Fan Broadband Noise Research Articles

Unsteady Simulations of a Fan/Outlet-Guide-Vane System: Broadband-Noise Computation

The fan broadband noise of a rotor–stator system representing a turbofan aeroengine is investigated using the improved delayed detached-eddy-simulation approach. The baseline configuration of the NASA source diagnostic test with the entire hard-wall nacelle is simulated at approach and takeoff conditions. In preceding studies, the simulation methodology for the aerodynamics and the treatment of turbulence was established; subsequently, the blade-passing-frequency tones were analyzed. In this study, the forward- and aft-radiating broadband noise associated with the rotor–stator interaction is investigated. The radiated sound power levels are calculated using two methods, and the results are compared with the experimental data: 1) the Ffowcs Williams and Hawkings method with a permeable surface enclosing the nacelle, which is determined through careful studies of the integration-surface design; and 2) the mode-extraction method applied in the fan duct, in which the cost function is tailored for the broadband noise. The power-level spectra from these two numerical methods generally agree, except for the forward-radiating noise in the takeoff condition. Compared with the experiment, both methods predict the spectra typically within 3 dB for both the inlet and the exhaust in the approach condition, but they underpredict the broadband levels in the takeoff condition. The azimuthal-mode decompositions in the inlet and exhaust ducts are also generally consistent with the experiment.

Comparison of two low-order models for the prediction of fan broadband noise

Two low-order methods for predicting the broadband noise downstream of a fan exit guide vane (FEGV) due to interaction with rotor wake turbulence are compared. The noise is predicted with a two-step approach in each method. The methods share the exact same first step. The unsteady vane surface pressure is computed using a strip approach. At each strip, the 2D flat-plate cascade response to a gust in the wave-number, frequency domain is calculated. An integral approach is used for this calculation. The two methods then differ in the second step. One method integrates the unsteady surface pressure together with the Green's function for a cylindrical annulus to obtain the downstream duct acoustic pressure modes. The other method computes rectilinear acoustic modes relevant to the cascade at each strip. In the second method, the total power is obtained by adding the acoustic power downstream of each strip. Both methods neglect mean flow swirl effects on acoustic propagation. The fan broadband noise workshop RC1 and FC2 cases are used as a platform to compare the two methods. When the FEGV inflow turbulent parameters are taken from different sources (e.g. hot-wire, CFD) the predictions from both methods respond similarly. The trends predicted with both methods based on turbulence level or rotor speed are similar. The trends with vane count predicted by the two methods do not match and it is unclear which prediction is correct. The actual spectrum predicted by the Green's function method for all of the cases is shown to be slightly closer to experimental results.

On the study of wavy leading-edge vanes to achieve low fan interaction noise

The application of wavy leading-edge vanes to reduce a single-stage axial fan noise is numerically studied. The aerodynamic and acoustic performance of the fan is numerically investigated using a hybrid unsteady Reynolds averaged Navier-Stokes (URANS)/acoustic analogy method (Goldstein equations). First, the hybrid URANS/Goldstein method is developed and successfully validated against experiment results. Next, numerical simulations are performed to investigate the noise reduction effects of the wavy leading-edge vanes. The aerodynamic and acoustic performance is assessed for a fan with vanes equipped with two different wavy leading-edge profiles and compared with the performance of conventional straight leading-edge vanes. Results indicate that a fan with wavy leading-edge vanes produces lower interaction noise than the baseline fan without a significant loss in aerodynamic performance. In fact, it is demonstrated that wavy leading-edge vanes have the potential to lead to both aerodynamic and acoustic improvements. The two different wavy leading-edge profiles are shown to successfully reduce the fan tone sound power level by 1.2 dB and 4.3 dB, respectively. Fan efficiency is also improved by about 1% with one of the tested wavy leading-edge profiles. Large eddy simulation (LES) is also performed for a simplified fan stage model to assess the effects of wavy leading-edge vanes on the broadband fan noise. Results indicate that the overall sound power level of a fan can be reduced by about 4 dB with the larger wavy leading-edge profile. Finally, the noise reduction mechanisms are investigated and analysed. It is found that the wavy leading-edge profiles can induce significant streamwise vorticity around the leading-edge protuberances and reduce pressure fluctuations (especially at locations of wavy leading-edge hills) and unsteady forces on the stator vanes. The underlying mechanism of the reduced pressure fluctuations is also discussed by examining the magnitude-squared coherence between the velocity and pressure fluctuations in the vicinity of the noise sources. Moreover, a reduction in the correlation level of the wall pressure fluctuations along the vane leading-edge is observed, as well as destructive phase interference along the vane leading-edge.

