Aerodynamic Noise Sources Research Articles

1E33 Numerical simulation for pantograph aerodynamic noise reduction(Environment and Energy)

It is necessary to reduce aerodynamic noise, when increasing the maximum operation speed of Shinkansen. In particular, aerodynamic noise from pantographs is a major aerodynamic noise source. In this study, we focused on aerodynamic noise from the upper part of pantograph, which is difficult to reduce with pantograph noise insulation panels. To investigate the noise generation mechanism, we conducted numerical simulation and analyzed the contribution of each noise source. From the analysis, we found that within a frequency range of 200Hz to 700Hz noise from the pantograph head support took up a dominant share of noise within the upper part of the pantograph. In addition it was found that the strong pressure fluctuation causing the noise from the pantograph head support was generated by interference between flow around the pantograph head and flow around the pantograph head support. To reduce pressure fluctuation, we conducted numerical simulations for some shapes by changing the positional relationship of the pantograph head and pantograph head support. From the above findings, we proposed an improved shape that embeds the pantograph head into the pantograph head support. By applying the "improved shape", noise from the pantograph head support was reduced over a wide frequency range and the strong pressure fluctuation on the pantograph head support that was observed in "standard shape" was reduced. We confirmed that the improved shape reduced the noise contribution ratio of the pantograph head support below a frequency of 800 Hz, and reduced the overall noise level of the upper part of pantograph by approximately 1dB.

Adaptive neuro-fuzzy methodology for noise assessment of wind turbine.

Wind turbine noise is one of the major obstacles for the widespread use of wind energy. Noise tone can greatly increase the annoyance factor and the negative impact on human health. Noise annoyance caused by wind turbines has become an emerging problem in recent years, due to the rapid increase in number of wind turbines, triggered by sustainable energy goals set forward at the national and international level. Up to now, not all aspects of the generation, propagation and perception of wind turbine noise are well understood. For a modern large wind turbine, aerodynamic noise from the blades is generally considered to be the dominant noise source, provided that mechanical noise is adequately eliminated. The sources of aerodynamic noise can be divided into tonal noise, inflow turbulence noise, and airfoil self-noise. Many analytical and experimental acoustical studies performed the wind turbines. Since the wind turbine noise level analyzing by numerical methods or computational fluid dynamics (CFD) could be very challenging and time consuming, soft computing techniques are preferred. To estimate noise level of wind turbine, this paper constructed a process which simulates the wind turbine noise levels in regard to wind speed and sound frequency with adaptive neuro-fuzzy inference system (ANFIS). This intelligent estimator is implemented using Matlab/Simulink and the performances are investigated. The simulation results presented in this paper show the effectiveness of the developed method.

Wind turbine noise measurement using a compact microphone array with advanced deconvolution algorithms

This paper experimentally investigates the noise from a large wind turbine (GE 1.5 MW) with a compact microphone array (OptiNav 24) using advanced deconvolution based beamforming methods, such as DAMAS and CLEAN-SC beamforming algorithms, for data reduction. Our study focuses on the ability of a compact microphone array to successfully locate both mechanical and aerodynamic noise sources on the wind turbine. Several interesting results have emerged from this study: (i) A compact microphone array is sufficient to perform a detailed study on wind turbine noise if advanced deconvolution methods are applied. (ii) Noise sources on the blade and on the nacelle can clearly be separated. (iii) Noise of the blades is dominated by trailing edge noise which is frequency dependent and is distributed along the length of the blade with the dominant noise source closer to the tip of the blade. (iv) The LP and DAMAS algorithms represent the distributed trailing edge noise source better than CLEAN-SC and classical beamforming. (v) Additional tonal noise produced during yawing operation is believed to be radiating from the tower of the wind turbine that acts like a resonator. (vi) Ground reflection is not believed to have a significant effect on noise source location estimates in this study.

