Experiments on the unsteady flow field and noise generation in a centrifugal pump impeller

This paper reports on an experimental investigation of large-scale flowfield instabilities in a pump rotor and the process of noise generation by these instabilities. Measurements of the fluctuating components of velocity and surface pressure were made with hot-wire probes and surface mounted pressure transducers on a seven bladed back swept centrifugal water pump impeller operating with air as the working fluid. The impeller was operated without a volute or scroll diffuser, thereby eliminating any sound generation from pressure fluctuations on the volute cutoff. Thus the study focused on flow field and noise components other than the blade passage frequency (and its harmonics). The primary goal of the study was to provide fundamental information on the unsteady flow processes, particularly those associated with the noise generation in the device. It was further anticipated that detailed flow measurements would be useful for the validation of future computational simulations.The measured data at the discharge show a jet-wake type of flow pattern which results in a strong vorticity field. The flow with high velocity found on the pressure side of the impeller tends to move to the low-pressure region present at the suction side of the passage as a form of roll-up around the blade trailing edge. This motion causes an unsteady flow separation at the suction side of the blade and consequently disturbs the flow in the adjacent passage. By interacting with the impeller blades near the trailing edges, this instability flow causes a periodic pressure fluctuation on the blade surface and generates noise by a trailing edge generation mechanism. The spectrum of surface pressure measured at the trailing edge of each blade reveals a cluster of peaks which were identified with azimuthal mode numbers. The correlation between the acoustic farfield pressure and the surface pressure on the impeller blade has proven that the azimuthal modes synchronized with the number of impeller blades generate noise much more efficiently than the other modes. The paper also clarifies the correlation between unsteady flowfield measurements, in both impeller and laboratory co-ordinates, with the radiated noise properties. Thus some light is shed on the noise generation mechanisms of this particular device.     

On combustion noise related to chemical reactions

An inhomogeneous pressure wave equation has been derived for chemically reacting multicomponent gas mixtures to predict the acoustic field associated with combustion. Out of three source terms for the generation of combustion noise—namely, fluctuations in the heat release rate, in the momentum flow rate and in the viscous and diffusive working—the first is examined to relate this acoustic source strength to the concentration and temperature fluctuations through the chemical kinetics of reaction processes. The relationships between the spectral features of combustion noise and the statistical aspects of these fluctuations are obtained. The essential feature that combustion noise has substantial low-frequency components is explained by the dependence of the contribution of reaction fluctuations on the integral of the time correlation with respect to the correlation time.     

Study on fracture of tungsten wire induced by acoustic cavitation at different hydrostatic pressures and driving electric powers

The near-solid wall multi-bubble cavitation is an extremely complex phenomenon, and cavitation has strong erosiveness. The melting point (about 3410 °C) of tungsten is highest among all pure metals, and its hardness is also very high (its yield strength is greater than 1 GPa). What would happen to pure tungsten wire under extreme conditions caused by collapsing cavitation bubbles at high hydrostatic pressure? In this paper, we have studied the fracture process of pure tungsten wire with diameter of 0.2 mm mounted at the focus of a standing acoustic wave produced by a spherical cavity transducer with two open ends placed in a near spherical pressure container, and also studied the macro and micro morphological characteristics of the fracture and the surface damage at different fracture stages of tungsten wire under various hydrostatic pressures and driving electric powers. The results have shown that the fracture time of tungsten wire is inversely proportional to avitation intensity with hydrostatic pressure and driving electric power, the higher the acoustic pressure caused by higher electric power, the shorter the fracture time. The possible fracture mechanisms of tungsten wire in this situation we found mainly contributed to asymmetrically bubbles collapse near the surface of tungsten wire, leading to tearing the surface apart; consequently cracks along the radial and axial directions of a tungsten wire extend simultaneously, classified as trans-granular fracture and inter-granular fracture, respectively. With the increase of cavitation intensity, the cracks tend to extend more radially and the axial crack propagation path becomes shorter, that is, mainly for trans-granular fracture; with the decrease of cavitation intensity, intergranular fracture becomes more obvious. When the hydrostatic pressure was 10 MPa and the driving electric power was 2 kW, the fibers became softener due to the fracture of the tungsten wire. The fracture caused by acoustic cavitation was different from conventional mechanical fracture, such as tensile, shear, fatigue fracture, on macro and micro morphology.     

