Study On The Acoustic Characteristics Of Silencers And Perforated Elements

Silencer is an efficient device to control the intake and exhaust noise of engines, and the prediction and analysis of acoustic attenuation performance is necessary for silencer design. The perforated elements are commonly used in silencers to improve the acoustic attenuation and flow characteristics. To predict the acoustic attenuation performance of silencers, the acoustic characteristics of perforated elements needs to be determined first. Therefore, the present thesis will investigate in detail the calculation methods of acoustic attenuation performance of silencers and the acoustic characteristics of perforated elements.Three kinds of methods for the prediction of silencer acoustic attenuation performance are developed, including the transfer matrix method based on the plane wave theory, finite element method and three-dimensional time domain CFD (Computational Fluid Dynamics) method. Based on the plane wave theory, the transfer matrices of several acoustic elements are derived with consideration of the flow and thermo-viscosity effect. A three-dimensional analytical approach is developed to calculate the duct end correction coefficient, and a curve-fitting expression for the end correction coefficients is obtained and used in the corrected plane wave theory to improve the accuracy of transfer matrix method. The finite element method for the prediction of silencer acoustic attenuation performance with consideration of the flow convected effect is developed to predict the transmission loss of perforated duct silencer. The numerical errors of finite element method for the acoustic computation are analyzed, and the wave decomposition method and four-pole parameter method for the calculation of silencer transmission loss by using the finite element method are discussed. The time domain CFD method is developed to predict the transmission loss of silencers. The numerical errors in the CFD simulation for sound propagation in duct are analyzed, the pulse method and wave decomposition method are combined with the time domain CFD approach to determine the transmission loss of silencers. The advantages and shortcomings of the three methods for the prediction of silencer acoustic attenuation performance are discussed.The acoustic characteristics of perforated plate are studied by using the numerical methods. For the case without flow, the acoustic impedance of perforated plate is calculated by using finite element method. The finite element model for determination of the acoustic impedance of perforation is first built, and then the effect of various structure factors on the acoustic thickness correction coefficient of perforated plate is investigated. A curve-fitting expression for the end correction coefficient is obtained based on the numerical results. For cases with flow, the acoustic impedance of perforated plate is studied with the time domain CFD method. The CFD models for the perforated plates with cross flow and grazing flow are built, and the effects of structure factors, cross flow and grazing flow on the acoustic resistance and reactance of the perforated plates are studied. Based on the numerical results, the curve-fitting expressions for the acoustic impedance of perforated plates with cross flow and grazing flow are obtained. Combining the curve-fitting expressions for the acoustic impedance of perforated plate without and with flow, the new models for acoustic impedance of perforated element without and with flow are presented. With the present models for the acoustic impedance of perforation, the transmission loss of perforated duct silencers is predicted, and the predictions are compared with measurements, and the good agreements demonstrate that the present models are suitable and accurate. The predictions with the present models are also compared with results models before, and the comparison demonstates that the present models are better.A testing bed for measuring the acoustic attenuation performance of silencer is designed and built, and the measurement method is studied. For the measurement of transmission loss of silencer with flow, the rapid sine sweep and sync time averaging techniques are used to increase the signal-to-noise ratio and improve the measurement accuracy. Using this testing bed, the transmission loss of prototype perforated duct silencers are measured with the two-load method for cases without and with flow. The measurements show good agreements with the predictions, which proves that the present models are accurate.Using the present models for the acoustic impedance of perforation, the transmission loss of perforated silencers with different structures and flow are predicted, and the effects of various structure factors and flow conditions on the transmission loss is examined, which may be used to guide the practical silencer designs.