Determination of Aerodynamic Sound Sources on Periodicity Noise Generated from a Micro Wind Turbine

To discuss the relationship between the periodicity noise and the aerodynamic noise source of a wind turbine, the characteristics of the actual wind turbine were measured by a wind tunnel; moreover, the flow around the impeller immersed in the uniform flow was solved by numerical simulation. In the observed wind turbine, when the mainstream velocity was 7.4 m/s or more, the power coefficient reached maximum at tip speed ratio 8. When the mainstream velocity increased in the driving condition of the maximum power point, the aerodynamic noise increased due to the periodicity noise in the vicinity of 1000 Hz and the broadband noise in the high-frequency domain. The noise sources were concentrated at the leading edge and trailing edge on the suction surface side. We experimentally indicated that the pressure fluctuation of the leading edge was the source of the periodicity noise.

1111 噴流拡散装置の低騒音化に対する基礎的研究

A muffler is attached to the exhaust duct of a diesel engine, and aerodynamic noise is generated by exhaust gas flows into the muffler. The jet flow diffusion device diffuses an air flow inside muffler. An air flow flowing into the jet flow diffusion device flow out outside through the perforated metal and slits. In this study, we performed measurements of the aerodynamic noise changing by perforated metal, the cone position and an attachment position of the support. Then, in order to clarify the aerodynamic noise sources, we performed measurements of the velocity fluctuation of the flow of air flowing out of the jet flow diffusion device. The result of the aerodynamic noise measurement experiment shows that the SPL increases when cone position is on the upstream side.

Simulation of Broadband Noise Sources of an Axial Fan under Rotating Stall Conditions

Study on the influence of rotating stall on the aerodynamic noise of axial fan has important value to warn of the occurrence of stall through monitoring the noise variations. The present work is to analyze the aerodynamic noise before and after the phenomenon of rotating stall by solving Navier-Stokes equations, coupled with the throttle condition and the broadband noise sources model. The impeller exit rotational Mach number and rotational Reynolds number are separately 0.407 and 8.332 × 106. The results show that the aerodynamic noise source of the fan is mainly the rotation noise under the design condition. The vortex noise accounts for the major part of fan noise after the occurrence of stall, and the maximum acoustic power level of the fan appears in the rotor domains. In the evolution process from the stall inception to the stall cell, the high noise regions of the rotor develop along the radial, circumferential, and axial directions, and the area occupied by high noise regions increases from 33% to 46% impeller channels area. On rotating stall condition, the high noise regions occupying about 46% impeller channels area propagate with the stall cell along the circumferential direction at a half of rotor speed.

Experimental and numerical analysis on noise reduction in a multi-blade centrifugal fan

In this work, analysis on noise source and reduction in a multi-blade centrifugal fan used for air-conditioners was carried out by experimental and numerical methods. Firstly, an experimental system using microphone mounted on volute surface for measuring surface pressure fluctuations of volute was designed and introduced, then surface pressure fluctuations of the whole volute for a multi-blade centrifugal fan were measured by this system, and the inlet noise for this fan was also obtained. And then, based on the experimental results, the aerodynamic noise source of the studied fan was analysed. The surface pressure fluctuations of the volute showed that there were largest surface pressure fluctuations near the volute tongue, and peaks appeared at the Blade Passing Frequency (BPF). The spectra of fan inlet noise showed that the peaks also appeared at BPF, and noise levels in a wide range of frequency were also larger. Secondly, the internal flow of the fan was simulated by commercial software under the same conditions with the experiment, and then the fluid flow and acoustic power field were obtained and discussed. The contours of acoustic power level showed that the larger noise was generated at the impeller area close to the outlet of scroll and at the volute tongue, which is same as that from experiment. Based on all of the results, we can find that the vortex noise is an important part of fan noise for the studied fan, and the rotation noise also cannot be neglected. Finally, several reduction methods that are thought to be effective based on experimental and numerical results were suggested.

