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.     

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。     

Reconstruction of the unsteady rotating forces of fan’s blade from far-field sound pressure

The aim of this paper is to investigate an inverse method used to evaluate the unsteady rotating forces acting on the fluid by the fan’s blade. A simple model based on the tonal noise produced by an axial fan and validated with a directivity experience is used to derive a discretized form of the direct problem and to simulate acoustic pressures at known spatial positions in the radiated field. The inversion of this direct problem is ill-conditioned and requires a regularization technique to stabilize the solution for small perturbations in the measured acoustic pressures. The reconstruction reveals that the conditioning of the inverse problem depends on the aeroacoustic source and the far-field microphones number as well as on the studied frequency. Tikhonov regularization can provide an appropriate regularization parameter leading to an accurate reconstruction of imposed unsteady rotating forces even with the presence of measurement noise.     

Numerical simulation of tonal fan noise of computers and air conditioning systems

Current approaches to fan noise simulation are mainly based on the Lighthill equation and socalled aeroacoustic analogy, which are also based on the transformed Lighthill equation, such as the wellknown FW-H equation or the Kirchhoff theorem. A disadvantage of such methods leading to significant modeling errors is associated with incorrect solution of the decomposition problem, i.e., separation of acoustic and vortex (pseudosound) modes in the area of the oscillation source. In this paper, we propose a method for tonal noise simulation based on the mesh solution of the Helmholtz equation for the Fourier transform of pressure perturbation with boundary conditions in the form of the complex impedance. A noise source is placed on the surface surrounding each fan rotor. The acoustic fan power is determined by the acoustic-vortex method, which ensures more accurate decomposition and determination of the pressure pulsation amplitudes in the near field of the fan.     

使用多孔蜗舌的贯流风机

将贯流风机的蜗舌替代为多孔板与容腔组合的结构,定性地研究其控制气动噪声的可行性。通过对气动特性和辐射声信号的实验测量,结合内部流场的非稳态雷诺平均数值模拟,分析了这种蜗舌改造对贯流风机的整体特性和内部流动特征的影响,结果表明多孔蜗舌对贯流风机的压力-流量曲线作用并不明显,但对风机的噪声有着重要的影响,改变蜗舌迎风面的穿孔率可以有效地控制贯流风机的噪声。     

A numerical study on the flow and sound fields of centrifugal impeller located near a wedge

Centrifugal fans are widely used and the noise generated by these machines causes one of the serious problems. In general, the centrifugal fan noise is often dominated by tones at blade passage frequency and its higher harmonics. This is a consequence of the strong interaction between the flow discharged from the impeller and the cut-off in the casing. However, only a few researches have been carried out on predicting the noise because of the difficulty in obtaining detailed information about the flow field and considering the scattering effect of the casing. The objective of this study is to understand the generation mechanism of sound and to develop a prediction method for the unsteady flow field and the acoustic pressure field of the centrifugal impeller. A discrete vortex method is used to model the centrifugal impeller and a wedge and to calculate the flow field. The force of each element on the blade is calculated by the unsteady Bernoulli equation. Lowson's method is used to predict the acoustic source. In order to consider the scattering and diffraction effects of the casing, Kirchhoff-Helmholtz boundary element method (BEM) is developed. The source of Kirchhoff-Helmholtz BEM is newly developed, so the sound field of the centrifugal fan can be obtained. A centrifugal impeller and wedge are used in the numerical calculation and the results are compared with the experimental data. Reasonable results are obtained not only for the peak frequencies but also for the amplitudes of the tonal sound. The radiated acoustic field shows the diffraction and scattering effect of the wedge.     

