基于积分方法的二维平行剪切层声远场预测

本文采用二维Ffowcs Williams＆Hawkings(FW-H)方程对平行剪切层远声场辐射特性进行了研究。近流场时间精确数据通过计算气动声学(Computational Aeroacoustics,CAA)技术数值模拟获得,声远场信息则通过FW-H方程对近流场内的可穿透积分面进行积分获得。该方法首先采用具有解析解的涡/尾缘干涉问题进行了校核,进一步采用CAA/FW-H匹配技术对二维平行剪切层声辐射问题进行了预测,计算结果表明,积分解与计算域内的CAA数值解吻合较好。     

基于伴随格林函数法射流噪声的研究

本文采用声类比法探讨了射流中小尺度湍流远场噪声特性.此声类比理论基于线化欧拉方程及声源互易原理,建立了关联声源和远场声压的伴随格林函数,适于声波折射现象及小尺度湍流噪声计算.声场的边界采用完全匹配层边界条件,有效地避免了声波产生数值反射.为减小频散及耗散误差,时空导数均采用高阶预测-校正格式离散控制方程.计算结果与前人的实验相一致.     

基于非结构网格CE/SE算法圆柱绕流气动声场模拟

本文采用时空守恒(CE/SE)格式求解Navier-Stoks方程与Euler方程,数值模拟了绕圆柱流动引发的气动噪声源及传播问题.对近声场与远声场采用了不同的控制方程和边界条件,分别得到了流场与声场解.将其与采用大涡模拟和FW-H方程得到的结果进行了对比,表明本文的数值方法能够很好地反映流场与声场的形态,能较好地模拟声场指向性及流场中的拟声现象,相比LES/FW-H方法能更精确地反映流场与声场的相互关系.     

三维串列双圆柱绕流气动流场及声场模拟

数值模拟三维串列双圆柱在不同间距比下湍流流态及其辐射声场.采用大涡模拟(LES)求解非定常不可压缩Navier-Stokes方程得到瞬时流场数据,从而得到声源相关数据,求解基于FW-H积分方程的Farassat-1A方程计算载荷噪声得到相应声场分布.通过对不同间距比下相应的声场及观测点的声压频谱图进行比较可以发现:随着间距的变化,流场呈现出3种不同的流态,其声场也呈现不同的特点,在临界间距比下,总噪声值最大.     

亚音主旋翼的噪声预测和声隐身分析

本文介绍了基于FW-H方程的亚音主旋翼噪声预测方法,并在螺桨噪声预测程序基础上发展了主旋翼噪声预测程序。本文采用商用CFD软件FINE／TURBO模拟直升机旋翼流场,为噪声预测程序提供所需要的桨叶表面载荷,并用算例验证了载荷数据的准确性和噪声预测程序的有效性。本文计算和讨论了亚音悬停条件下的辐射噪声,并重点分析了不同形状桨叶对辐射噪声的影响,结果表明采用合理的薄翼型叶尖、尖削叶尖及线性扭转桨叶都可以降低辐射噪声,为通过改变桨叶形状降低旋翼辐射噪声提供了合理途径。     

空气中声源的水下匹配场定位

通过水下布放的垂直线列阵采集空中声源在水下激发的测量声场,采用声场波数积分模型(OASES模型)对空中声源激发的水下声场建模,计算出拷贝声场,将二者进行匹配处理从而对空中声源目标定位。首先通过数值仿真验证了匹配场处理技术对空中声源的测距能力,并通过引入宽带匹配场处理器平滑掉距离上的周期性旁瓣。最后分析南海某海域的空气声试验数据,采用常规匹配场方法对700 m以内的32组空中声源目标进行定位,测距结果与GPS计算的收发间实际距离相比,大多数情况下是一致的,在较远距离由于信噪比降低,测量结果容易出现偏差。     

采用球面波叠加法还原自由声场的方法

为解决非自由声场中近场声全息重建时,干扰声在目标声源表面产生的散射影响,提出一种基于球面波叠加法的自由场还原技术。该技术首先采用基于球面波叠加法的声场分离技术获得向内和向外传播的声场,然后以目标声源的表面导纳作为边界条件,实现目标声源辐射声和散射声的分离,从而获得等效于自由声场的测量条件。该技术为准确实现非自由声场中的噪声源识别创造了条件。文中首先详细描述了该技术的基本原理,并提出一种最优球面波展开项数选取方法,最后通过数值仿真说明了该技术的有效性。结果表明:在频率较低时,散射声影响较小,采用声场分离技术和自由场还原技术效果相当;但随着频率升高,散射声影响逐步增强,必须采用自由场还原技术才能准确获得目标声源辐射声。      

