Near field acoustic solution of a rotating point source in frequency domain

I.IntroductionAerodyntalcnoiseofturbomaChinrynotoalythectstheenvironmentbutalsosometimesresultsinstructuredamage.Aeroacoustics,asaintewhsciPlineofacousticsandaerodynamics,isattractingmorandmoreatteDtionandresearch.TherearetwoimportalltcontentsonthenoiseproducingmechAnsmsinaeroacousticproblem.OneisaboutthesourcesofnoiseinturbomachinryGenetallytherearethreetyPesofsources:thickness,forceandturbulencestress,correspondingrespectivelyTomonopole,dipoleandquadruplesourcesThelatertwokindsofsourcesh…     

Sound radiation from point sources in circular motion

The moving-source approach used by Morfey and Tanna for broad-band sound radiation from a point force in circular motion is adopted in this paper to evaluate the sound radiated in the far field due to point sources of random time variation rotating uniformly in a circle at subsonic speed. It is shown that the sound pressure results (overall and spectral density) for the volume velocity source can be easily extracted from the corresponding results for a rotating point force. An expression for the sound field of a point volume displacement source in arbitrary motion is derived and this is applied to the special case of uniform circular motion. The effects of acceleration of the sources due to circular motion on the radiated sound are established. Applications include noise from tip jet rotors, compressors and marine propellers.     

Spectrum of the sound produced by a jet impinging on the gas-water interface of a supercavity

An analysis is made of the sound generated by the impingement of an air jet on the gas-water interface of a supercavity. The water is in uniform low Mach number motion over the interface. The interface is rippled by the jet, which produces an unsteady surface force on the water that behaves as a dipole or monopole acoustic source, respectively, at high and low frequencies. In a first approximation the very large difference in the gas density and that of water implies that the surface force is similar to that occurring when a jet impinges on a rigid wall. Data from recent measurements by Foley (2009, Ph.D. Dissertation, Department of Mechanical Engineering, Boston University) of the frequency spectrum of the surface force produced by the impact of a turbulent jet on a wall are used to formulate an analytical representation of the spectrum and thence to predict the sound produced in water when the same jet impinges on the cavity interface. The prediction is used to estimate the characteristics of gas jet impingement noise for an experimental supercavitating vehicle in use at the Applied Research Laboratory of Penn State University.     

Direct computation and aeroacoustic modelling of a subsonic axisymmetric jet

A numerical algorithm for acoustic noise predictions based on solving Lilley's third order wave equation in the time-space domain is developed for a subsonic axisymmetric jet. The sound field is simulated simultaneously with the source field calculation, which is based on a direct solution of the compressible Navier-Stokes equations. The computational domain includes both the nearfield and a portion of the acoustic farfield. In the simulation, the detailed sound source structure is provided by the nearfield direct numerical simulation (DNS), while the sound field is obtained from both the DNS and the numerical solution to the non-linear Lilley's equation. The source terms of Lilley's equation are used to identify the apparent sound source locations in the idealized axisymmetric low-Reynolds number jet. The sound field is mainly discussed in terms of instantaneous pressure fluctuations, frequency spectra, acoustic intensity and directivity. A good agreement is found between the predictions from the axisymmetric Lilley's equation and the DNS results for the sound field. Limitations and perspectives of the simulation are also discussed.     

Low frequency sound produced by a ventilated supercavitating vehicle

An analytical investigation is made of the low frequency noise produced by gas jet impingement on the gas-water interface of a ventilated supercavity. Enclosure within a supercavity enables an underwater vehicle to attain high-speed forward motion. Whereas high frequency components of the cavity self-noise can interfere with the vehicle guidance system, low frequency sound tends to radiate in the water to large distances from the cavity. A canonical mathematical problem is examined that extends a previous study by Foley et al. (Journal of Sound and Vibration329 (2010) 415-424.) of sound generation by a specially modified, model scale supercavitating vehicle involving gas jet impingement at normal incidence to the interface. Our analysis determines the influence on low frequency sound production of cavity aspect ratio and the manner in which the efficiency of sound production increases with decreasing distance of the jet impact region from the circular ‘cavitator’ at the vehicle nose, where the supercavity is formed.     

