Aeroacoustic performance evaluation of axial flow fans based on the unsteady pressure field on the blade surface

Because of the European and global regulation concerning acoustic emission, the goal of manufacturers is to substantially decrease the noise radiated by turbomachines, and in particular axial fans, without degrading their aerodynamic performances. High rotation speed and increasing power add to the overall difficulties. The theoretical study of this paper consists of two parts: (1) an aerodynamic approach based on the vortex surface method and (2) an aeroacoustic approach which mainly concerns the prediction of the tonal noise using the Ffowcs Williams and Hawkings (FW-H) equation. One of the main goals is the evaluation of the unsteady aerodynamic forces applied on the fan blades. A 2D software analysis, based on the vortex surface method (or potential flow method), was carried out. That process enabled evaluation of the potential flow around a mobile grid; first in a steady mode, and secondly in an unsteady mode by introducing an upstream disturbance in the form of an inlet velocity variation. The sources of noise corresponding to the zones with high force fluctuation amplitude are located initially on the blade surface. These fluctuating forces are used to predict the tonal noise radiated by the fan in far field by means of the FW-H equation. Two axial flow fans were used in this study. The theoretical results will be compared to the experimental ones in order to evaluate the aeroacoustic performances of the fans.     

Identification of noise sources on a residential split-system air-conditioner using sound intensity measurements

This paper presents the results of experimental studies of the noise of a residential split-system air-conditioner unit. The compressor and condenser and associated fans were removed from the unit and did not form part of the studies. Care was taken with the unit to separate the inlet and exhaust noise from the noise radiated from the cabinet. The measurements were made with a two-microphone sound intensity probe and these resulted in sound power level data. The sound power levels produced by radiation from the inlet, exhaust and cabinet were obtained for five different volume flow rates. The effect on the sound power generated by removing the coil was investigated. Measurements and subjective studies show that the low frequency sound is predominantly radiated from the exhaust and inlet. At high frequency, the cabinet noise dominates.     

Effect of inlet duct contour and lack thereof on the noise generated of an axial flow fan

This study concerns the unsteady flows in turbomachines in general, and the aeroacoustics of fans in particular. The principal objective of this paper is the determination of the influence of the upstream environment on the acoustic and aerodynamic behavior of axial fans. After analysis of the various sources of noise present in turbomachines, interest is focused on the influence of the disturbances of the velocity field at the suction. Accordingly, the effect of the presence of a contoured duct and a lack thereof at the inlet of an axial flow fan is analyzed . The results show the strong involvement of the upstream turbulence level in the generation of the noise, and in particular, of broadband noise.     

The noise of cooling fans used in heavy automotive vehicles

This paper is concerned with the reduction of noise from automotive cooling systems. A comparison is made between the use of axial flow and centrifugal fans and formulae presented for obtaining the octave sound power for each type of fan. The disadvantages of centrifugal fan installations are highlighted and axial fan design configurations are examined with the object of providing optimized systems. Experimental results are presented for different axial fans and comparisons made of the noise measurements with the ingested flow distortions measured by means of a hot wire anemometer. The results indicate the limits of maximum noise reduction which manufacturers may expect using existing fan designs and also indicates the methods for achieving maximum noise reduction for these configurations.     

Sound generated in the vicinity of the trailing edge of an upper surface blown flap

Far field noise data indicated that for practical upper surface blown flap configurations, the noise radiated below the flap is dominated by the noise generated in the vicinity of the trailing edge. The sound field caused by turbulent mixing in the trailing edge wake is investigated experimentally and theoretically. Hot wire measurements were made downstream of the trailing edge to determine the gross turbulent mixing characteristics of the flow. This information is used as input to a theoretical analysis of the sound field. Favorable agreement is found between predicted and measured far field noise directivity at various frequencies and noise power spectra in various directions.     

冷却用弯掠轴流风机气动噪声预测与试验分析

采用Navier-Stokes方程和标准k-ε湍流模型,对五种不同结构的弯掠轴流风机A/B/C/D/E进行稳态数值模拟,结合给定的性能要求筛选出B/E风机。以稳态流场为基础,采用大涡模拟(LES)与基于Lighthill声类比的FW-H模型相结合的方法对B/E风机进行非定常计算和气动噪声预测,并将预测结果与试验数据进行了对比验证。分析了以叶轮表面作为噪声源时B/E风机的涡流噪声频谱特性,研究了风机三维非定常内部流场中旋涡分布特性,并探讨了风机旋转区域内部声压级的分布规律。     