The impact of flow induced loads on snap-through behavior of acoustically excited, thermally buckled panels

The analysis, and ultimately the design, of air-breathing hypersonic cruise-type vehicles is hampered by the inability to accurately capture the coupled fluid-thermal-structure interactions. There are few laboratory experiments that have investigated the interactions of compliant surface panels and hypersonic flow. The vast majority of experimental studies are limited to replicating only parts of the physical mechanisms and couplings because of the difficulty in imposing the complete transient, hypersonic environment. Studies of hot-structures, thermal protection systems, or exotic material and structural configurations, typically neglect vibration induced fluctuating pressures, shock-boundary layer interaction, the effect of transition, and separated flow. Conversely, hypersonic wind-tunnel experiments purposely neglect the influence of non-rigid structure on the aforementioned high-speed flow effects in an effort to better understand the nature of the high-speed effects. The goal of this study is to implement a simple computational model that seeks to incorporate many of the fluid-thermal-structure interactions inherent in hypersonic flow. This is accomplished using simplified aerothermal and aerodynamic theories in conjunction with a simply supported von Kármán panel in cylindrical bending. Comparisons are made between the inclusion and exclusion of self-induced and forced fluctuating pressures, as well as the fidelity required in computing the temperature distributions. Results indicate that self-induced pressure fluctuations, which arise from interactions of a vibrating structure with an ambient mean flow, significantly impact the response of panels to random acoustic loadings. Additionally, the inclusion of forced fluctuating pressure loadings can significantly reduce the onset time of panel flutter. It is surmised that experimental investigations must combine thermal loads, as well as both forced and self-induced fluctuating pressures.     

金属烧结丝网在内场消声中的应用

利用试验手段研究暂冲式风洞稳定段内安装不同规格烧结金属丝网对风洞上游控制阀后气流噪声和湍流度抑制作用。试验结果表明:多层金属烧结丝网可在全频段内大幅度降低上游气流的噪声,最大可达21 dB;消声量与金属烧结丝网无量纲的压力损失系数成正比,压力降与金属烧结丝网层数呈现出非线性叠加的结果。另外发现烧结金属烧结丝网对气流速度脉动亦具有突出的抑制效果。例如,试验段马赫数Ma=1:5时,120目26层+160目26层组合烧结金属丝网出口气流速压脉动幅值减小为入口来流的18%,湍流度由11.7%降至3%。因此金属烧结丝网适合于暂冲式风洞的内场降噪。     

Calculation of acoustic scattering of a nonrigid surface using physical acoustic method

In this paper the physical acoustic method or the Kirchhoff approxima-tion is extended to treat the scattering of a nonrigid surface in order to estimatethe target strength of targets with absorbing coatings.By using the locally planewave approximation,the relationship between the sound pressure and its normalderivative on the surface can be represented by the plane wave reflectioncoefficient and the acoustic impedance of the surface.The resulting modifiedKirchhoff approximation involves the plane wave reflection coefficient.For aimpedance sphere,a comparison between the physical acoustic method and theexact solution shows that the physical acoustic method still is a good approxima-tion at higher κα values.     

A study of noise reduction by acoustic shock waves

I.IntroductionAcousticshockwavcs(ASW)isanimportantphcnomcnoninnonlinearacoustics.Experimentalrcsultshavcshownthatwhenanaircraftcngincinletopcratesneartheson-iccondition,vcrystrongnoisegcncratedbythcfanscanbcreduccdgreat1yowingtothcformationofASWatthcthroatofthcin1etll].ASWisadiscontinuityofacousticvaria-bles,whichisdifTcrcntfromthcshockwavesoccurringinhighspcedsteadyflowinducts.Theformer'sintensityismuch1cssthanthelattcr's.Furthcrmorc,thepositionandintensityofASWisalwayschangedwithtime.l…     

PSYCHO-CIRCULATORY RESPONSES CAUSED BY LISTENING TO MUSIC, AND EXPOSURE TO FLUCTUATING NOISE OR STEADY NOISE

This study investigated the effect of steady noise, fluctuating noise and music on circulatory function. Pulse-wave and blood pressure were continuously measured in 35 healthy young females who listened to three types of music or were exposed to steady noise or fluctuating noise, synchronized with each type of music with respect to intensity variations. The pulse-wave did not change during any exposure conditions. Regarding blood pressure, several modes were observed. The critical level for a blood pressure change was estimated to be 54 L_Aeqduring exposure to steady noise. The frequency of high-intensity peaks in the mode of sound fluctuation was associated with elevation in blood pressure. The blood pressure change was analyzed by distinguishing the intensity variation in sound fluctuation from other attributes of music. The effects of music on blood pressure were modified not only by the melody and timbre of the music but also by emotional responses during listing.     