An unsteady aerodynamic formulation for efficient rotor tonal noise prediction

An aerodynamic/aeroacoustic solution methodology for predction of tonal noise emitted by helicopter rotors and propellers is presented. It is particularly suited for configurations dominated by localized, high-frequency inflow velocity fields as those generated by blade–vortex interactions. The unsteady pressure distributions are determined by the sectional, frequency-domain Küssner–Schwarz formulation, with downwash including the wake inflow velocity predicted by a three-dimensional, unsteady, panel-method formulation suited for the analysis of rotors operating in complex aerodynamic environments. The radiated noise is predicted through solution of the Ffowcs Williams–Hawkings equation. The proposed approach yields a computationally efficient solution procedure that may be particularly useful in preliminary design/multidisciplinary optimization applications. It is validated through comparisons with solutions that apply the airloads directly evaluated by the time-marching, panel-method formulation. The results are provided in terms of blade loads, noise signatures and sound pressure level contours. An estimation of the computational efficiency of the proposed solution process is also presented.     

Numerical and experimental investigations of the unsteady aerodynamics and aero-acoustics characteristics of a backward curved blade centrifugal fan

A numerical study of the aerodynamic and aeroacoustic behaviors of a backward curved blade centrifugal fan was conducted under two important flow conditions: BEP and 1.3 × BEP. Three-dimensional numerical simulations of the complete unsteady flow field for the whole impeller-volute configuration were used to determine the aeroacoustic sources. To locate the unsteady flow and perturbations, the near field wall pressure fluctuations at different strategic points on the volute were computed using the URANS approach. Thus the intensities and positions of the aeroacoustic sources were identified by analyzing frequency spectra. The aeroacoustic sources caused by fluctuations in the interactions of the flows leaving the impeller and volute were close to the volute tongue, and the most effective noise sources related to the flow rate were near the impeller shrouds. In addition, the unsteady flow variables provided by CFD calculations were used as inputs in the Ffowcs Williams-Hawkings equation to estimate the noise tones of the fan. The aeroacoustic calculation results showed that the volute noise was much larger than the blade noise, and the noise mainly propagated from the outlet duct of the fan. Moreover, to account for the noise propagation, three calculation methods were used by applying different solid boundaries. Compared with the other methods, the FEM method, which accounted for the complex solid boundaries, produced good agreement and showed that the complex solid boundaries cannot be neglected in aeroacoustic predictions. The calculation results showed good agreement with the experimental results.     

Experimental and numerical studies on the discrete noise about the cross-flow fan with block-shifted impellers

The experimental and numerical studies have been carried out to investigate the blade passage frequency (BPF) noise of a cross-flow fan (CFF) with the block-shifted impeller. Firstly, the aeroacoustic and aerodynamic features about the five different block-shifted impellers have been obtained experimentally. Secondly, the dynamic pressure sensors were put in the noise generating surfaces to investigate the pressure fluctuations generated by the shifted blocks in the near-field through the cross-correlation analysis. Thirdly, the two-dimensional (2D) unsteady flow field has been simulated by commercial CFD software and the vortex flow patterns and the unsteady force of the blade have been analyzed to detect the noise source about the CFF. Finally, the noise properties about the CFF were predicted by a hybrid method through the Farassat’s equation and the surface pressure fluctuations were provided by the CFD simulations. A simplified theory model has also been built up at the same time. The comparisons are made between the results of hybrid method and the theory model to validate the correctness of the noise prediction methods. The accuracy of these results was also evaluated by the corresponding experimental ones. The results indicate that the impellers with different block-shifted angles are the same in aerodynamic performance but different in the BPF noise. The relations between the shifted angles and the BPF noise levels have been predicted and discussed for the noise reduction.     

