流管实验装置中声传播计算的模态方法

流管实验装置是测量有流动情况下航空发动机消声短舱内声衬声阻抗的主要装置。本文发展了一种解析的模态匹配方法进行在平均流有声衬条件下矩形流管中声传播的计算。用同伦方法求解特征值问题,并与用环绕积分求解的结果进行比较。声场通过轴向阻抗间断面的声压和声质点速度积分相等计算。第一个算例是无流动、硬壁、有限长、考虑端口反射的情况,并与北航流管实验台测量数据进行了对比;第二个算例为有流动情况下有限长声衬管道不考虑端口反射的声场计算,它与文献中NASA流管实验结果和CAA计算结果符合得很好。     

含旋流圆环形管道声传播的有限元计算

本文发展了计算包含旋流的圆环形管道声传播问题的有限元数值模型,并且对等熵和均熵流动做了讨论.经典的声学有限元方法大多求解只包含一个变量的对流波动方程,但是对于包含旋流等非均匀流的圆环形管道声传播问题,不能简化成只含有一个变量的对流波动方程.本文尝试用直接求解耦合方程组的有限元方法方法研究管道声传播问题.文章分别研究了刚性壁面、不同阻抗的声衬壁面对管道声传播的影响,同时还对不同阶的声模态包括传播模态和截止模态在管道中的传播规律做了分析.计算的结果和正模态解析方法符合得很好,从而验证了有限元模型的正确性和可行性.     

等价分布源方法新发展

本文以广义Lighthill方程和广义Green函数为基础给出任意截面管道声传播计算模型,同时应用等价分布源方法建立了包含声衬影响的声传播模型,该方法避开了复杂的管道特征值计算,并在此基础上以积分变换发展了等价分布源方法,免去了声源及观察点匹配时产生的奇异性,使该方法大大简化,易于工程实际计算.此外,本文还给出了特殊截面管的算例.     

丁羟衬层固化状态的激光超声实时监测方法研究

衬层是连接固体火箭发动机壳体和推进剂的重要组成部分,其粘接状态决定着推进剂-衬层-壳体粘接界面的完整性,进而影响固体发动机的安全可靠性。针对衬层粘接结构的混合态和非连续阻抗特性,研究了激光超声在衬层结构中的传播规律,搭建了实验装置;以烧蚀机制激发超声波为手段,通过提取超声波在衬层中的传播时间和相对声衰减量,采用超声波透射法对衬层的固化过程进行状态表征,提出了用于求解衬层中纵波渡越时间的标定时间差值法,建立了非连续阻抗的相对声衰减模型。实验结果表明,采用标定时间差值法获得的渡越时间,以及采用相对声衰减模型计算得到的声衰减量能够很好地表征衬层的固化过程。     

发动机圆形管道内噪声衰减谱模态分析

本文应用模态分析方法建立了剪切流存在条件下,发动机多段声衬圆形管道声传播工程计算模型,对管内各模态频谱和总噪声衰减频谱进行了算例计算,并与有关文献试验数据进行了对比。结果表明,多段声衬圆形管道中声传播工程计算方法是可行的,从而为发动机前短舱管内声传播研究提供了一种模态分析工程预测方法。     

有限长管道声衬的参数优化设计研究

本文应用已经过验证的有限长管道声学模型进行声衬参数的优化设计,研究了固定工况下,优化半径、声衬长度和声衬位置对阻抗优化参数的影响,其中声衬长度和声衬位置对阻抗优化参数的影响和应用无限长管道声学模型进行优化设计时的影响明显不同.本文讨论得到的结论对声衬参数优化设计有重要的指导价值.     

耗散量子隧道系统中的局域—退局域转变

通过考虑声子基态变迭积分对隧道参量的重整化效应,本文提出一种新的方法来研究玻色型耗散量子隧道系统中的局域-退局域转变。研究表明,已有的理论结果主要建立在位移声子态近似的基础上,而本文的方法可以用来研究其它声子基态(如位移压缩声子态)对局域-退局域转变的影响。 关键词：     

积分方程法与求解谐振频率的声散射

表面Ｈｅｌｍｈｏｌｔｚ积分公式可以有效地求解物体的辐射声场和散射声场，但并不是在任何频率下都能得到满意的结果。当波数等于或接近目标内部问题的特征值时，将产生非唯一解，严重影响求解的准确性。本文根据混合Ｈｅｌｍｈｏｌｔｚ积分公式，用最小平方正交法有效地求解谐振频率的声散射。文中以圆柱和椭圆柱的声散射为例进行了计算，并讨论了目标内附加计算点的选取问题。     