Study on the aerodynamic noise of internal flow of regenerative flow compressors for a fuel-cell car

The regenerative flow compressor used in fuel-cell cars generates high aerodynamic noise, which is the main source of noise. Compared with the research on centrifugal or axial turbomachinery, research on the noise of regenerative flow compressors is far from adequate. This paper presents the on-going work on it at Tongji University based on both experimental and computational works. In this study, a three-dimensional unsteady computational fluid dynamic model of the compressor was constructed with the large eddy approach. The pressure fluctuation, vortex noise source and Ffowcs William-Hawkings (FW-H) method were used to analyze the characteristics of the aerodynamic noise sources. Additionally, the far-field aerodynamic noise generated by the internal flow of the compressor was predicted using the aeroacoustic finite element method. The simulation results were validated with the experimental data. It was found that combining the fluid dynamic model and aeroacoustic finite element analysis promising results for aerodynamic noise prediction of compressors could be produced. The effects of the impeller parameters on the aerodynamic noise of the compressor were also studied.

Effect of the Backlight Angle on the Aero-Acoustics of the C-Pillar

In this paper, the effect of the backlight angle on the aerodynamic and acoustic parameters of the Ahmed model is investigated using numerical methods. The numerical results are validated by the experimental results available in the literature. The comparison reveals good agreement between the simulations and experiments. After validation of the aerodynamic model, aerodynamic noise on the c-pillar is studied and aerodynamic noise sources are identified. The critical backlight angles considering the aerodynamic and acoustic parameters are obtained.

On aerodynamic noises radiated by the pantograph system of high-speed trains

Pantograph system of high-speed trains become significant source of aerodynamic noise when travelling speed exceeds 300 km/h. In this paper, a hybrid method of non-linear acoustic solver (NLAS) and Ffowcs Williams-Hawkings (FW-H) acoustic analogy is used to predict the aerodynamic noise of pantograph system in this speed range. When the simulation method is validated by a benchmark problem of flows around a cylinder of finite span, we calculate the near flow field and far acoustic field surrounding the pantograph system. And then, the frequency spectra and acoustic attenuation with distance are analyzed, showing that the pantograph system noise is a typical broadband one with most acoustic power restricted in the medium-high frequency range from 200 Hz to 5 kHz. The aerodynamic noise of pantograph systems radiates outwards in the form of spherical waves in the far field. Analysis of the overall sound pressure level (OASPL) at different speeds exhibits that the acoustic power grows approximately as the 4th power of train speed. The comparison of noise reduction effects for four types of pantograph covers demonstrates that only case 1 can lessen the total noise by about 3 dB as baffles on both sides can shield sound wave in the spanwise direction. The covers produce additional aerodynamic noise themselves in the other three cases and lead to the rise of OASPLs.

Fly-over aircraft noise measurement campaign at Montreal-Trudeau airport using a microphone array

Engines being quieter due to high by-pass ratios, the airframe noise, produced for instance by landing gears or high-lift devices, has become a significant contributor to the total noise radiated by aircraft during approach and landing. As part of the investigations carried out to understand noise generation mechanisms, the beamforming techniques developed over the last decade and applied to microphone array measurements have shown to be effective tools for localization and quantification of these aerodynamic noise sources. In order to validate their in-house beamforming softwares, Bombardier Aerospace and the Groupe d’Acoustique de l’Université de Sherbrooke have conducted a 5-day measurement campaign in June 2012. The 95-microphone array was located on the roof of a building next to the Montreal-Trudeau airport. Aircraft position was determined by two high-definition cameras, both synchronized with the microphone array by inter-range instrumentation group time codes generators. This paper summarizes the measurement campaign. The aircraft tracking tool and the beamforming algorithms used to characterize the noise sources are presented. Several Bombardier CRJ fly-overs were recorded during this test. Beamforming results obtained for different airlines are compared in order to evaluate the repeatability of the method.