Fluid dynamics of a flow excited resonance,Part II: Flow acoustic interaction

This is the second of two companion papers in which the physics and detailed fluid dynamics of a flow excited resonance are examined. The approach is rather different from those previously used, in which stability theory has been applied to small wavelike disturbances in a linearly unstable shear layer, with an equivalent source driving the sound field which provides the feedback. In the approach used here, the physics of the flow acoustic interaction is explained in terms of the detailed momentum and energy exchanges occurring inside the fluid. Gross properties of the flow and resonance are described in terms of the parameters necessary to determine the behaviour of the feedback system. In this second paper it is shown that two relatively distinct momentum balances can be considered in the resonator neck region. One can be identified with the vortically induced pressure and velocity fluctuations and the other with the reciprocating potential flow. The fluctuating Coriolis force caused by the interaction of the potential and vortical flows is shown to be the only term in the linearized momentum equation which is not directly balanced by a fluctuating pressure gradient. This force provides the mechanism for the exchange of the mean energies associated with the mean and fluctuating momenta, respectively. A source and sink of energy are identified in which mean energy associated with fluctuating momentum is extracted from and returned to the mean flow, respectively. The imbalance between the source and sink is responsible for both the radiated acoustic power and the power carried away by the vortices as they convect downstream. This radiated acoustic power and vortically convected power, and the source and sink powers, are all of the same order of magnitude. With the vortex shedding and reciprocating potential flow “phase locked” the amplitude of the steady state oscillations is determined by the condition that the net power produced in the resonator neck (the source power less the sink power) is equal to the sum of the radiated acoustic power and that carried by the vortices.     

Aerodynamic sound production in low speed flow ducts

Measurements of spoiler aerodynamic noise, generated in a low velocity flow duct and radiated from an open exhaust termination, have been made in the form of sound power spectra. The individual $\text{1}\text{3}$ octave power measurements are satisfactorily collapsed (within ± 3 dB) with the aid of derived theoretical scaling laws. Non-dimensional spectra are presented which permit generalized predictions of flow noise for bluff bodies, including splitter attenuators, mounted in low speed flow ducts.     

NOISE GENERATED BY A COANDA WALL JET CIRCULATION CONTROL DEVICE

An analysis is made of the production of sound by a hydrofoil with a Coanda wall jet circulation control (CC-) device. Three principal sources are identified in the vicinity of the trailing edge of the hydrofoil. The radiation at very low frequencies is dominated by “curvature noise” generated by the interaction of boundary layer turbulence with the rounded trailing edge of the CC-hydrofoil; this is similar in character and magnitude to the low-frequency component of the conventional trailing edge noise produced by a hydrofoil of the same chord, but with a sharp trailing edge. Higher frequency sound is produced principally at the Coanda jet slot. “Passive slot noise” is caused by the “scattering” by the slot lip of nearfield pressure fluctuations in the turbulent boundary layer of the exterior mean flow past the slot. This is of comparable intensity to high frequency, sharp-edged trailing edge noise. However, the acoustic spectrum is greatly extended to much higher frequencies if the Coanda jet is turbulent; the sound produced by the interaction of this turbulence with the lip tends to dominate the spectrum at frequencies f (Hz) greater than about U_j/h, where h is the slot width and U_jthe Coanda jet speed. Sample numerical results are presented for a typical underwater application that indicate that at this and higher frequencies the slot noise can be 20 dB or more greater than conventional trailing edge noise, although the differences become smaller as the thickness of the slot lip increases.     

开式轴流风扇气动噪声预测

本文采用LES/FW-H的匹配方法,研究了开式轴流风扇内部旋涡流动特征及其与叶片表面干涉引起的气动噪声之间的联系,同时进行了远场噪声预测,探讨了叶轮不同表面辐射噪声时的频谱分布特征.研究结果表明,开式轴流风扇吸力面附近形成的叶尖涡和前缘分离涡在吸力面叶片表面相应位置形成大压力波动,形成主要噪声源;叶片吸力面的辐射噪声可以通过改善吸力面附近的旋涡流动来降低;低速轴流叶轮由叶轮壁面辐射的噪声以宽频成分为主.     

亚音速轴流风扇后掠叶片定子与湍流互作用宽频辐射噪声预报

针对亚音速轴流风扇后掠叶片定子的宽频辐射噪声问题,介绍并推导了叶栅宽频辐射声功率计算公式,通过该公式计算后掠叶片定子的宽频辐射声功率级,并从湍流入流和叶栅响应的角度揭示后掠角对定子辐射噪声的影响机理。在此基础上,考虑到实际风扇定子工作在转子尾流中的情况,采用Gauss尾流模型模拟转子尾流,建立转子尾流湍流波数谱模型,推导得到定子叶片与转子尾流互作用的宽频辐射声功率计算公式。通过与NASA风扇试验模型对比得到,考虑转子尾流的定子叶栅宽频辐射声功率计算公式能够较好的预报后掠定子宽频辐射声功率。最后,针对试验风扇模型,分析叶片安装角、叶片弦长对后掠叶片定子辐射噪声的影响。      

Nature of inlet turbulence and strut flow disturbances and their effect on turbomachinery rotor noise

Results of an investigation in which turbomachinery rotor sound spectra were correlated with aerodynamic measurements of the inlet turbulence, strut wake, and vortex flow strengths are reported. Aerodynamic measurements included mean velocity profiles, turbulence intensity, and axial length scales. Inlet turbulence data indicate that the major effect of flow contraction appears to be the elongation of turbulent eddies. Eddies of this size dominate the blade passing frequency (BPF) tones. Decreasing eddy size by use of a grid revealed vortex flow strength to be the second major sound source. A doubling of vortex flow strength produced a 6 dB increase in the SPL of the first BPF. The sound pressure level showed less than a 2 dB change with doubling of strut wake turbulence intensity or velocity defect. A discussion of the relative importance of various sources of noise due to flow non-uniformities at the inlet is given.     