Sound radiation from inflow turbulence in axial flow fans

The sound radiated when inflow turbulence is present in axial flow fans has been investigated. Theoretically, two noise radiating mechanisms can be identified: (i) interaction of turbulence with the rotor potential field results in a quadrupole-type volume source distribution, producing “flow-interaction” noise; (ii) impingement of turbulence on the blades results in a dipole-type (fluctuating force) surface source distribution, producing “fluctuating lift” noise. A theoretical expression for the flow interaction sound power in the upstream radiation field has been developed, in terms of parameters that can be experimentally determined by near field flow measurements involving spatial cross-correlations of the fluctuating axial velocity, with respect to both radial and circumferential position. Both these measurements and radiated sound pressure measurements have been made for eight- and ten-bladed rotors of relatively low tip Mach number (< 0·3). The sound pressure measurements revealed the occurrence of band-spreading of discrete tones at the blade passing frequency and its harmonics, as would be theoretically predicted for quadrupole-type sources here. The theoretical predictions and the measurements, respectively, of the sound power radiated upstream were compared. The results indicated that, for the fans tested, the “fluctuating lift” noise strongly predominated over the “flow-interaction” noise. The observed sound power levels were consistent with levels estimated from the theory.     

Slat noise modeling and prediction

This paper presents a model for aircraft slat noise prediction, based on the theory of aerodynamic sound generation and the first principles of source flow physics. Starting from the theory of acoustic analogy, the noise from the high Reynolds number and low Mach number flows in the slat cove region is formulated as a general solution by the method of dimensional analysis, far-field asymptotic expansion and statistical modeling. The solution relates the far-field noise spectrum to the surface pressure statistics, the characteristic length and time scales in the surface pressure statistics and Green's function that accounts for the sound-flow coupling and propagation effects. The general solution is then used to extract scaling laws and correlation models for the individual functional dependences between the far-field noise and various parameters, including the slat noise spectral shape, its Mach number dependence and its far-field directivity. The simple scaling laws and correlation models are validated by test data and serve as building blocks to construct a slat noise prediction model.     

低速开式空腔自激反馈流场结构与流致噪声的风洞试验研究

本文主要针对低速开式空腔流动自激振荡产生噪声问题,在0.55 m×0.4 m航空声学风洞开展了不同低马赫数(0.1/0.15/0.2/0.25)条件下长深比为2的空腔腔内流场结构和噪声特性风洞试验研究。通过利用高频粒子图像测速技术捕捉腔内流场结构,分析了腔内声波传递路径;完成空腔远场噪声和壁面压力测试,分析了噪声自激振荡模态和简正波模态,并对空腔壁面脉动压力和远场噪声进行压/声相关性研究。结果表明:空腔内部除主涡外,在腔口前缘处剪切涡与腔口后缘处碰撞涡明显存在;在875 Hz,1288 Hz,1875 Hz,2050 Hz四个频率附近出现了由声腔共振所致的单频噪声;壁面压力与远场噪声密切相关,在壁面压力主频位置有明显单频噪声出现。      

浅海舰船噪声声强流的垂直方向性

浅海环境中,确定性声源的多途声信号干涉使得接收点处声强流的方向发生改变,不再与声源位置处的声强流方向一致。只测量声场的标量声强时,无法得到接收点处声强流的垂直方向性,而基于简正波矢量场建模和仿真,可获得理想条件下宽带点声源激发声场声强流的垂直方向性。本文采用单矢量水听器进行海上实验,获得了海洋环境噪声和干扰条件下舰船噪声声强流的垂直方向性。仿真和实验结果表明:远场条件下,浅海干涉现象引起接收点处声强流的方向(极角)随频率和距离变化,其时间-频率分布呈现与LOFAR谱干涉条纹相似的条纹,声强流的极角值主要分布在70?～110?范围内。     

ACOUSTIC IDENTIFICATION OF COHERENT STRUCTURES IN A TURBULENT JET

Fluid-dynamic events associated with noise generation in a subsonic jet are educed by conditioning in-flow velocity and pressure signals on farfield sound measurements. The jet is located in an anechoic chamber, and farfield noise measurements are performed simultaneously with in-flow anemometric and acoustic measurements at a number of distances x from the nozzle (0?x/D?20, with D the jet diameter). The experimental data are then analyzed with a conditional averaging procedure using peaks in the acoustic signal as a trigger. An analysis of the method is developed and supported by numerical simulations. The averaging procedure permits the identification of the average time signatures of in-flow velocity and pressure associated with noise-generating coherent structures in the flow, their position at the emitting instants and their temporal statistics. The physical properties of the events associated with the averaged time signatures are then discussed.     

矩形腔体流场模拟及噪声研究

用大涡模拟方法对低速湍流引起的矩形腔体内流动进行了模拟,并应用FW-H声学类比方程分析了由流动诱发的气动噪声.数值模拟观察到了涡结构的脱体及腔体内部的自激振荡过程,通过分析得出了由流动诱发噪声的声压-频率曲线.研究发现在流速30 m/s时,流动噪声声压级在60 dB以下,348.48 Hz及其高次谐波是噪声的主要来源,流场与声场表现出耦合关系,辐射声场具有明显的方向性.腔体噪声的风洞实验研究得到了与数值模拟吻合的结果.     