旋转运动点声源近场声学频域解

本文在任意运动点源声辐射频域解基础上，得到旋转运动声源近场声学的一个近似解。如果旋转半径相对于场声波是紧致的，此解就可以表示为在远场解基础上叠加三项近场修正项。根据此解，本文得到适合于近场的自由空间Ｇｒｅｅｎ函数，由此讨论了声源旋转导致的声场方向性特征，研究了声源位置及源频率以及旋转频率等对近场声学结构的影响。     

Source characterization of a subsonic jet by using near-field acoustical holography

In the present study, patch near-field acoustical holography was used in conjunction with a multireference, cross-spectral sound pressure measurement to visualize the sound field emitted by a subsonic jet and to predict its farfield radiation pattern. A strategy for microphone array design is described that accounts for the low spatial coherence of aeroacoustic sources and for microphone self-noise resulting from entrained flow near the jet. In the experiments, a 0.8-cm-diameter burner was used to produce a subsonic, turbulent jet with a Mach number of 0.26. Six fixed, linear arrays holding eight reference microphones apiece were disposed circumferentially around the jet, and a circular array holding sixteen, equally spaced field microphones was traversed along the jet axis to measure the sound field on a 30-cm-diameter cylindrical surface enclosing the jet. The results revealed that the jet could be modeled as a combination of eleven uncorrelated dipole-, quadrupole-, and octupole-like sources, and the contribution of each source type to the total radiated sound power could be identified. Both the total sound field reconstructed in a three-dimensional space and the farfield radiation directivity obtained by using the latter model were successfully validated by comparisons to directly measured results.     

The distribution of headways in traffic noise simulation

In recent years researchers in jet turbulence and jet noise have become increasingly interested in what is termed “large scale coherent jet structures”. There is now considerable evidence that azimuthally coherent structures can be generated by acoustically forcing a jet from upstream. However, the evidence for such structures in unforced jets, except close to the nozzle at low Reynolds numbers, is, at best, circumstantial. The role of such structures in subsonic jet noise production is also completely unproven. In an attempt to establish a link between azimuthally coherent structures and the jet noise field a number of experimenters have recently made azimuthal cross-correlation measurements of either the near field pressure or far field noise, and used the observed coherence to infer the existence of an azimuthally coherent source field. The term azimuthally coherent is used here to infer that the source region is dominated by low order azimuthal components, with relatively little contribution coming from the higher azimuthal components. The purpose of this paper is to question the interpretation of that data. Specifically the sound field generated by a simple ring source with various types of azimuthal coherence is considered theoretically. It is shown that the azimuthal coherence of both the near and far field pressures is principally a function of the Helmholtz number and in many cases of practical interest is relatively insensitive to any coherent structure of the source.     

On the interaction of a sound pulse with the shear layer of an axisymmetric jet

The behavior of a sound pulse from a simulated source in a jet is investigated both experimentally and numerically. Both approaches show that in the low and medium frequencies the far field acoustic power exhibits a marked amplification as the flow velocity increases. Experimentally this changes to an attenuation at the higher frequencies which cannot be computed by the numerical model. This amplification is traced to shear noise terms which trigger the instability waves that are inherent within the flow.     