Turbulent boundary-layer noise: direct radiation at Mach number 0.5

AbstractBoundary layers constitute a fundamental source of aerodynamic noise. A turbulent boundary layer over a plane wall can provide an indirect contribution to the noise by exciting the structure and a direct noise contribution. The latter part can play a significant role even if its intensity is very low, explaining why it is difficult to measure. In the present study, the aerodynamic noise generated by a spatially developing turbulent boundary layer is computed directly by solving the compressible Navier–Stokes equations. This numerical experiment aims at giving some insight into the noise radiation characteristics. The acoustic wavefronts have a large wavelength and are oriented in the direction opposite to the flow. Their amplitude is only 0.7 % of the aerodynamic pressure for a flat-plate flow at Mach 0.5. The particular directivity is mainly explained by convection effects by the mean flow, giving an indication about the compactness of the sources. These vortical events correspond to low frequencies and thus have a large lifetime. They cannot be directly associated with the main structures populating the boundary layer such as hairpin or horseshoe vortices. The analysis of the wall pressure can provide a picture of the flow in the wavenumber–frequency space. The main features of wall pressure beneath a turbulent boundary layer as described in the literature are well reproduced. The acoustic domain, corresponding to supersonic wavenumbers, is detectable but can hardly be separated from the convective ridge at this relatively high speed. This is also due to the low frequencies of sound emission as noted previously.

Investigation of noise sources in high-speed trains

In this paper, the quantification of the external noise sources in high-speed trains is discussed. A thorough understanding of the underlying causes of noise generation in high-speed trains is needed to develop effective noise control measures. However, because high-speed trains produce a complex array of sounds, it is very difficult to determine each individual source of noise. In this study, the delay-and-sum beamforming method, which uses microphone arrays, was used to separate the noise sources and analyze the sound characteristics of high-speed trains. A new microphone array with 96 microphones was designed to measure the noise produced by high-speed trains. Performance verification tests were conducted to ensure the reliability of the results obtained from the array. Then, the system was used to measure the sounds produced by Korean high-speed trains traveling at speeds between 150 and 300 km/h. Sound maps were then produced using the beamforming technique. The study determined that the majority of the noise produced by the high-speed trains originated from the front nose, bogie, pantograph and inter-coach spacing. Finally, the beampower spectra of the aerodynamic noise sources originating in the front nose, pantograph and inter-coach spacing were deduced from frequency conversion. From these results, the aerodynamic noise characteristics of the major sources of noise in high-speed trains were determined.

Numerical Analysis of Aerodynamic Noise of a New High-Speed Train

Along with the raising of the train speed, aerodynamic noise of the high-speed train is generated more and more significantly and their reduction has become one of the key factors to control noise of the high-speed train. Aerodynamic noise radiated from the high-speed train surface was analyzed numerically. The mathematical and physical models of the three dimensional flow field of the high-speed train were established and the external steady and unsteady flow fields of the high-speed train were calculated by using the standard "k-ε" turbulence model and large eddy simulation (LES) respectively. On the basis of the steady flow field, aerodynamic noise sources on the car body surface of the high-speed train are calculated by using the broadband noise source model. On the basis of the unsteady flow field, the time domain characteristics of fluctuating pressures on the car body surface are analyzed. The sound pressure level on the surface pressure demonstrating is calculated and the flow field of some critical parts is analyzed.

A computational fluid dynamics study of crosswind effects on vehicle A-pillar aero-acoustics

The surface pressure fluctuations due to the flow separation in the A-pillar region on the front side windows of a passenger car is a dominant source of aerodynamic noise. Aerodynamically generated noise adversely affects vehicle occupant’s comfort and safety (due to fatigue caused by long time noise exposure). The size and magnitude of the A-pillar flow separation mainly depend on the local A-pillar and windshield geometry and yaw angles. The primary objective of this was to develop a computational fluid dynamic (CFD) and computational aero-acoustics (CAA) model to simulate aerodynamic flow and noise generation in the A-pillar region of a simplified 40% scale vehicle with elliptical windshield radius at ±15° yaw angles. Observations made from the CFD and CAA results showed trailing edge and boundary layer noise were generated, caused by turbulent airflow separation behind the A-pillar region. The trailing edge noise was dominant in the leeward region while boundary layer noise was seemed to dominate in the windward region. High sound pressure levels were observed on the vehicle body, close to the noise source, which gradually decreased as the aerodynamic noise propagated further from the vehicle body.