开口/封闭薄壳体声辐射和散射的统一边界积分方程解法

推导了在二维和三维空间下开口和封闭薄壳体在任意阻抗边界条件下声辐射和散射的统一边界积分方程.相对于以前的求解方法,该方程求解声辐射和散射问题具有相同的影响矩阵,能够同时求解薄壳体气动和振动噪声的辐射和散射现象,以及分析壳体声阻抗对声波传播的影响.推导的方程可以应用于叶轮机械、管道等噪声和消声器消声性能的预测等方面.在此方程基础上,可以进一步考虑运动边界和运动介质对声辐射和散射的影响. 关键词： 薄壳体 声阻抗 积分方程 边界元方法     

非均匀无序系统的声子局域化

本文根据电、声子类比关系,由重整化群方法在电子局域化问题上的应用结果,给出了非均匀无序系统的声子局域化信息。发现非均匀无序系统的声子频谱上可存在三条迁移率边。 关键词：     

Broadband noise reduction by circular multi-cavity mufflers operating in multimodal propagation conditions

In this study, sound propagation through a circular duct with non-locally lining is investigated both numerically and experimentally. The liner concept is based on perforated screens backed by air cavities. Dimensions of the cavity are chosen to be of the order or bigger than the wavelength so acoustic waves within the liner can propagate parallel to the duct surface. This gives rise to complex scattering mechanisms among duct modes which renders the muffler more effective over a broader frequency range. This work emanates from the Cleansky European HEXENOR project which aim is to identify the best multi-cavity muffler configuration for reduction of exhaust noise from helicopter turboshaft engines. Here, design parameters are the cavity dimensions in both longitudinal and azimuthal directions. The best cavity configuration must in addition fit weight specifications which implies that the number of walls separating each cavity should be chosen as small as possible. To achieve these objectives, the scattering matrix of the lined duct section is obtained experimentally for two specific muffler configurations operating in multimodal propagation conditions. The good agreement with numerical predictions serves to validate the perforate plate impedance model used in our calculation. Finally, given an incident acoustic pressure which is representative of typical combustion noise spectrum, the best cavity configuration achieving the maximum overall acoustic Transmission Loss is selected numerically. The study also illustrates how the acoustic performances are dependent on the nature of the incident field.     

A displacement-pressure finite element formulation for analyzing the sound transmission in ducted shear flows with finite poroelastic lining

In the present work, the propagation of sound in a lined duct containing sheared mean flow is studied. Walls of the duct are acoustically treated with absorbent poroelastic foams. The propagation of elasto-acoustic waves in the liner is described by Biot's model. In the fluid domain, the propagation of sound in a sheared mean flow is governed by the Galbrun's equation. The problem is solved using a mixed displacement-pressure finite element formulation in both domains. A 3D implementation of the model has been performed and is illustrated on axisymmetric examples. Convergence and accuracy of the numerical model are shown for the particular case of the modal propagation in a infinite duct containing a uniform flow. Practical examples concerning the sound attenuation through dissipative silencers are discussed. In particular, effects of the refraction effects in the shear layer as well as the mounting conditions of the foam on the transmission loss are shown. The presence of a perforate screen at the air-porous interface is also considered and included in the model.     

A straightforward method for wall impedance eduction in a flow duct

The development of the advanced liner technology for aeroengine noise control necessitates the impedance measurement method under realistic flow conditions. Currently, the methods for this need are mainly based on the inverse impedance eduction principle, confronting with the problems of initial guess, high computation cost, and low convergence. In view of this, a new strategy is developed that straightforwardly educes the impedance from the sound pressure information measured on the duct wall opposing to the test acoustic liner embedded in a flow duct. Here, the key insight is that the modal nature of the duct acoustic field renders a summed-exponential representation of the measured sound pressure; thus, the characterizing axial wave number can be readily extracted by means of Prony's method, and further the unknown impedance is calculated from the eigenvalue and dispersion relations based on the classical mode-decomposition analysis. This straightforward method is simple in its basic principle but remarkably has the advantages of ultimately overcoming the drawbacks inherent to the inverse methods, incorporating the realistic multimode nonprogressive wave effects, high computational efficiency, possibly reducing the measurement points, and even avoiding the necessity of the duct exit impedance that bothers perhaps all the existing waveguide methods.     