Mode detection in turbofan inlets from near field sensor arrays

Knowledge of the modal content of the sound field radiated from a turbofan inlet is important for source characterization and for helping to determine noise generation mechanisms in the engine. An inverse technique for determining the mode amplitudes at the duct outlet is proposed using pressure measurements made in the near field. The radiated sound pressure from a duct is modeled by directivity patterns of cut-on modes in the near field using a model based on the Kirchhoff approximation for flanged ducts with no flow. The resulting system of equations is ill posed and it is shown that the presence of modes with eigenvalues close to a cutoff frequency results in a poorly conditioned directivity matrix. An analysis of the conditioning of this directivity matrix is carried out to assess the inversion robustness and accuracy. A physical interpretation of the singular value decomposition is given and allows us to understand the issues of ill conditioning as well as the detection performance of the radiated sound field by a given sensor array.     

水下翼型结构流噪声实验研究

为测量流激水下翼型结构的流噪声,提出了一种混响箱测量方法。在重力式水洞中搭建了一套实验测量系统,利用混响箱法测量了水下翼型结构模型的辐射声功率。在此基础上研究了流速及结构参数(厚度、肋、声学覆盖层)对其辐射声功率的影响。结果表明:当流速小于5 m/s时,辐射声功率随流速的6次方增长,符合偶极子的辐射规律;当流速大于5 m/s时,辐射声功率随流速的10土1次方规律增长,不再按偶极子的规律辐射;若对水下翼型结构模型加厚、加环肋及外部敷设黏弹性材料,均可在一定程度上抑制流噪声。此研究方法可对水下复杂结构的辐射声功率测量及结构优化设计提供一定的参考。      

Numerical study on reduction of the elevated structure noise by surface absorption on plate girders

With the aim to propose a reasonable and effective countermeasure for the elevated structure noise, the sound field radiated by a steel plate girder, which is the main source of the elevated structure noise, have been theoretically analysed. In the present study, steel plate girders are modelled as infinitely long elastic plate strips placed in parallel and numerical examples on the sound field radiated by the steel plate girders are shown. In the analysis of the radiated sound field, the equivalent source method is employed. Effect of the surface absorption on the sound field radiated by the girders is discussed through numerical examples. The results show that the surface absorption is effective for reduction of the radiated sound field, especially in the area that increase of sound pressure due to reflection by adjacent plate girders is observed. Furthermore, to design for reasonable countermeasure, variation of the noise reduction effect due to changing the pattern of surface absorption area on plate girders is classified.     

Influence of geometrical parameters upon the sound power level of centrifugal fans

The formula for sound power radiated to the environment expressed as a function of tip speed does not take into account the influence of the impeller shape. Thus it may be used for noise assessment of geometrically similar fans only, since in this case the proportionality coefficient appearing in the formula remains constant. When determining the sound power as a function of the absolute velocity of gas exhaust from the impeller a formula was derived that covers the dependence of sound power or its level on the geometrical parameters of the impeller.     

Effect of blade wrap angle on efficiency and noise of small radial fan impellers—A computational and experimental study

Radial impellers have several technical applications. Regarding their aerodynamic performance, they are well optimized nowadays, but this is in general not true regarding acoustics. This work was therefore concerned with analyzing the flow structures inside isolated radial impellers together with the far-field sound radiated from them in order to optimize the aerodynamic and acoustic performance. Both numerical and experimental techniques were applied in order to study the effect of varying wrap angle and otherwise identical geometric configuration on aerodynamics and acoustics of the radial impellers. The results give a detailed insight into the processes leading to sound generation in radial impellers. Measurements were performed using laser Doppler anemometry for the flow field and microphone measurements to analyze the radiated noise. In addition, unsteady aerodynamic simulations were carried out to calculate the compressible flow field. An acoustic analogy was employed to compute far-field noise. Finally, the phenomena responsible for tonal noise and the role of the wrap angle could be identified. Using this knowledge, design guidelines are given to optimize the impeller with respect to the radiated noise. This work shows that improved aerodynamic efficiency for isolated impellers does not automatically lead to a smaller flow-induced sound radiation.     

Comparative Study of the Propagation of Jet Noise in Static and Flow Environments

In order to analyze the effect of the background flow on the sound prediction of fine-scale turbulence noise, the sound spectra from static and flow environments are compared. It turns out that, the two methods can obtain similar predictions not only at 90 deg to the jet axis but also at mid- and high frequencies in other directions. The discrepancies of predictions from the two environments show that the effect of the jet flow on the sound propagation is related to low frequencies in the downstream and upstream directions. It is noted that there is an obvious advantage of computational efficiency for calculating in static environment, compared with that in flow environment. A good agreement is also observed to some extent between the predictions in static environment and measurements of subsonic to supersonic. It is believed that the predictions in static environment could be an effective method to study the propagation of the sound in jet flow and to predict the fine scale turbulence noise accurately in a way as well.