双声源位置和强度识别的三维声强方法

目前在远场识别声源空间位置和强度缺乏行之有效的方法。针对此问题,提出采用四传声器进行三维声强测量,从而构建出声强、声源坐标和声功率的非车线性方程组,求解方程得出声源空间坐标和强度的方法。以3个三维声强探头对两个同频率单极子声源的识别为例,分别利用数值仿真和半消声室内的实验进行方法验证,并对声源的识别空间分辨率做了测试,得出角度识别最大误差为3.83°,为真实值的8.5%,距离识别最大误差0.1 m,为真实距离的10%。结果表明采用该方法空间坐标和声功率识别均具有很高的准确度,双声源的空间位置分辨力也优于远场声全息方法。      

Roles of initial condition and vortex pairing in jet noise

This is a study of the effect of initial condition on sound generated by vortex pairing in a low Mach number, cold air jet (0·15 ⩽ M ⩽ 0·35). Data has been taken, both flow velocity fields and sound pressure far fields, in a quality anechoic facility, with careful documentation of the effect of initial condition on the sound field of jets of two different geometries (i.e., circular and elliptic). Explanations are presented for most of the observed effects by applying Möhring's theory of vortex sound to vortex filament models of coherent structures in the jets. The explanations also draw upon experience with coherent structure dynamics. The sound source of interest here is that associated with the pairing of shear layer vortices. The evolution of these vortices is greatly affected by the initial condition as is their resultant sound field. The elliptic jets with laminar boundary layers show azimuthal directivity, namely, sound pressure levels in the minor axis plane were greater than in the major axis plane. This difference decreases as the nozzle boundary layer undergoes natural transition with increasing jet speed. When the nozzle boundary layer is tripped, making it fully turbulent and removing the shear layer mode of pairing, the elliptic jet sound fields become nearly axisymmetric. What appears to be the most acoustically active phase of vortex pairing has been modeled, and the resulting sound field calculated for the circular jet. Supporting evidence is found in the experimental data for the validity of this model. The model explains the connection between the initial condition and the far field sound of jets. Interestingly, a general result of Möhring's theory is that motions of vortex rings (of any arbitrary shape) can produce only axisymmetric sound fields if the rings remain in a plane. This implies that the observed asymmetric directivity of the laminar elliptic jet sound field must be due to non-planar ring motions of the vortical structures. The primary contribution of this paper is to examine quantitatively the role of vortex pairing in the production of jet noise; the results are used to reemphasize that “pairing noise” cannot be dominant in most practical jet sound fields, contrary to claims by other researchers.     

基于联合波叠加法的浅海信道下圆柱壳声辐射研究

针对浅海信道下弹性结构声辐射预报尚无高效可靠的研究方法,提出了一种浅海信道下弹性结构声辐射快速预报的联合波叠加法.该方法结合了浅海信道传输函数、多物理场耦合数值计算法和波叠加法理论,运用该方法可对浅海信道下弹性结构辐射声场进行快速预报.经数值法和解析解法验证后,从信道下辐射源、环境影响和辐射声场测量的角度研究分析了浅海信道下弹性圆柱壳的声辐射特性,阐释了进行浅海信道下结构声辐射研究的必要性.研究结果表明,仅在低频浅海信道下弹性结构可近似等效为点源,信道上下边界对声场产生显著的耦合影响,高频段的空间声场指向性分布尤为明显,垂直线列阵进行信道下结构辐射声功率测量时,测量结果受到信道环境边界和潜深的影响较大.     

Propagation and radiation of sound in a finite length duct

An analysis has been carried out to investigate sound transmission in an unflanged circular duct of finite length. A new formulation for calculating the generalized radiation impedance of the opening of a finite length duct with a spinning source inside is presented. The effect of interference between the duct apertures at its two ends is identified in the calculations of radiation impedance, reflection coefficient, and the far field radiation pattern. The results show that the degree of interference between the two ends reduces for high frequency waves and/or for long ducts. Further, the interference effects on the far field for spinning sound are more serious than those for non-spinning sound.     

