerodynamic noise. Several studies on the effect of wind turbine noise on human health have linked wind turbine noise with annoyance and sleep disturbance. Thus, there is a need to reduce these noise emissions which can be achieved by targeting the specific noise sources. Techniques for mitigation of trailing edge noise, tip noise and leading-edge inflow noise have been discussed along with recent developments.

Selection of a leading edge noise prediction method for PNoise

Wind energy expansion is worldwide followed by various limitations, i.e. land availability, the NIMBY (not in my backyard) attitude, interference on birds migration routes and so on. This undeniable expansion is pushing wind farms near populated areas throughout the years, where noise regulation is more stringent. That demands solutions for the wind turbine (WT) industry, in order to produce quieter WT units. Focusing in the subject of airfoil noise prediction, it can help the assessment and design of quieter wind turbine blades. Considering the airfoil noise as a composition of many sound sources, and in light of the fact that the main noise production mechanisms are the airfoil self-noise and the turbulent inflow (TI) noise, this work is concentrated on the latter. TI noise is classified as an interaction noise, produced by the turbulent inflow, incident on the airfoil leading edge (LE). Theoretical and semi-empirical methods for the TI noise prediction are already available, based on Amiet’s broadband noise theory. Analysis of many TI noise prediction methods is provided by this work in the literature review, as well as the turbulence energy spectrum modeling. This is then followed by comparison of the most reliable TI noise methodologies, qualitatively and quantitatively, with the error estimation, compared to the Ffowcs Williams-Hawkings solution for computational aeroacoustics. Basis for integration of airfoil inflow noise prediction into a wind turbine noise prediction code is the final goal of this work.

CABARET solutions on graphics processing units for NASA jets: Grid sensitivity and unsteady inflow condition effect

The GPU CABARET method for solving the Navier–Stokes equations coupled with the Ffowcs Williams–Hawkings scheme for far-field noise predictions is applied for conditions of the NASA SHJAR experiment corresponding to Set Point 3 and 7 in accordance with Tanna's classification. The questions addressed include the sensitivity of the flow and noise spectra solutions to the grid resolution and the inflow condition at the nozzle exit. To study the grid sensitivity, several “hand-made” multi-block curvilinear grids are considered along with a simple hanging-nodes-type grid that was automatically generated with OpenFOAM, whose solutions are cross-verified. To study the effect of the inflow jet condition, the flow and noise solutions based on the laminar inflow condition for Set Point 7 case are compared with the same based on modifying the interior nozzle geometry with a turbulence grid to generate the initial unsteadiness inside the nozzle so that both the centerline velocity fluctuations and the jet Mach number at the nozzle exit are preserved in accordance with the experiment. The numerical solutions obtained are compared with the experimental data and reference LES solutions available in the literature.

Comparison of some wind turbine noise emission models coupled to BEM aerodynamics

Noise is an important design driver for onshore wind turbines. Therefore, there is a need to include validated noise prediction models within wind turbine and wind plant design procedures. However, the literature presents a multitude of different models and formulations, combined with a severe lack of publicly available experimental data. The present work presents a comparison between semi-empirical frequency-domain methods coupled to blade element momentum (BEM) aerodynamics, which is the typical approach used for the transient aeroelastic analysis of wind turbines. Among the various noise sources, only turbulent boundary layer – trailing edge and turbulent inflow noise models have been considered in the present analysis. The noise models are benchmarked in terms of emission spectra, highlighting differences and discrepancies. A time-domain Ffowcs Williams-Hawkings formulation is also coupled to the same BEM model, investigating its ability in predicting low frequency emissions. Thanks to this study, a few inconsistencies among published noise models are identified and reported.

Noise emission from wind turbines in wake - Measurement and modeling

The influence of the wake of an upstream turbine impinging another one located further downstream is studied focusing on the latter’s noise emission. Measurement data are investigated in the form of surface pressure fluctuations acquired using microphones flush-mounted in a wind turbine blade near its tip, characterizing the noise sources. Numerical results from a wind turbine noise model are also included in the analysis. The wind speed deficit and increased turbulence levels of the wake flow are clearly observed. Surface pressure measurements strongly support the fact that turbulent inflow noise is increased. However, numerical results show that the wake velocity deficit reduces noise in certain circumtances. This can compensate, or even sometime more than compensate, the additional noise emission expected as a result of the wake turbulence. Furthermore, noise amplitude modulation appears to increase when the turbine is impacted by the wake flow.