Impact of cyclostationarity on fan broadband noise prediction

One of the dominant noise sources of modern Ultra High Bypass Ratio (UHBR) engines is the interaction of the rotor wakes with the leading edges of the stator vanes in the fan stage. While the tonal components of this noise generation mechanism are fairly well understood by now, the broadband components are not. This calls to further the understanding of the broadband noise generation in the fan stage. This article introduces a new extension to the Random Particle Mesh (RPM) method, which accommodates in-depth studies of the impact of cyclostationary wake characteristics on the broadband noise in the fan stage. The RPM method is used to synthesize a turbulence field in the stator domain using a URANS simulation characterized by time-periodic turbulence and mean flow. The rotor-stator interaction noise is predicted by a two-dimensional CAA computation of the stator cascade. The impact of cyclostationarity is decomposed into various effects, which are separately investigated. This leads to the finding that the periodic turbulent kinetic energy (TKE) and periodic flow have only a negligible effect on the radiated sound power. The impact of the periodic integral length scale (TLS) is, however, substantial. The limits of a stationary representation of the TLS are demonstrated making this new extension to the RPM method indispensable when background and wake TKE are of comparable level. Good agreement of the predictions with measurements obtained from the 2015 AIAA Fan Broadband Noise Prediction Workshop are also shown.

プロペラファンの翼間相対流れ場と広帯域騒音に関する研究

プロペラファンの翼間相対流れ場と広帯域騒音に関する研究

Three-dimensional analysis of vane sweep effects on fan interaction noise

The lifting surface method is an efficient solution for fully three-dimensional aerodynamic response of an annular cascade to the unsteady disturbances. Based on this response function, a prediction model for fan tonal and broadband interaction noise has been established. Three dimensional effects, including primarily the annular geometry and the radial non-uniformity of the upwash, can be fully taken into account. In this paper, a thorough analysis of vanes sweep effects is carried out by particularly considering the great dependence of the annular cascade aerodynamic response and modal acoustic field upon the radial phase profiles of incident disturbances. For fan tonal noise, different control behaviors of the forward and backward swept vanes are observed when the radial non-uniformity in the incident gust is introduced. The argument suggests that sweep should be selected so as to increase the wake intersections per vane, until it is larger than the number of cut-on radial acoustic modes. For fan broadband noise, the backward sweep succeeds in reducing the sound power level for a wide range of frequencies. Due to the statistical average effect, the efficiency relies much on the shape of the turbulence spectrum. And the sweep angle should be large enough to guarantee a preferable reduction to the fan broadband noise.

The Impact of Low-Speed Fan Design on Noise: An Exploratory Study

The steady evolution since the 1950s toward higher bypass ratio engines has enhanced the acoustic role of the fan compared to the jet. This paper addresses the following question: Does a further decrease in fan pressure ratio (FPR) and rotor tip speed provide a significant reduction of fan broadband and tonal noise? The paper presents two conceptual parametric studies conducted with a fast analytical aerodynamic and acoustic prediction tool. The tool includes an aerodynamic fan design model which provides the quantities necessary to assess the tradeoff between efficiency and noise at given thrust conditions. The fan acoustic model has a theoretical formulation for broadband and tonal noise sources which is not based on empirical correlations; it is applied on conventional and contrarotating fan concepts. The first study proposes a variation of the design FPR and evaluates for each concept its impact on noise at three acoustic off-design points. The results obtained, which are in line with a past NASA study, indicate that the optimum pressure ratio in terms of fan noise is well below the fuel-burn optimum. Significant noise reductions of the broadband and tonal interaction components can be achieved with fans operating in a fully subsonic domain. Alternatively, designing at higher speed and pressure ratio near the fuel-burn optimum may invite to consider the contrarotating fan as a candidate: it performs very well in terms of buzz-saw and broadband noise compared to the conventional fan. The second study addresses the variation of design rotor tip speed at constant FPR. Although reduced tip speed may suppress buzz-saw noise, the increased loading related to it implies large blade solidities and wakes which causes a significant increase in broadband noise. Thus, there is an optimum loading that will depend on the severity of fan inflow distortion and on the onset of buzz-saw noise. Here again these conclusions confirm some experimental work performed by NASA on two different fans, and by Rolls-Royce on a third one.