空气中声源的水下匹配场定位

通过水下布放的垂直线列阵采集空中声源在水下激发的测量声场,采用声场波数积分模型(OASES模型)对空中声源激发的水下声场建模,计算出拷贝声场,将二者进行匹配处理从而对空中声源目标定位。首先通过数值仿真验证了匹配场处理技术对空中声源的测距能力,并通过引入宽带匹配场处理器平滑掉距离上的周期性旁瓣。最后分析南海某海域的空气声试验数据,采用常规匹配场方法对700 m以内的32组空中声源目标进行定位,测距结果与GPS计算的收发间实际距离相比,大多数情况下是一致的,在较远距离由于信噪比降低,测量结果容易出现偏差。      

轴对称受限射流冲击尖端障碍物流声特性

本文研究了障碍物位于流场不同位置时轴对称射流冲击尖劈障碍物流动和声学特性。应用大涡模型(LES)数值模拟了受限射流流动特性和FW-H方程数值积分求解了远声场噪声频谱和声压级特性,并与自由射流模拟结果进行对比分析。模拟结果表明当障碍物位于自由射流流场转折界面处,自由射流涡环配对失败,使得自由射流主要噪声源被破坏,受限射流声源为障碍物产生的偶极子声源;当障碍物位于自由射流流场充分发展区,自由射流主要声源涡环配对完成,声源为自由射流段四极子声源和障碍物产生的偶极子声源,且随着离喷嘴距离增加,障碍物处流体流动速度减小,远声场相同位置声压级值逐渐减小。     

管道后传声数值模拟的不稳定波抑制

在管道后传声的数值模拟中,必须考虑平均流剪切层的散射效应,然而在非均匀剪切流动下时域求解线化欧拉方程会面临Kelvin-Helmholtz不稳定波产生和放大的难题。已有的不稳定波抑制技术通常很难获得令人满意的结果。本文采用一种混合方法,首先引入有限时段的宽频声源波包将声波和不稳定波分离,进而采用声源滤波器技术对不稳定波进行抑制。数值验证算例选择半无限长轴对称环形硬壁直管道,采用计算气动声学方法时域求解2.5维线化欧拉方程,无背景流动的数值解与解析解符合很好,验证了程序的精度与可靠性,非均匀流动算例则表明所采用波包加声源滤波器混合方法对不稳定波抑制效果明显,对声场影响很小,充分显示了该方法的精度与可行性。     

空腔流动的大涡模拟及气动噪声控制

大涡模拟(LES)和三维的Ffowcs Williams-Hawkings声学比拟方法相结合,研究空腔过流的一种噪声控制措施.空腔的底板/后墙使用多孔壁板,因此流体可以穿透空腔壁面,多孔效果使用Darcy压力-速度关系模拟.声源流场由LES计算,声辐射和远声场由声学比拟获得.结果表明,这种措施有效地减弱了空腔内的压力脉动和远场声辐射,低频脉动Rossiter模数对应的波动幅值被有效抑制,声源中偶极子占优项大幅度减小,从而抑制了声辐射.     

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.     

Scattering of a spherical wave from the coupling of two half-planes

Summary  The diffraction of a spherical acoustic wave from the juncture of pressure release (soft) and locally reacting (absorbing) half-planes in a fluid moving at subsonic velocity is examined. This consideration is important because the point sources are regarded as better substitutes for real sources than line sources/plane waves. The integral representation of the field is obtained using integral transforms and the Wiener-Hopf technique. The factorization of the kernel function in the Wiener-Hopf functional equation is accomplished. The analytic solution of the integrals is obtained by employing asymptotic methods and the far field is presented. The effect of the Mach number is shown explicitly on the diffracted field.     