Analysis of liner performance using the NASA Langley Research Center Curved Duct Test Rig

The NASA Langley Research Center Curved Duct Test Rig (CDTR) is designed to test aircraft engine nacelle liner samples in an environment approximating that of the engine on a scale that approaches the full scale dimensions of the aft bypass duct. The modal content of the sound in the duct can be determined and the modal content of the sound incident on the liner test section can be controlled. The effect of flow speed, up to Mach 0.5 in the test section, can be investigated. The results reported in this paper come from a study to evaluate the effect of duct configuration on the acoustic performance of single degree of freedom (SDOF) perforate-over-honeycomb liners. Variations of duct configuration include: asymmetric (liner on one side and hard wall opposite) and symmetric (liner on both sides) wall treatment; inlet and exhaust orientation, in which the sound propagates either against or with the flow; and straight and curved (outlet is offset from the inlet by one duct width) flow path. The effect that duct configuration has on the overall acoustic performance is quantified. The redistribution of incident mode content is shown, in particular the mode scatter effect that liner symmetry has on symmetric and asymmetric incident mode shapes. The Curved Duct Test Rig is shown to be a valuable tool for the evaluation of acoustic liner concepts.     

充水管道声分隔片消声性能研究Ⅰ.理论分析

本文对在充水管道中加入声分隔片的消声结构的消声性能进行了理论研究,用模式匹配法计算了有限和蔗声分隔片的传递损失和功率反射系数。研究结果表明,声分隔片的消声效果优于通常所采用在管壁加吸声衬怪的结构,并且其功率反射系数很小。     

Effect of flow on quasi-one-dimensional acoustic wave propagation in a variable area duct of finite length

The general equation for the velocity potential of quasi-one-dimensional acoustic wave motion in a variable area, finite duct with one-dimensional flow is derived by using a perturbation technique. The non-linear second-order partial differential equation is linearized and then solved, by either a power series expansion method or the Runge-Kutta fourth-order method, for harmonic time dependence. The boundary condition taken at the duct mouth is that of matching the impedance of the duct sound field to that of the radiation field at the duct opening. Three axial Mach number variations along the duct axis are considered and the results obtained are compared with those for the case of constant Mach number, to determine the influence of the axial velocity gradient on sound propagation. The effect of flow on the radiation impedance is also considered.     

Sound wave propagation in ducts whose walls are lined with a porous layer backed by cellular cavities

This paper concerns propagation and attenuation of sound waves through acoustically lined ducts. For a cylindrical duct whose liner consists of a point-reacting porous material layer backed by cellular cavities, the admittance formula derived by taking into account a wave motion within the liner is applied to an analysis of waves propagating downstream. For the point-reacting liner of fixed porous material properties, influences of the porous layer thickness, cellular cavity depth, mean flow profile, and three dimensionality of the duct (i.e., cylindrical or plane) on the attenuation are examined. The results show a significant role of the porous layer thickness. For the cylindrical duct, attenuation spectra evaluated from this analysis are compared with those given by the widely used semi-empirical formula.     

Sound generation and transmission in flow ducts with axial temperature gradients

This article describes a one-dimensional, linearized, analysis of fundamental mode sound generation and propagation in rigid-walled flow ducts with axial temperature variation. An acoustic wave equation, including damping effects and volume sources, is derived and its solution (in the absence of sources) by a numerical technique and an approximate analytical method is discussed. The “forced” wave equation is then solved (the existence of an oscillating solution to the “unforced” equation being assumed) for sound generation by a side-branch volume source in an infinite duct, and the results are applied to a duct of finite length. Reasonably good agreement is obtained between measurements and predictions of the sound pressure field in a flow duct, away from the source region.     

PIV and LDV evidence of hydrodynamic instability over a liner in a duct with flow

The acoustical behavior and the flow in a rectangular lined channel with grazing flow have been investigated. The liner consists of a ceramic structure of parallel square channels and is locally reacting. In the absence of flow, the liner has a classical behavior: the acoustic transmission coefficient has a minimum at the resonance frequency of the resonators. When the Mach number of the grazing flow increases, the material behavior becomes unclassical in the sense that its acoustic transmission increases strongly around the resonance frequency. To connect this behavior with flow features, the flow itself in the vicinity of a liner has been measured by means of laser velocimetry. Periodic structures have been observed along the liner that are phase-locked with the incident sound wave. The axial and transverse velocity of these structures bear the typical features of an instability. In particular, the wavelength, convection speed, and growth rate are given. This is the first time that an aeroacoustic instability resulting from the interaction of flow and sound over a liner is measured.