Prediction of the broadband noise of a low-speed axial fan by CFD similations

An extensive research program has been conducted for several years by CETIAT with scientific and industrial partners to predict one of the major broadband noise sources of low-speed axial fans, namely the blade trailing-edge noise. In the first part of the paper, comparisons of measured and predicted fan sound power spectra due to this mechanism are presented and discussed. The second part of the paper is devoted to the prediction of the wall-pressure fluctuation spectra on the blades of the test fan, which are the main input data of the trailing-edge noise model used. The prediction is made with an empirical model using results of RANS simulations of the flow in the blade boundary layer.

F-19 プロペラファンの翼先端相対流れ場と広帯域騒音に関する研究

F-19 プロペラファンの翼先端相対流れ場と広帯域騒音に関する研究

Predictions of Fan Broadband Noise Using Lifting Surface Method

This paper adopts the three-dimensional lifting surface method as a new cascade-response function to predict fan broadband interaction noise. The stator is modeled as an annular cascade, and the vanes are assumed to be zero-thickness plates with no stagger angle. The unsteady loading spectrum on the stator vanes is considered under a fully three-dimensional condition rather than calculated by the strip theories that are widely used in the existing analytical broadband fan noise models. The sound power spectrum is calculated in an annular duct using numerical integration weighted by statistical turbulence spectrum. The present theory is verified by calculating the category 4 benchmark problem of the Third Computational Aeroacoustics Workshop for the response function and by comparing the predictions of a benchmark test for the broadband formulations of the sound power spectra. Then, the predictions of the inlet and exhaust sound power spectra for the NASA source diagnostic test are presented, which both show good agreement with the experimental results. The discrepancies indicate the importance of further corrections for stagger and swirling mean flow effects in realistic predictions. In the end, the variation trend of the sound power with different vane numbers is well captured, which indicates that this effect mainly refers to the propagation of cut-on acoustic modes in an annular duct. This model has the capability for further studies on the vane swept or lean effects.

Aerodynamic and acoustic analysis of an industrial fan

The efforts to predict noise radiation for an industrial fan using direct computational fluid dynamics (CFD) simulation is presented in this paper. Industry has been using CFD tool to guide fan design in terms of efficiency prediction and improvement. However, the use of CFD tool for aero-dynamic noise prediction is very limited in the past, partly due to the fact that research in aero-acoustics field was not practical for industry application. With the most recent technologies in CFD field and increasing computational power, the industry application of aero-acoustics becomes much more promising. It is demonstrated here that fan tonal noise and broadband noise at low frequencies can be directly predicted using an Overset grid system and high order finite difference schemes with acceptable fidelity.

Different experimental methods for the measurement of broadband noise sources in ducts

Aeroengine broadband fan noise is a major contributor to the community noise exposure from aircraft. It is currently believed that the dominant broadband noise mechanisms are due to interaction of turbulent wake from the rotor with the stator, and interaction of the turbulent boundary layers on the rotor blades with the trailing edge. Two different methods are presented that enable the separation of different broadband noise sources in turbomachinery ducts, one with respect to modal decomposition developed by DLR, and the other with focused beamformer technique developed in university of Southampton. Different measurement mechanisms are displayed to explain the merits, faults, and requirements.

복합 CAA 방법과 FRPM 기법을 이용한 냉장고 얼음제조용 원심팬의 광대역 소음 예측

In this paper, prediction of centrifugal fan was conducted through combination the hybrid CAA method which was used to predict the fan noise with the FRPM technique which was used to generate the broadband noise source. Firstly, flow field surround the centrifugal fan was computed using the RANS equations and noise source region was deducted from the computed flow field. Then the FRPM technique was applied to the source region for generation of turbulence which satisfies the stochastic features. The noise source of the centrifugal fan was modeled by applying the acoustic analogy to the synthesized flow field from the computed and generated flow fields. Finally, the broadband noise of the centrifugal fan was predicted through combination the modeled noise source with the linear propagation which was realized using the boundary element method. It was confirmed that the proposed technique is efficient to predict the tonal and broadband noises of centrifugal fan through comparison with the measured data.