Identification and Suppression of Noise Sources Around High Speed Trains

Aerodynamic noise would become a significant limiting factor as the running speed of high speed trains increases, and should be taken into consideration during the design of high speed trains. In the present work, the Nonlinear Acoustics Solver (NLAS) has been adopted to investigate the aerodynamic noise of high speed trains. The region around the streamlined head of the leading car and the trailing car, the bogie region, the upstream zone and the wake zone are discussed and the distribution characteristics of aerodynamic noise sources around a high speed train is exhibited. The acoustic characteristics of the streamlined head could be contaminated due to the unreasonable arrangement of cab windows or cowcatchers. As a result, different designs of cab windows and cowcatchers are investigated in this paper and advice on low noise design is given. Based on the above analysis, a new design of the streamlined head is proposed. Numerical results reveal that the noise level of the new streamline has been greatly suppressed. The analysis would aid in identification of noise sources around high speed trains and design of high speed trains with low noise.

Acoustical device using helmholtz resonator for the high-speed train noise barrier

This study is primarily aimed at developing a measure to overcome the limited shielding performance of the noise barriers for the high-speed train. Up to the train speed of 300km/h and more the noise incidence angle and the source height change due to the pronounced aerodynamic noise source parts located at the higher positions compared to the height of the conventional rolling noise source. By means of the experimental analysis on the sound radiation characteristics and the sound pressure distribution around the noise barrier, a prototype of an acoustical attachment is porduced based on the analytical model calculation and numerical analysis. The principle of the Helmholtz resonator is used to optimize the acoustical impedance on the surface of the upper dege of the noise barrier. Using the model it was possible to find an appropriate acoustical property for the impedance according to the target frequency,sound incidence angle relative to the barrier and the receiver position. In this paper the results of the experiment in an anechic room and from the out door experiment are shown and discussed.

Improving the resolution of three-dimensional acoustic imaging with planar phased arrays

This paper examines and compares two methods of improving the quality of three-dimensional beamforming with phased microphone arrays. The intended application is the detection of aerodynamic noise sources on wind turbines. Both methods employ Fourier based deconvolution. The first method involves a transformation of coordinates that tends to make the response to a point source, the point spread function, more shift invariant. The result is a significant improvement in sound source imaging in the transformed coordinate system. However, the inverse transformation to Cartesian coordinates introduces range dependent resolution limitations because of the irregular distribution of the focal points. The second method combines the transformation of coordinates with an alternative scanning technique. This method can be used in near field three-dimensional acoustic imaging to produce maps free of sidelobes and with constant resolution. The robustness of the proposed methods is validated both with computer simulations and experimentally.

Aerodynamic Sound and Flow Field Generated from Combinational Inclined Tapered Cylinder

This study deals with the aerodynamic sound, especially the Aeolian tone. One of the problems with the aerodynamic sound, noise reduction along a railway line is significant engineering issue for further speed-up of a high-speed train. In particular, a pantograph is one of the main aerodynamic noise sources of a high-speed train. As one of the methods for the aerodynamic noise reduction, this study proposes the tapered cylinder by a pantograph. In this paper, the aerodynamic sound and the wake flow of the combinational inclined circular cylinder and tapered cylinder are experimentally and numerically investigated in knuckle upstream and knuckle downstream. Also, we suggest that the surface of grooves such as wood grain interacts with the aerodynamic sound generated from the model. Here, it is considered that the comparison of aerodynamic sound between the model with grooves and without grooves. Consequently, reduction of the maximum aerodynamic sound is possible for the tapered cylinder in the case of knuckle downstream. Furthermore, the noise reduction method is effective that the model surface with grooves such as wood grain.

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.