Vertical distribution of traffic noise—A preliminary study

This paper examines the effect of flight on the sound radiated by a high frequency source embedded in a constant area jet pipe in the presence of flow. Ray acoustics theory and classical results for sound transmission at an interface of relative motion are used. The diffraction of sound at the nozzle lips, the inhomogeneity and irregularity of the interface and the possibility of instability waves being triggered by the incident sound are neglected. Some of the waves characterized by wave-fronts pointing upstream are shown to be convected downstream by the flow and to illuminate the forward arc after refraction at the jet interface. The amount of energy emitted by the source, which is trapped inside the flow, depends only on internal jet pipe conditions. However, the portion of the forward arc which is illuminated by this energy, is a function of flight speed. The radiation into the ambient atmosphere at rest of a basically omnidirectional source peaks at the edge of the downstream zone of silence and falls off rapidly when the observation angle is increased. The flight to static comparison reveals an interesting forward arc amplification due to flight but this occurs in a range of angles where the radiation is basically rather feeble.     

Spatial cross-correlation of acoustic pressures in steady and decaying reverberant sound fields

From the equation for the steady state sound pressure distribution produced in a rectangular reverberation chamber by a point source, and by using the usual high frequency approximations, it is shown that, for a random source position, the cross-correlation function for two points not too far apart approaches that of Cook et al. in the reverberant field of the chamber. When the same approach is used on the equation for sound pressure decay when the point source excitation is cut off, the cross-correlation function obtained for the initial portion of the decay corresponds with that determined experimentally by Balachandran and Robinson.     

高精度球面阵聚焦声源定位方法研究

提出了一种高精度高空间分辨率球面阵聚焦声源定位方法——虚拟源法。该方法通过球面阵波束扫描获得实际声源的空间聚焦谱,并假定各扫描点为虚拟声源,将实际声源聚焦谱看作是全体虚拟源共同作用的结果,由此得到各虚拟源对声场的贡献量,从而可实现声源精确定位。仿真研究分析了频率,阵列孔径,声场模态阶数,信噪比等参数对声源定位性能的影响,并与常规算法进行对比。结果显示,该方法不受频率和阵列孔径的限制,避免了空间“混淆”,能够进行高精度高分辨率声源定位,并具有良好的背景噪声抑制能力。      

Suppression effect of jet flow on pulsating pressure of cavity using scale-adaptive simulation model

The suppression of the aerodynamic noise in the cavity has a great significance to solve relevant puzzles of weapon bays.Acoustic field of the standard cavity model is simulated by using the computational fluid dynamics technology based on scale-adaptive simulation(SAS)model.The results obtained by the proposed method in this paper show reasonable agreement with experiments.On the basis of this,effect of different jet flow rates on the time-averaged variables,turbulent kinetic energy,root mean square(RMS)of sound pressure,sound sources distribution and the pulsating pressure distribution in the cavity is studied.The analysis shows that the jet flow has great influence on the cavity flow field and the distribution of pulsating pressure RMS by changing the morphology of the shear layer.The most obvious of these measures is spout4 configuration,the influence mainly in the form of reducing the pulsating pressure of the whole cavity and changing the sound pressure level in the far field.The results show that different jet flow rates have different control effects on pulsating pressure in the cavity and sound pressure level in the far field.Furthermore,the jet flow rates and the suppression effect on the pulsating pressure have no linear relation.     

Computation of the sound field of a jet from characteristic turbulence parameters measured by crossed beam techniques

The sound field radiating from a jet is strongly dependent upon the turbulence in the jet. To describe the sound sources in a hot jet, a method has been developed, based upon the measurement of infra-red radiation of the jet, and a hybrid processing of the measured signal, which enables the computation of characteristic properties of the turbulence at various points inside the jet (convection speed, integral length scale, life time and intensity of turbulence).From these quantities the acoustic far field can be mapped, as a function of the polar distribution of the spectrum, the turbulence intensity and the total sound pressure field.     

The sound field of a rotating monopole in a plug flow

A theoretical study is performed on the sound field generated by a rotating point monopole in a jet flow, the mixing layer of which is simulated by a velocity discontinuity. Its sound in the far field is compared to the sound field generated by a rotating monopole in a uniform flow in the absence of a velocity discontinuity, which makes it possible to estimate the size of the sound refraction effect.