Random particle methods applied to broadband fan interaction noise

Predicting broadband fan noise is key to reduce noise emissions from aircraft and wind turbines. Complete CFD simulations of broadband fan noise generation remain too expensive to be used routinely for engineering design. A more efficient approach consists in synthesizing a turbulent velocity field that captures the main features of the exact solution. This synthetic turbulence is then used in a noise source model. This paper concentrates on predicting broadband fan noise interaction (also called leading edge noise) and demonstrates that a random particle mesh method (RPM) is well suited for simulating this source mechanism. The linearized Euler equations are used to describe sound generation and propagation. In this work, the definition of the filter kernel is generalized to include non-Gaussian filters that can directly follow more realistic energy spectra such as the ones developed by Liepmann and von Kármán. The velocity correlation and energy spectrum of the turbulence are found to be well captured by the RPM. The acoustic predictions are successfully validated against Amiet’s analytical solution for a flat plate in a turbulent stream. A standard Langevin equation is used to model temporal decorrelation, but the presence of numerical issues leads to the introduction and validation of a second-order Langevin model.

Fan broadband noise shielding for over-wing engines

Increasingly demanding community noise targets are promoting noise performance ever higher on the list of airliner design drivers. In response, significant noise reductions are being made, though at a declining rate—it appears that a whole airframe approach is now needed to achieve significant further gains. As a possible step in this direction, over-wing engine installations are considered here, which use the airframe itself as a noise shield. The paper is the account of an experimental investigation of the comparative shielding performances of a range of relative engine positions on such a layout. Using the statistical modelling technique Kriging, we build an approximation of the noise shielding metric as a function of the position of the engines above the wing—this can serve as the input to multi-disciplinary design trade-off studies. We then compare the results found with the results of applying simple half-barrier diffraction theory to the same problem. We conclude that the latter could be considered as a first order, conceptual design tool, though it misses certain features of the design merit landscape identified by the experiment presented here.

An Experimental and Numerical Study on Wake Vortex Noise of a Low Speed Propeller Fan

In order to clarify the relationship between the aerodynamic noise and the flow regime around the rotating blade of a propeller fan operated at the maximum efficiency point and the off-design point, the characteristics of fans with different solidity impellers were analyzed experimentally. At the off-design point, the broadband noise of the high solidity fan was much larger than that of the low solidity fan because the relative velocity increased according to the solidity and the noise sources increased because of the number of blades. In the case of the low solidity fan, the broadband noise due to wake vortex shedding was generated at the off-design point in the low flow rate domain and the maximum efficiency point because the relative flow around the blade separated easily.

통계적 난류합성 모델을 이용한 원심홴 내부 광대역 소음 예측

The internal broadband noise of a centrifugal fan in a household refrigerator is predicted using hybrid CAA techniques based on stochastic turbulent synthetic model. First, the unsteady flow field around the centrifugal fan is predicted using computational fluid dynamics(CFD) method. Then, the turbulent flow field is synthesized by applying the stochastic turbulent synthetic technique to the predicted flow field. The aerodynamic noise sources of the centrifugal fan are modeled on a basis of the synthesized turbulent field. Finally, the internal broadband noise of the centrifugal fan is predicted using the boundary element method(BEM) and the modeled sources. The predicted noise spectrum is compared with the experimental data. It is found that the predicted result closely follows the experimental data. The proposed method can be used as an effective tool for designing low-noise fans without expensive computational cost required generally for the LES and DNS simulations to resolve the turbulence flow field responsible for the broadband noise.

NUMERICAL ASSESSMENT OF TWO-CHAMBER MUFFLERS WITH PERFORATED PLUG/NON-PLUG TUBES UNDER SPACE AND BACK PRESSURE CONSTRAINTS USING SIMULATED ANNEALING