CAA broadband noise prediction for aeroacoustic design

The current status of a computational aeroacoustics (CAA) approach to simulate broadband noise is reviewed. The method rests on the use of steady Reynolds averaged Navier-Stokes (RANS) simulation to describe the time-averaged motion of turbulent flow. By means of synthetic turbulence the steady one-point statistics (e.g. turbulence kinetic energy) and turbulent length- and time-scales of RANS are translated into fluctuations having statistics that very accurately reproduce the initial RANS target-setting. The synthetic fluctuations are used to prescribe sound sources which drive linear perturbation equations. The whole approach represents a methodology to solve statistical noise theory with state-of-the-art CAA tools in the time-domain. A brief overview of the synthetic turbulence model and its numerical discretization in terms of the random particle-mesh (RPM) and fast random particle-mesh (FRPM) method is given. Results are presented for trailing-edge noise, slat noise, and jet noise. Some problems related to the formulation of vortex sound sources are discussed.     

A moving medium formulation for prediction of propeller noise at incidence

This paper presents a time domain formulation for the sound field radiated by moving bodies in a uniform steady flow with arbitrary orientation. The aim is to provide a formulation for prediction of noise from body so that effects of crossflow on a propeller can be modeled in the time domain. An established theory of noise generation by a moving source is combined with the moving medium Green's function for derivation of the formulation. A formula with Doppler factor is developed because it is more easily interpreted and is more helpful in examining the physic of systems. Based on the technique presented, the source of asymmetry of the sound field can be explained in terms of physics of a moving source. It is shown that the derived formulation can be interpreted as an extension of formulation 1 and 1A of Farassat based on the Ffowcs Williams and Hawkings (FW–H) equation for moving medium problems. Computational results for a stationary monopole and dipole point source in moving medium, a rotating point force in crossflow, a model of helicopter blade at incidence and a propeller case with subsonic tips at incidence verify the formulation.     

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.     

A hybrid approach to reconstruct transient sound field based on the free-field time reversal method and interpolated time-domain equivalent source method

A hybrid approach is presented in the current work, which reconstructs the transient sound field radiated from the two-dimensional sources with unknown locations and sizes, by combining the free-field time reversal method and the interpolated time-domain equivalent source method (TDESM). In the first step of the proposed method, the time reversal focusing algorithm is performed to estimate the source locations on the source plane. And then, the interpolated TDESM is applied to reconstruct the transient sound field on the reconstruction plane by assuming that the equivalent sources are located near the estimated source locations found in the previous step. The proposed technique, in principle, requires fewer microphones in the measurement since the equivalent sources are only placed in the vicinity of the ‘real’ sound sources. Reconstruction of the transient sound field radiated from the dual-planar-piston model is studied by numerical simulation for feasibility demonstration. A measurement of the sound fields radiated from two baffled loudspeakers is performed in the anechoic chamber, which shows that a better reconstruction result can be achieved by using the proposed hybrid scheme than the original interpolated TDESM with relatively the same number of sampling channels.     

基于时域多普勒修正的运动声全息识别方法

由于运动声源测量信号中多普勒效应的存在,一般声全息方法无法直接使用,而阵列信号波束成形处理方法无法进行定量分析.本方法建立了基于测量面、辐射面和全息面的运动学几何关系,提出了声源与测量信号之间的非线性时间映射方法,基于运动声源的声源特征函数,构造了消除多普勒效应的全息面时域声压分布.全息重建得到运动声源表面有效声压分布,实现了对主要声源处声压幅值的定量估计.实际运动声源的测量实验结果证明了该方法的有效性.     

A wave field synthesis approach to reproduction of spatially correlated sound fields

This article discusses an open-loop wave field synthesis (WFS) approach for the reproduction of spatially correlated sound fields. The main application concerns laboratory reproduction of turbulent boundary layer wall pressure on aircraft fuselages and measurement of their sound transmission loss. The problem configuration involves reconstruction of random sound pressure distributions on a planar reproduction surface using a planar array of reproduction monopoles parallel to the reproduction plane. In this paper, the WFS formulation is extended to sound fields with imposed time and spatial correlation properties (or equivalently imposed cross-spectral density in the frequency and wave number domains). Numerical examples are presented for the reproduction of a propagating plane wave, diffuse acoustic field and wall pressure in subsonic or supersonic turbulent boundary layers. The reproduction accuracy is examined in terms of the size of the source plane and reproduction plane, their separation, and the number of reproduction sources required per acoustic wavelength. While the reproduction approach cannot reconstruct sub-wavelength correlation scales of subsonic turbulent boundary layers, it effectively reconstructs correlation scales larger than the acoustic wavelength, making it appropriate for diffuse acoustic field and supersonic turbulent layers.