双倒易边界元法应用于预测三维势流管道和消声器声学特性

将双倒易边界元法应用于预测具有三维势流存在时管道及消声器的声学特性,阐述了其基本原理与数值过程.与传统边界元方法相比,该方法考虑了声学控制方程中气流马赫数二阶小量的影响,因此适用于具有较高马赫数亚音速流的情况.使用双倒易边界元法预测有气流存在时管道和变截面膨胀腔的四极参数,并与一维解析解和传统边界元法结果进行了比较,从而验证了该方法的正确性.利用双倒易边界元计算并分析了不同结构类型消声器的传递损失,结果表明,三维流对复杂结构的消声器声学性能的影响是不可忽略的.     

具有线性温度梯度的管道及消声器声学特性的边界元法计算

采用摄动法将具有线性温度梯度介质中的声传播方程化为Ｈｅｌｍｈｏｌｔｚ方程，然后用边界元法进行计算，由边界元法计算出消声器的四极参数，从而预测传递损失等消声量。文中计算了直管段的四极参数及膨胀腔的传递损失，并与一维理论结果进行了比较。     

用边界无法与四负载法联合预测排气消声器的插入损失

本文提出了将边界元法与四负载法相结合用于预测排气消声器插入损失的新方法，建立了具有低马赫数流时消声器四极参数求值的边界元计算方法，并将四负载法扩展到排气消声系统声源阻抗的求值。文中对两个消声器的插入损失进行了数值计算和实验测量，其预测结果与实测结果吻合良好。     

管道及消声器学特性的边界元法计算

从流体力学的基本方程出发，导出等截面管内具有平均流和线性温度梯度时的三维声传播方程，然后采用迭代法将其化成可用边界积分方程表示的形式，最后用边界元法求解，由边界元法计算消声器的四极参数，从而可预测传递损失等声学特性，文中计算了膨胀腔内传递损失，并与一维理论结果进行了比较。     

消声器传递损失预测的边界元模态展开混合方法

将边界元法和解析方法结合形成一种混合方法用于计算消声器的传递损失,消声器被划分成若干个子结构,解析方法和边界元方法被分别用于计算规则结构和不规则结构的阻抗矩阵,不同子结构之间通过阻抗矩阵连接起来。为减少计算时间,采用一种基于模态配点法的简化方法。对单级膨胀腔、双级膨胀腔和穿孔管阻性消声器的传递损失进行了计算,混合方法计算结果与解析方法和三维数值方法计算结果吻合良好。分析了混合方法的计算效率并与传统子结构方法进行了比较,混合方法能明显节省计算时间。     

消声器传递损失预测的边界元模态展开混合方法

将边界元法和解析方法结合形成一种混合方法用于计算消声器的传递损失,消声器被划分成若干个子结构,解析方法和边界元方法被分别用于计算规则结构和不规则结构的阻抗矩阵,不同子结构之间通过阻抗矩阵连接起来。为减少计算时间,采用一种基于模态配点法的简化方法。对单级膨胀腔、双级膨胀腔和穿孔管阻性消声器的传递损失进行了计算,混合方法计算结果与解析方法和三维数值方法计算结果吻合良好。分析了混合方法的计算效率并与传统子结构方法进行了比较,混合方法能明显节省计算时间。      

消声器声学性能预测的子结构快速多极子边界元法

鉴于快速多极子边界元法的应用主要局限于单区域声学问题计算,发展基于子结构技术的快速多极子边界元法以计算多区域声场问题,介绍基本原理、具体实施过程以及优缺点.以带有插进口管的膨胀腔消声器为例,应用子结构快速多极子边界元法和传统边界元法计算其传递损失,通过与实验测量结果的比较,验证方法的有效性和计算精度.研究表明,快速多极子边界元法与传统边界元法相比,节点数越多,其在节省计算时间,减少计算量等方面的优势越明显.     

具有平均流的膨胀腔声学特性的三维边界元分析

本文将边界元法应用于具有平均流的管道及膨胀腔声学特性的三维分析，获得了具的平均流介质中声传播问题的边界积分方程和基本解。采用九节点二次等参单元离散边界表面并对物理量插值，对奇异积分采用极坐标变换法和间接法联合来消除奇异性，在棱边角点处区分不同方向的质点振速。文中对有无平均流时直管的四极参数及膨胀腔的传递损失进行了计算，并与一维理论及其它方法计算结果进行了比较。     

消声管道声学性能计算的二维等几何有限元方法及应用

将等几何有限元方法应用于消声管道的声学性能计算,使用二维等几何有限元方法求解管道截面的声学特征值,考虑了存在穿孔边界和吸声材料边界的情况,进而使用特征值计算消声管道的传递损失。对包覆式消声管道进行传递损失的计算,结果与二维有限元方法吻合较好.对圆形截面的特征值计算结果表明,在计算量相同的情况下,等几何有限元方法取得了比传统有限元方法更好的计算精度.在不同结构参数条件下对消声管道的声学性能进行计算,结果与三维数值方法吻合良好。方法能够在宽频范围内较好地预测消声管道的传递损失。      