Recently, research on new techniques of single-chamber mufflers equipped with perforated tubes has been addressed. However, research work on shape optimization of multichamber silencers hybridized with perforated plug/non-plug tubes along with work on the maximal allowable back pressure of mufflers has been neglected. Therefore, we will not only analyze the sound transmission loss (STL) of a spaceconstrained two-chamber muffler hybridized with perforated plug/non-plug tubes but also optimize the best design shape under a specified pressure drop. In this paper, both the numerical decoupling technique and simulated annealing (SA) for solving the coupled acoustical problem of perforated plug/ non-plug tubes are used. A numerical case in eliminating a broadband fan noise is also introduced. To verify the reliability of SA optimization, optimal noise abatements for the pure tone (500 Hz) are exemplified. However, before the SA operation can be carried out, the accuracy of the mathematical model must be checked using the experimental data. Results indicate that the maximum STL is precisely located at the desired target tones. The optimal result of one case study for eliminating broadband noise also revealed that the overall noise reductions with respect to the muffler, which are under various maximum allowable pressure drops (100, 200, 300, 400, 500 (Pa), and infinity) can reach 68.4, 52.7, 45.4, 40.4, 36.0, and 33.2 dB. Furthermore, both the pressure drop and the acoustical performance increase when the diameters (at inlet tubes as well as perforated holes), the perforated ratio, and the length of perforated tube decrease. Consequently, a successful approach used for the optimal design of the twochamber mufflers equipped with perforated plug/non-plug tubes under space and back pressure constrained conditions has been demonstrated.

Application of frequency-domain linearized Euler solutions to the prediction of aft fan tones and comparison with experimental measurements on model scale turbofan exhaust nozzles

Although it is widely accepted that aircraft noise needs to be further reduced, there is an equally important, on-going requirement to accurately predict the strengths of all the different aircraft noise sources, not only to ensure that a new aircraft is certifiable and can meet the ever more stringent local airport noise rules but also to prioritize and apply appropriate noise source reduction technologies at the design stage. As the bypass ratio of aircraft engines is increased – in order to reduce fuel consumption, emissions and jet mixing noise – the fan noise that radiates from the bypass exhaust nozzle is becoming one of the loudest engine sources, despite the large areas of acoustically absorptive treatment in the bypass duct. This paper addresses this ‘aft fan’ noise source, in particular the prediction of the propagation of fan noise through the bypass exhaust nozzle/jet exhaust flow and radiation out to the far-field observer. The proposed prediction method is equally applicable to fan tone and fan broadband noise (and also turbine and core noise) but here the method is validated with measured test data using simulated fan tones. The measured data had been previously acquired on two model scale turbofan engine exhausts with bypass and heated core flows typical of those found in a modern high bypass engine, but under static conditions (i.e. no flight simulation). The prediction method is based on frequency-domain solutions of the linearized Euler equations in conjunction with perfectly matched layer equations at the inlet and far-field boundaries using high-order finite differences. The discrete system of equations is inverted by the parallel sparse solver MUMPS. Far-field predictions are carried out by integrating Kirchhoff's formula in frequency domain. In addition to the acoustic modes excited and radiated, some non-acoustic waves within the cold stream-ambient shear layer are also captured by the computations at some flow and excitation frequencies. By extracting phase speed information from the near-field pressure solution, these non-acoustic waves are shown to be convective Kelvin–Helmholtz instability waves. Strouhal numbers computed along the shear layer, based on the local momentum thickness also confirm this in accordance with Michalke's instability criterion for incompressible round jets with a similar shear layer profile. Comparisons of the computed far-field results with the measured acoustic data reveal that, in general, the solver predicts the peak sound levels well when the farfield is dominated by the in-duct target mode (the target mode being the one specified to the in-duct mode generator). Calculations also show that the agreement can be considerably improved when the non-target modes are also included, despite their low in-duct levels. This is due to the fact that each duct mode has its own distinct directionality and a non-target low level mode may become dominant at angles where the higher-level target mode is directionally weak. The overall agreement between the computations and experiment strongly suggests that, at least for the range of mean flows and acoustic conditions considered, the physical aeroacoustic radiation processes are fully captured through the frequency-domain solutions to the linearized Euler equations and hence this could form the basis of a reliable aircraft noise prediction method.

Numerical simulation for fan broadband noise prediction

In order to elucidate the broadband noise of fan, the numerical simulation of fan operating at two different rotational speeds is carried out using the three-dimensional unsteady Reynolds-averaged Navier-Stokes (URANS) equations. The computed results are compared to experiment to estimate its accuracy and are found to show good agreement with experiment. A method is proposed to evaluate the turbulent kinetic energy in the framework of the Spalart-Allmaras one equation turbulence model. From the calculation results, the turbulent kinetic energy is visualized as the turbulence of the flow which leads to generate the broadband noise, and its noise sources are identified.