Numerical Mode Matching Approach For Acoustic Attenuation Performance Calculation Of Silencers

Silencer is widely used to control the intake and exhaust noise of engines,and the calculation and analysis of acoustic characteristics is the foundation for silencer design.The 3-D analytical method is only applicable for the acoustic attenuation performance calculation of the silencer with regular shaped cross-section.The 3-D numerical method may be used for solving the silencer wirh arbitrary shaped cross-section,but for the high-frequency calculation it is restricted by the mesh size and the performance of computer.Therefore,the numerical mode matching(NMM)method is studied in this thesis to calculate the acoustic attenuation performance of the complicated silencers with arbitrary shaped cross-section.The numerical mode matching method for solving the three-dimensional acoustic filed of silencers is introduced.The two-dimensional finite element method is employed to calculate the transversal modes of the silencers,and the mode matching technique is utilized to derive the transmission loss.The acoustic attenuation performance of the expansion chamber silencers,the perforated tube silencers and the silencers with mean flow are investigated by using the numerical mode matching method.The transmission loss results from the numerical mode matching method,three-dimensional finite element method(FEM),and experiment show good agreements,which verifies the accuracy of the present NMM method and the computational codes.The comparison between the computational speed of the NMM method and the 3-D FEM demonstrates the advantages of the proposed numerical mode matching method.Then the NMM method is used to investigate the effects of configuration parameters,porous material,mean flow and higher order modes on the acoustic characteristics of silencers,and the methods for optimizing the acoustic attenuation performance of silencers are analyzed from the modal point of view.By locating the inlet/outlet duct on the nodal lines of the higher modes,the modes may be suppressed,and the acoustic attenuation frequency bound may be widened.Decreasing the hole diameter or increasing the porosity may enhance the modal frequencies and the acoustic attenuation performance of the perforated tube reactive or dissipative silencers.The bigger density of the porous materials may bring better acoustic attenuation performance of perforated tube silencers.With bigger Mach number,the perforated tube reactive silencers present bigger TL at mid-high frequencies,and the perforated tube dissipative silencers exhibit worse acoustic attenuation performance at whole frequency range.For the more complicated silencers,the coupling method based on the substructure is proposed.The basic idea is that:the silencer is divided into several substructures,and applying the 3-D methods to calculate the transfer matrixes or impedance matrixes of the substructures.Combining with the continuity conditions at the interfaces of the substructures,the integer transfer matrix and impedance matrix of the silencer are solved,thereby the transmission loss is derived.For the double-chamber silencers,the transmission loss results from the proposed coupling method agree well with the 3-D FEM predictions,which varifies the proposed method.In order to verify the validity of acoustic performance predictions of silencers from the present NMM,the wave decomposition method and two-load method are used to measure the transmission loss of two perforated tube silencers without mean flow.Measurement results agree well with the predictions from proposed methods,which shows that the measurement method and the numerical method are both valid.