单个球形生物质颗粒热解过程的数值模拟

本文对单个球形生物质颗粒的热解过程进行数值模拟．利用双倒易边界元法和四阶龙格-库塔方法分别对非线性的导热方程和化学反应动力学方程组进行求解．讨论了生物质颗粒的大小与环境温度对热解时间和热解产物中相关组分质量分数的影响．     

Application of dual reciprocity boundary element method for prediction of acoustic characteristics of ducts and silencers with three-dimensional potential flow

The Dual Reciprocity Boundary Element Method （DRBEM） is applied to predict the acoustic characteristics of ducts and silencers with three-dimensional potential flow, and the basic principle and numerical procedure of the proposed method are introduced. Compared to the Conventional Boundary Element Method （CBEM）, the DRBEM takes into account the second order terms of flow Mach number in the acoustic governing equation, which is suitable for the situations with higher Mach number subsonic flow. The four-pole parameters of a duct and a varying cross-sectional area expansion chamber are predicted with the DRBEM, and the predictions are compared with the one-dimensional analytical solutions and the CBEM results. The comparisons demonstrated that the present method is valid. Transmission loss of silencers with different structures was also calculated with the DRBEM. The results showed that the influence of the three-dimensional flow on the acoustic characteristics of silencers with complex structures is not negligible.     

Analysis and design of pod silencers

Parallel baffle mufflers or split silencers are used extensively in heating, ventilation and air conditioning systems for increased attenuation of noise within a short or given length. Acoustic analysis of rectangular parallel baffle mufflers runs on the same lines as that of a rectangular duct lined on two sides. This simplification would not hold for circular configurations. Often, a cylindrical pod is inserted into a circular lined duct to increase its attenuation (or transmission loss), thereby making the flow passage annular and providing an additional absorptive layer on the inner side of this annular passage. This configuration, called a pod silencer, is analyzed here for the four-pole parameters as well as transmission loss, making use of the bulk reaction model.The effect of thin protective film or a highly perforated metallic plate is duly incorporated by means of a grazing-flow impedance. Use of appropriate boundary conditions leads to a set of linear homogeneous equations which in turn lead to a transcendental frequency equation in the unknown complex axial wave number. This is solved by means of the Newton-Raphson method, and the axial wave number is then used in the expressions for transmission loss as well as the transfer matrix parameters. Finally, results of a parametric study are reported to help the designer in optimization of a pod silencer configuration within a given overall size for minimal cost.     

Evaluation of the errors in the measurements of silencer characteristics

This paper is concerned with an evaluation of the errors committed when measuring the transmission loss and the flow noise of silencers; these errors are mainly due to both source and end reflection. It is shown that in measuring the transmission loss, the direct method is more likely to give small errors than the substitution method. Yet whenever the substitution method is used special attention must be paid to the reflection coefficient toward the source. Moreover, it appears necessary to use rather long ducts in order to avoid the effects of axial resonances.     

The study of reactive silencers by shape and parametric optimization techniques

The aim of this work is to show the application of shape and parametric optimization techniques in the study of reactive silencers with extended inlet and outlet ducts. Parametric optimization is employed to evaluate the appropriate size of the inlet and outlet ducts. Shape optimization is employed to establish the proper profile of these ducts in order to improve the acoustic features of these mufflers in a specific frequency range. The objective function used in the optimization processes is defined through the average transmission loss (TL) for the desired frequency range. This type of objective function is strongly non-linear and the genetic algorithm, GA, was chosen as a mathematical method for determining the maximum of this function. The Finite Element Method with an axisymmetric formulation along with the modified four-parameter method are used to calculate the TL(ω). The Hermite polynomials were used in the shape optimization in order to obtain local boundary approximations with C¹ continuity. The results showed the optimization efficiency of the inlet ducts profile for acting in specific frequency ranges with gains up to 20 dB with respect to silencers without shape optimization. The numerical analyses agree well with experimental results.     

Acoustic attenuation analysis of silencers with multi-chamber by using coupling method based on subdomain division technique

For multi-chamber silencers with non-uniform cross-sections at axial direction, the coupling method based on subdomain division technique is proposed. The silencer is divided into several subdomains which impedance matrixes are calculated by using suitable 3-D methods. Combining with the continuity conditions at the interfaces to solve the integral impedance matrix of the silencer, thereby the transmission loss is derived. For the double-chamber silencers, the transmission loss and computational speed results from the proposed method, the numerical mode matching (NMM) method, 3-D finite element (FE) method predictions and experimental measurements are compared, which verifies the accuracy and efficiency.     