襟翼侧缘噪声机理及修型降噪设计

襟翼侧缘噪声是飞机起降阶段机体噪声的重要噪声源。采用极大涡模拟对襟翼侧缘非定常流场进行数值模拟,分析其噪声产生机理.基于此,提出了两种襟翼侧缘修型方式,应用虚拟渗透面的Ffowcs Williams and Hawkings(FW-H)声比拟方法将修型构型的远场噪声频谱特性和指向性与基准构型对比分析,研究其降噪效果。通过流场和声场的数值模拟表明,襟翼侧缘噪声属于宽频噪声。不同的襟翼侧缘形状改变了流场形态、侧缘涡结构以及涡系的发展过程,进而对声源分布和远场噪声特性产生影响。结果表明:在给定的5°计算迎角下,两种襟翼侧缘修型方式在保证增升装置的原有升阻气动特性的前提下,能达到减小全场总声压级1~2 dB的降噪效果。      

电子器件散热风扇气动噪声管道声学模态截止控制技术

深度学习输入特征的选择直接影响其分类性能,为了进一步提高基于深度学习的鸟类物种识别模型的分类性能,该文提出一种多特征融合识别方法。该方法首先通过短时傅里叶变换、梅尔倒谱变换和线性调频小波变换分别计算得到鸣声信号的3种语图样本集,然后分别利用3种语图样本集训练3个基于VGG16迁移的单一特征模型,将3个模型的输出进行自适应加权求和实现融合,并修正了加权交叉熵函数以克服样本不平衡的问题,最后对语图进行分类实现鸟类物种的识别。以ICML4B鸣声库的35种鸟类为研究对象,对比了4种模型的平均识别准确率(MAP),结果表明特征融合模型较单一特征模型的MAP最大提高了0.307;选择输入语图的持续时间分别为100 ms、300 ms以及500 ms,对比不同持续时间下4种模型的测试MAP值,结果表明持续时间为300 ms时4种模型的MAP值均为最高;对比了不同信噪比下4种模型的识别效果,多特征融合模型的识别准确率随着信噪比的下降降低最少。说明在选择合适的语图持续时间后,该文提出的特征融合模型能得到更高的识别准确率,具有一定的抗噪能力,且训练参数少,更适合于少样本鸟类的识别。     

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.     

轴流式弯掠动叶变工况气动──声学性能的实验研究

本文介绍了变工况下，两台具有相同几何参数（动叶弦长除外）的轴流试验风机进行的气动一声学性能的实验研究。一台动叶是常规（径向）动叶，另一台是弯掠动叶（其弦长在中往处增加２０．４％。实验表明：在相同转速条件下，弯掠动叶轴流风机与径向动叶风机相比，不仅提高了气动效率（３％左右），降低了气动噪声（２～４ｄＢ（Ａ）（在最高效率点处，弯掠动叶与径向动叶相比压升增加了１３．１％，流量增加了５％），而且还大幅度地扩大了稳定工作范围（２０％左右）。弯掠动叶在旋转失速工况下性能曲线平坦，并且其旋转失速特征参数明显不同于径向动叶。弯掠动叶的总声压级和噪声频谱也表明了弯掠动叶具有低噪声的特点。     

An experimental study of airfoil instability tonal noise with trailing edge serrations

This paper presents an experimental study of the effect of trailing edge serrations on airfoil instability noise. Detailed aeroacoustic measurements are presented of the noise radiated by an NACA-0012 airfoil with trailing edge serrations in a low to moderate speed flow under acoustical free field conditions. The existence of a separated boundary layer near the trailing edge of the airfoil at an angle of attack of 4.2 degree has been experimentally identified by a surface mounted hot-film arrays technique. Hot-wire results have shown that the saw-tooth surface can trigger a bypass transition and prevent the boundary layer from becoming separated. Without the separated boundary layer to act as an amplifier for the incoming Tollmien–Schlichting waves, the intensity and spectral characteristic of the radiated tonal noise can be affected depending upon the serration geometry. Particle Imaging Velocimetry (PIV) measurements of the airfoil wakes for a straight and serrated trailing edge are also reported in this paper. These measurements show that localized normal-component velocity fluctuations that are present in a small region of the wake from the laminar airfoil become weakened once serrations are introduced. Owing to the above unique characteristics of the serrated trailing edges, we are able to further investigate the mechanisms of airfoil instability tonal noise with special emphasis on the assessment of the wake and non-wake based aeroacoustic feedback models. It has been shown that the instability tonal noise generated at an angle of attack below approximately one degree could involve several complex mechanisms. On the other hand, the non-wake based aeroacoustic feedback mechanism alone is sufficient to predict all discrete tone frequencies accurately when the airfoil is at a moderate angle of attack.     