利用源强密度声辐射模态重建声场

为了利用声场中少量测点声压数据精确重建复杂结构的辐射声场,提出了源强密度声辐射模态分析理论和声场重建公式.在结构表面定义的空间上,利用以源强密度分布函数为参量的结构辐射声功率泛函表达式定义了一个线性自伴正辐射算子,该算子的特征函数为结构的源强密度声辐射模态.然后通过对矩形平板和带有半球帽的圆柱体的源强密度声辐射模态的分析,证明了源强密度声辐射模态具有空间滤波特性,并利用该性质建立了声场重建公式.球体仿真和平板实验验证了所提出的声场重建方法的可行性和稳健性.基于源强密度声辐射模态的声场重建方法简单,利用较少测点数据就可以获得较高的声场重建精度,特别适合于复杂结构的低频声场重建.     

Computation of aeolian tone from a circular cylinder using source models

The generation of aeolian tones from a two-dimensional circular cylinder situated in a uniform cross-flow is investigated. The major emphasis here is placed on identifying the important noise generation mechanisms. Acoustic-viscous splitting techniques are utilized to compute modelled acoustic source terms and their corresponding acoustic fields. The incompressible Reynolds averaged Navier-Stokes equation is used to compute the near-field viscous flow solution, from which modelled acoustic source terms are extracted based on an approximation to the Lighthill’s stress tensor. Acoustic fields are then computed with an acoustic solver to solve the linearized Euler equations forced by the modelled source terms. Computations of the acoustic field based on the approximated Lighthill’s stress tensor are shown to be in good agreement with those computed from the surface dipole sources obtained using Curle’s solution to the acoustic analogy. It is shown in this paper that the stress tensor source term in the streamwise direction makes a comparable, but slightly larger contribution to the overall radiated field, compared with that due to the stress tensor in the direction normal to the mean flow. In addition, it is shown that shear sources, which arise due to the interaction between the fluctuating velocity and the background steady mean velocity, make the greatest contribution to the acoustic field, while the self-noise sources, which represents the interaction between the fluctuating velocities, is shown to be comparably negligible.     

振荡射流提高翼型升力的机理研究

本文数值模拟了施加振荡射流以及相应定常吸气条件下的翼型分离流动。对振荡射流改善翼型升力的机理进行了研究。结果表明,翼型表面施加的振荡射流能够控制流动分离的形态,提高分离区流体的湍流度,增强分离区内部流体,以及与主流的动量和能量交换,增强近壁区流体的动能,降低翼型吸力面压力,而对压力面无显著影响,因而翼型的升力得到提高。     

A numerical study of transient flow around a cylinder and aerodynamic sound radiation

The fully 3D turbulent incompressible flow around a cylinder and in its wake at a Reynolds number Re = = 9×10⁴ based on the cylinder diameter and Mach number M = 0.1 is calculated using Large Eddy Simulations (LES). Encouraging results are found in comparison to experimental data for the fluctuating lift and drag forces. The acoustic pressure in far-field is commutated through the surface integral formulation of the Ffowcs Williams and Hawkings (FWH) equation in acoustic analogy. Five different sound sources, the cylinder wall and four permeable surfaces in the flow fields, are employed. The spectra of the sound pressure are generally in quantitative agreement with the measured one though the acoustic sources are pseudo-sound regarding the incompressible flow simulation. The acoustic component at the Strouhal number related to vortex shedding has been predicted accurately. For the broad band sound, the permeable surfaces in the near wake region give qualitative enough accuracy level of predictions, while the cylinder wall surface shows a noticeable under-prediction. The sound radiation of the volumetric sources based on Lighthill tensors at vortex shedding is also studied. Its far-field directivity is of lateral quadrupoles with the weak radiations in the flow and cross-flow directions.     

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.     

开孔结构流致噪声的数值模拟和机理分析

飞机机体表面的开孔设计会形成空腔结构,产生空腔流致噪声。空腔噪声的控制需要彻底认识其流动和噪声机理。以飞机的功能性开孔为例,通过半经验公式分析了其空腔噪声频率随速度的变化规律,预测了出现流声共振的工况。空腔发生流声共振时,特定频率的纯音噪声会被放大。为此,采用脱体涡模拟方法开展了开孔结构流声共振的三维非定常数值计算,分析了其流场和声场特性。其中,数值方法的准确性通过圆形空腔标模计算进行验证。结果表明,在一定速度下剪切层内的扰动将诱发空腔深度方向声模态,出现流声共振现象。此时,剪切层表现为强烈的周期性上下拍动,空腔底部和后缘区域的局部压力脉动幅值较大,声波主要由空腔后缘向上游方向辐射,上游噪声大于下游。