Shape optimization of multi-chamber cross-flow mufflers by SA optimization

It is essential when searching for an efficient acoustical mechanism to have an optimally shaped muffler designed specially for the constrained space found in today's plants. Because the research work of optimally shaped straight silencers in conjunction with multi-chamber cross-flow perforated ducts is rarely addressed, this paper will not only analyze the sound transmission loss (STL) of three kinds of cross-flow perforated mufflers but also will analyze the optimal design shape within a limited space.In this paper, the four-pole system matrix used in evaluating acoustic performance is derived by using the decoupled numerical method. Moreover, a simulated annealing (SA) algorithm, a robust scheme in searching for the global optimum by imitating the softening process of metal, has been adopted during shape optimization. To reassure SA's correctness, the STL's maximization of three kinds of muffles with respect to one-tone and dual-tone noise is exemplified. Furthermore, the optimization of mufflers with respect to an octave-band fan noise by the simulated algorithm has been introduced and fully discussed. Before the SA operation can be carried out, an accuracy check of the mathematical model with respect to cross-flow perforated mufflers has to be performed by Munjal's analytical data and experimental data.The optimal result in eliminating broadband noise reveals that the cross-flow perforated muffler with more chambers is far superior at noise reduction than a muffler with fewer chambers. Consequently, the approach used for the optimal design of noise elimination proposed in this study is certainly easy and efficient.     

A comparison between analytic and numerical methods for modelling automotive dissipative silencers with mean flow

Identifying an appropriate method for modelling automotive dissipative silencers normally requires one to choose between analytic and numerical methods. It is common in the literature to justify the choice of an analytic method based on the assumption that equivalent numerical techniques are more computationally expensive. The validity of this assumption is investigated here, and the relative speed and accuracy of two analytic methods are compared to two numerical methods for a uniform dissipative silencer that contains a bulk reacting porous material separated from a mean gas flow by a perforated pipe. The numerical methods are developed here with a view to speeding up transmission loss computation, and are based on a mode matching scheme and a hybrid finite element method. The results presented demonstrate excellent agreement between the analytic and numerical models provided a sufficient number of propagating acoustic modes are retained. However, the numerical mode matching method is shown to be the fastest method, significantly outperforming an equivalent analytic technique. Moreover, the hybrid finite element method is demonstrated to be as fast as the analytic technique. Accordingly, both numerical techniques deliver fast and accurate predictions and are capable of outperforming equivalent analytic methods for automotive dissipative silencers.     

Transmission loss prediction of reactive silencers using 3-D time-domain CFD approach and plane wave decomposition technique

A predictive method is proposed to determine the transmission loss of reactive silencers using the three-dimensional (3-D) time-domain computational fluid dynamics (CFD) approach and the plane wave decomposition technique. Firstly, a steady flow computation is performed with a mass-flow-inlet boundary condition, which provides an initial condition for the following two unsteady flow computations. The first unsteady flow computation is conducted by imposing an impulse (acoustic excitation) superimposed on the constant mass flow at the inlet of the model and then adding the non-reflecting boundary condition (NRBC) when the impulse completely propagates into the silencer. The second unsteady flow computation is conducted for the case without acoustic excitation at the inlet. The time histories of pressure and velocity at the upstream monitoring point as well as history of pressure at the downstream monitoring point are recorded during the two transient computations. The differences between the two unsteady flow computational results are the corresponding acoustic quantities. Therefore, the incident sound pressure signal is obtained by using plane wave decomposition at upstream, while the transmitted sound pressure signal is just the sound pressure at downstream. Finally, those two sound pressure signals in the time-domain are transformed into the frequency-domain by Fast Fourier Transform (FFT) and then the transmission loss (TL) of silencer is determined. For the straight-through perforated tube silencers with and without flow, the numerical results agree well with the published measurements.     

均匀流直通穿孔消声器的声学特性分析

将数值模态匹配法(NMM)扩展应用于计算有均匀流存在时直通穿孔管抗性和阻性消声器的声学特性,编写了相应的计算程序。对于圆形同轴穿孔管抗性和阻性消声器,应用数值模态匹配法计算得到的传递损失结果与实验测量结果吻合良好,从而验证了计算方法和计算程序的正确性。进而应用数值模态匹配法研究了运流效应和穿孔阻抗以及穿孔管偏移对穿孔管抗性和阻性消声器传递损失的影响。研究结果表明,马赫数越高,穿孔管抗性消声器在中高频的消声量越高,阻性消声器在整体频段内的消声性能越差;低马赫数时运流效应对穿孔管抗性消声器的影响可以忽略,马赫数较高时运流效应和穿孔阻抗的影响比较明显;对于穿孔管阻性消声器,穿孔阻抗对消声器声学特性的影响比运流效应的影响小,但是与真实值的差别不可忽略;穿孔管偏移对消声器声学特性的影响与频率和消声器结构均相关。