Finite element acoustic simulation based shape optimization of a muffler

This paper describes a methodology which combines finite element analysis and Zoutendijk’s feasible directions method for mufflers shape design. The main goal is to obtain the dimensions of the acoustic muffler with the transmission loss (TL), being maximized in the frequency range of interest. The improved four parameters method is used for TL evaluations and the Helmholtz’s equation is solved numerically with the finite element method (FEM). The quadratic triangular finite element meshes are adequately constructed to control the pollution error and the optimization problem is solved using the Zoutendijk’s feasible directions method due to robustness and efficiency for problems with nonlinear constraints. Numeric experiments performed with circular expansion chambers with extended inlet and outlet show results for constrained and unconstrained shape optimization.     

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.     

Predictions and experimental studies of the tail pipe noise of an automotive muffler using a one dimensional CFD model

The tail pipe noise from a commercial automotive muffler was studied experimentally and numerically under the condition of wide open throttle acceleration in the present research. The engine was accelerated from 1000 to 6000 rpm in 30 s at the warm up condition. The transient acoustic characteristics of its exhaust muffler were predicted using one dimensional computational fluid dynamics. To validate the results of the simulation, the transient acoustic characteristics of the exhaust muffler were measured in an anechoic chamber according to the Japanese Standard (JIS D 1616). It was found that the results of simulation are in good agreement with experimental results at the 2nd order of the engine rotational frequency. At the high order of engine speed, differences between the computational and experimental results exist in the high revolution range (from 5000 to 6000 rpm at the 4th order, and from 4200 to 6000 rpm at the 6th order). According to these results, the differences were caused by the flow noise which was not considered in the simulation. Based on the theory of one dimensional CFD model, a simplified model which can provide an acceptable accuracy and save more than 90% of execution time compared with the standard model was proposed for the optimization design to meet the demand of time to market.     

新型高频消音器对Whoosh噪声的优化

为了诊断匹配涡轮增压器的汽油车型急加速过程中产生的Whoosh噪声,并分析噪声产生的原理,确定噪声的频率特性以及噪声产生的工况,本文通过对Whoosh噪声在进气系统中贡献量的分析,按照"源-路径-响应"原则,设计出频率相应的高频穿孔消音器并将其插入到进气系统中,从噪声传递路径上进行优化与控制。通过整车道路客观数据分析和主观驾评,确定该方案切实可行,可推广至多款增压车型上应用。     

不同声强宽带噪声激励下结构参数对穿孔板消声器吸声特性影响的实验研究

高声压级激励下,由于非线性效应的存在,穿孔板消声器的吸声特性将发生改变,而改变量的大小与穿孔板的结构参数(穿孔率,孔径,板厚)密切相关。本文设计搭建了实验平台来研究结构参数的变化对穿孔板消声器的吸声特性的影响。根据实验结果发现:随着声压级的升高,由于穿孔板结构的非线性加剧,其声阻抗将发生变化,导致穿孔板消声器的吸收峰值降低,但吸收频带却拓宽了;在穿孔率一定的情况下,孔径越小的穿孔板消声器更适合低声压级环境工作;在孔径一定的情况下,穿孔率越低的穿孔板消声器也更适合低声压级环境工作。     

An algorithm for the efficient acoustic analysis of silencers of any general geometry

Transfer matrix analysis provides a very efficient means to analyse the linear plane-wave acoustic performance of silencer systems in the frequency domain. However, the nature of the algorithm for combination and reduction of the matrices is different for different combinations of elements. This paper describes an efficient algorithm for acoustic analysis of any general silencer system. The basic format of the algorithm is the identification of sub-systems of two-port acoustic elements. Computational time is also reduced by recording the order in which all of the elements are analysed and the sub-systems are reduced. Examples of the analysis of several complex silencer systems are presented. The gain in efficiency over a general global matrix approach is exceptional.     

High-frequency response of a calculation methodology for gas dynamics based on Independent Time Discretisation

A calculation methodology to solve the one-dimensional governing equations system is presented. This calculation methodology is based on the Independent Time Discretisation (ITD) of the ducts composing the system. The purpose is the improvement of the trade-off between the accuracy and the computational cost that the current 1D gas dynamic models can yield. The ITD methodology is applied to the specific problem of noise prediction in internal combustion engines in order to evaluate its performance in the frequency domain. The application of the ITD methodology to the well-known acoustic configurations which are representative of the main attenuation mechanisms in commercial mufflers shows its ability. The potential is evaluated in terms of reduction of the computational cost and the accuracy and robustness provided by the results as a function of the spatial mesh size and the family of finite difference numerical method applied.     

Expansion-chamber muffler for impulse noise of pneumatic frictional clutch and brake in mechanical presses

A compound expansion-chamber muffler, which consists of a sound absorbing chamber and a switch valve, the chamber integrating structural features of impedance muffler and micropunch plate muffler, is proposed to diminish impulse exhaust noise of pneumatic friction clutch and pneumatic friction brake (PFC/B) in mechanical presses. The structure decreases the impulse exhaust noise of PFC/B over 30 dB(A). A one-dimensional flow model is applied to study the aerodynamic characteristics of compound exhaust process of the single acting cylinder and muffler because the exhaust time is a critical factor for application of muffler in PFC/B. The volume of sound absorbing chamber is found to be an important design parameter to minimize the exhaust resistance of pneumatic cylinder. Experiments are also conducted to validate analytical results. Then the effects of diameter of exhaust ducts and volume of muffler on the exhaust time are discussed in detail. The proposed one-dimensional computational method, which considers the coupling of air-flow field and sound field, gives satisfactory results for the preliminary design of an expansion-chamber muffler. This method has been applied to an existing model HKM3-40MN to reduce its impulse noise.     

An investigation on the acoustic characteristics of microperforated muffler

I.lntroductionTheac0usticperformanccofmicropcrforatedmumerhasbeengreatlynoticedinre-ccntyears.Especially,itshighsi1encingva1ueandbroadsi1encingfrequencyrangeenableittobeusedwidelyinmanyyiclds,suchasvehicleexhaustsystem,ventilator.Butitisdifficulttodesignagoodsilcnccrbecauseoritscomp1exacousticperformanccwithinPerforatedtubes.Thegoverningwaveequationofmicroperforatedmufflerisnotlinearduetothenonuniformmassflowofgasaswc11asthetcmpcraturegradientalongthePerforatedducts.Inordertoutilizethegoodsi…     

Simulations and experiments for hybrid noise control systems

The purpose of this study is to explore the effects of sound elimination in a cylindrical duct by combining a reactive muffler and active noise control (ANC) system. Besides the exploration via experiment of the combined noise control system, a Grey prediction based on Grey theory is also applied to ANC for this hybrid system.In the experiment for this system, a combined adaptive algorithm is adopted. The results of sound elimination are compared between cases with ANC systems installed before the muffler and after the muffler. The results indicate that the sequence of arrangement of muffler can influence the results of active noise control. According to the results of experiment and simulation, the effect of noise reduction in ANC system is influenced extremely by reference signal received. The transmission loss and insertion loss in this system are also discussed in details. Besides, the experimental results indicate that the hybrid system has the advantages over a traditional muffler when the muffler is not designed for the frequency of the noise. Furthermore, the mathematic simulation for acoustic field in a cylindrical duct with a muffler is performed in order to verify the experiment results. Finally, Grey theory is applied to estimate the expected signals in order to perform a computer simulation of Grey prediction to explore effects of the ANC system. The results indicate that application of Grey theory gives a good control for the hybrid system.