Optimization Of Sound Transmissionloss Through High-speed Train Extrusions Based On 2.5D FEM-BEM

With the development of high-speed railways,the requirement for high speed train ride comfort has been becoming increasingly serious.As one of the main indicators of ride comfort,the interior noise of the train has attracted more attention than ever before.Panels of train carriages vibrate and radiate sound into the coach due to excitation of exterior noise and vibration,resulting the fact that improving the sound transmission loss(STL)through the panels is an effective way to provide a good acoustic environment inside the carriage.Therefore,this thesis is focused on the STL optimization for train carriage panels,in which the lightweight and strength of the panels are also considered.Main works of the thesis are listed as follows.Firstly,vibro-acoustical model of an infinitely long plate is established based on the2.5D finite element method(2.5D FEM)and the 2.5D boundary element method(2.5D BEM).The 2.5D FEM-BEM is validated by comparison with the analytical solution of vibro-acoustics of the plate.With the 2.5D FEM-BEM,dispersion relation and wave propagation of the plate are investigated.Effects of incident angle of the acoustic plane wave,damping loss factor,thickness,density,Young’s modulus and Poisson’s ratio on sound radiation and STL are analyzed.The results show that the plate exhibits modal behavior in the finite dimension,but wave propagation in the infinitely long dimension.Characteristic waves include the in-plane compressional wave and the out-of-plane flexural wave,propagating with the cross-section modal shape.Increasing the incident angle will improve the STL of the plate and widen the frequency band of ‘acoustic short circuit’.The STL consists of three sections and they are controlled by the stiffness,mass and coincidence of the plate,respectively.The STL in the stiffness control section is mainly affected by Young’s modulus and thickness of the plate,and the STL in the mass control section is dominantly influenced by the thickness and density of the plate.Damping loss factor and Poisson’s ratio has negligible impact on the general trend of the STL.Therefore,the influence of the aforementioned parameters on the STL of the plate indicates that,the equivalent Young’s modulus,density and thickness are the main parameters to be determined when treating the aluminum extrusion from train carriages as an equivalent homogeneous plate.Secondly,vibro-acoustics of an aluminum extrusion commonly used in train carriages are investigated by the 2.5D FEM-BEM.The 2.5D FEM-BEM is again validated with STL measurements of the extrusion published in other literatures.In calculation,dispersion relation and wave propagation of the extrusion are analyzed.Effects of incident angle of the acoustic plane wave,boundary condition and thicknesses of surface panels and stiffeners of the extrusion are studied.The results show that cross-sectional deformation is contributed by global modes at low frequencies,while dominated by local modes of surface panels and stiffeners at high frequencies.Frequencies of STL dips includes cut-on frequencies of dispersion curves and the frequency of intersection between the in-plane projection of the acoustic dispersion curve and the structural dispersion curve.Boundary condition only affects vibro-acoustics of the extrusion at low frequencies.The STL of the extrusion changes as a function with thicknesses of surface panels and stiffeners while continuous,derivable and concave-convex properties of the function are difficult to be found.Therefore,it provides useful proposals for selection of optimal algorithm in the STL optimization of the extrusion.Finally,a method for treating the extrusion as an equivalent homogeneous plate is developed based on the flexural dispersion relation of the extrusion.By using the 2.5D FEM-BEM model of the equivalent plate,setting the equivalent density as optimal objective,treating the STL and flexural stiffness of the equivalent plate and thicknesses of surface panels and stiffeners as constraints,the computationally efficient model for STL optimization of the extrusion is established.In the optimization,the gradient based-active set-multi start(GB-AS-MS)algorithm is adopted.The results show that the diffuse STL of the equivalent plate is slightly larger than that of the extrusion across frequencies considered.The difference arises from the over-estimated Young’s modulus and thickness of the equivalent plate,while it is feasible to qualitative investigation of STL of the extrusion.Besides that,it provides an efficient way to STL optimization of the extrusion.Among the triangle-stiffened,rectangle stiffened and trapezoid-stiffened extrusion,the last one is the best since after optimization,it possesses the middle mass and is more stable compared with the rectangle stiffened one having the smallest mass.