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.     

轴流风扇叶片端导叶作用的研究

本文采用数值方法研究了叶片端导叶对轴流风扇性能的影响。通过与普通开式轴流风扇比较,分析了叶片端导叶对内部流动作用的机理.数值计算结果表明:叶片端导叶的安装位置将影响轴流风扇气动效率,安装叶片端导叶不能提高风扇静压升,但是在压力面安装时能有效地减小风扇叶顶泄漏流与主流的掺混损失;在设计流量下,压力面安装叶片端导叶使泄漏涡的作用范围较小,涡核更靠近吸力面;吸力面安装叶片端导叶弱化了泄漏涡的强度但没有减小泄漏涡的作用范围。     

贯流风机变斜式叶轮和常规直叶轮的对比研究

本文对贯流风机变斜式叶轮和常规直叶轮进行了三维数值模拟和实验研究。结果表明贯流风机内部偏心涡的位置沿轴向具有明显的三维分布特征,两者偏心涡的圆周位置沿轴向不断变化,而直叶轮偏心涡的径向位置沿轴向的分布几乎没有变化。变斜式叶轮中偏心涡的位置更加靠近叶轮内圆周和风机的蜗舌。采用变斜式叶轮可以降低叶片通过频率噪声并改善音质。为验证本计算方法的可靠性,计算的流量和压力特性曲线和实验结果进行了比较,吻合良好。     

Investigation of higher spanwise Helmholtz resonance modes in slender covered cavities

Cavity aeroacoustic noise is relevant for aerospace and automotive industries and widely investigated since the 1950s. Most investigations so far consider cavities where opening length and width are of similar scale. The present investigation focuses on a less investigated setup, namely cavities that resemble the door gaps of automobiles. These cavities are both slender (width much greater than length or depth) and partially covered. Furthermore they are under influence of a low Mach number flow with a relatively thick boundary layer. Under certain conditions, these gaps can produce tonal noise. The present investigation attempts to reveal the aeroacoustic mechanism of this tonal noise for higher resonance modes. Experiments have been conducted on a simplified geometry, where unsteady internal pressures have been measured at different spanwise locations. With increasing velocity, several resonance modes occur. In order to obtain higher mode shapes, the cavity acoustic response is simulated and compared with experiment. Using the frequency-filtered simulation pressure field, the higher modes shapes are retrieved. The mode shapes can be interpreted as the slender cavity self-organizing into separate Helmholtz resonators that interact with each other. Based on this, an analytical model is derived that shows good agreement with the simulations and experimental results.     

Prediction of aerodynamic noise reduction by using open-cell metal foam

As the speed of high-speed train (HST) increases continuously, aerodynamic noise has become more remarkable compared with the wheel/rail noise, which affects the inhabited environment along the railway and the riding comfort. This paper preliminarily investigates the feasibility of using open-cell metal foam covering layer to reduce the low Mach number aerodynamic noise generated by the flow around a circular cylinder which is the typical section of pantographs. The aerodynamic noises radiated from the circular cylinder with and without metal foam are calculated. The hybrid method combining two-dimensional large eddy simulation (LES) with Ffowcs Williams–Hawkings (FW–H) equation is employed. The calculated Strouhal number, time-averaged drag coefficient, base pressure and overall sound pressure level agree well with some available experimental data. Then, the influences of metal foam porosity, pore density, thickness of covering layer and the speed of train on the aerodynamic noise and the aerodynamic forces are investigated, and some detailed comparisons of flow field are made. The numerical results indicate that as a passive scheme, the open-cell metal foam with high porosity can modify the flow, adjust the vortex shedding frequency and regularize the wake, leading to a significant reduction of aerodynamic noise. The results are expected to provide useful information for the control of aerodynamic noise using this new material.