Coupling of an aeroacoustic model and a parabolic equation code for long range wind turbine noise propagation

This study proposes to couple a source model based on Amiet's theory and a parabolic equation code in order to model wind turbine noise emission and propagation in an inhomogeneous atmosphere. Two broadband noise generation mechanisms are considered, namely trailing edge noise and turbulent inflow noise. The effects of wind shear and atmospheric turbulence are taken into account using the Monin-Obukhov similarity theory. The coupling approach, based on the backpropagation method to preserve the directivity of the aeroacoustic sources, is validated by comparison with an analytical solution for the propagation over a finite impedance ground in a homogeneous atmosphere. The influence of refraction effects is then analyzed for different directions of propagation. The spectrum modification related to the ground effect and the presence of a shadow zone for upwind receivers are emphasized. The validity of the point source approximation that is often used in wind turbine noise propagation models is finally assessed. This approximation exaggerates the interference dips in the spectra, and is not able to correctly predict the amplitude modulation.

Experimental Validation of a Wind Turbine Turbulent Inflow Noise Prediction Code

This paper describes the comparison of a semi-empirical wind turbine noise prediction code to a large volume of acoustic data gathered on a full-scale 2.3 MW wind turbine. Particular focus is placed on the turbulent inflow noise model due to the relatively small amount of validation that has been performed previously. Two novel techniques are developed in order to effectively validate the model. The first is that the turbulence is characterized in the acoustic model using a single quantity, the turbulence dissipation rate, instead of the combination of turbulence intensity and integral length scale as has been used in previous studies. The second development is a method of measuring the turbulence dissipation rate using blade-mounted accelerometers such that the turbulence dissipation rate in the vicinity of the blade is accurately determined. Using these methods, it is shown that the flat-plate airfoil analytical turbulence noise model underpredicts the turbulent inflow noise levels by 3–5 dB, whereas semi-empirical corrections for finite-thickness airfoils reduce prediction errors to less than 3 dB. Comparisons are also made on the basis of scaling with blade tip-speed, scaling with turbulence dissipation rate, and noise directivity.

Modification of population based arterial input function to incorporate individual variation

This technical note describes how to modify a population-based arterial input function to incorporate variation among the individuals. In DCE-MRI, an arterial input function (AIF) is often distorted by pulsated inflow effect and noise. A population-based AIF (pAIF) has high signal-to-noise ratio (SNR), but cannot incorporate the individual variation. AIF variation is mainly induced by variation in cardiac output and blood volume of the individuals, which can be detected by the full width at half maximum (FWHM) during the first passage and the amplitude of AIF, respectively. Thus pAIF scaled in time and amplitude fitting to the individual AIF may serve as a high SNR AIF incorporating the individual variation. The proposed method was validated using DCE-MRI images of 18 prostate cancer patients. Root mean square error (RMSE) of pAIF from individual AIFs was 0.88±0.48mM (mean±SD), but it was reduced to 0.25±0.11mM after pAIF modification using the proposed method (p<0.0001).

Effect of atmospheric stability and low level jets on wind turbine noise

Numerous studies from the literature have shown that strong wind shear occurs frequently in stable atmospheres, typically at night. This phenomenon can be associated to low level jets, characterized by a wind speed profile with a maximum at a few hundred meters above the ground. This study investigates the effect of such wind speed profiles on wind turbine noise using a physically-based model. The predictions are obtained by coupling an aeroacoustic source model based on Amiet’s theory and a parabolic equation code for acoustic propagation in an inhomogeneous atmosphere. Two important broadband noise generation mechanisms are considered, namely trailing edge noise and turbulence inflow noise, and the coupling method takes into account the fact that wind turbine blades are moving and extended sources. In order to obtain realistic wind speeds for the simulations, a simple model of nocturnal low-level jet is used, with input parameters based on measurements acquired in Cabauw observatory in the Netherlands. Predictions of the overall sound pressure levels and of the third octave band noise spectra will be presented up to a distance of 1km to show the noise variations due to the wind shear effect.

Numerical Prediction of the Sources and the Modal Content of the Acoustic Field in a Radial Compressor Outflow

In large modern turbochargers, transonic compressors often constitute the main source of noise, with a frequency spectrum typically dominated by tonal noise at the blade passing frequency (BPF) and its harmonics. Inflow BPF noise is mainly generated by rotor locked shock fronts. Outflow noise, while also dominated by BPF tones, is linked to more complex source mechanisms. Its modal structure and the relationships between sources and modal sound pressure levels (SPL) are less well understood, and its numerical analysis is, in general, significantly more complex than for compressor inflows. To shed some light on the outflow acoustic characteristics of radial machines, transient simulations of a 360 deg model of a radial compressor stage, including its vaned diffuser and volute, were carried out. Four increasingly finer grids were used for this purpose. On all grids, numerical damping had detrimental effects on prediction quality. A simple and mathematically sound method is proposed to account for this damping. With it, the global outflow acoustic power level (PWLg) is predicted to within an accuracy of 2 dB of the experimental result on the finest grid. This shows that satisfactory accuracy can be obtained with state-of-the-art computational fluid dynamics (CFD) codes if care is taken with the simulation setup. The simulations are further validated with experimental data from 17 transient wall pressure sensors.