Vibro-acoustics Of Multi-media Composite Structures Of High-speed Train Body

Chinese high-speed railway is developing in the direction of higher speed and more environmental friendliness.The network is widely distributed across complex environments such as a wide range of temperature and frozen soil,which gradually deteriorate the interior noise.However,a high demand of life quality of passengers puts forward a stricter requirement for the interior acoustic environment of a high-speed train.Therefore,the interior noise has become one of the key problems that significantly affect,or even restrict a further development of high-speed railways.The interior noise of a high-speed train mainly comes from vibration and structural noise of wheel/rail systems,aerodynamic noise of bogie and pantograph regions.When these vibration and noise propagate to the body surface of a high-speed train,the body structure will be excited to vibrate and therefore radiate sound into the vehicle.Actually,a high-speed train body is a multi-media composite structure that is laminated by plate or shell structures(i.e.aluminium extrusions,wooden floors and decoration plates),continuous solids(i.e.damping layers and wooden supports),porous media(i.e.sound absorbent materials)and fluids(i.e.inner and outer air).The longitudinal dimension of the structure is much larger than other two dimensions and its cross-section geometry and material properties are uniformly distributed along the longitudinal direction,making it a so-called two and half dimensional(2.5D)structure.Therefore,establishing a 2.5D vibro-acoustic model of the composite structure,investigating its vibro-acoustics and exploring vibration and noise controlling measures can provide a tool of vibro-acoustic design and a theoretical basis for high-speed train manufacturers,and are of great benefits to the reduction of interior noise.In terms of the sound radiation and sound insulation of a baffled composite structure from a high-speed train body,which is assumed to be infinitely long,the following works are mainly carried out.(1)The sound transmission loss(STL)of a segment of a composite structure from a high-speed train body is measured by the sound pressure method.The STLs of the structure with different inner porous layers are compared,which highlights the significance of the elasticity of porous material skeletons on vibro-acoustics of the structure.This indicates that the elasticity of the skeleton should be considered in the following vibro-acoustic model.Through hammer testing,the damping loss factor of the aluminium extrusion,i.e.the outer layer of the composite structure,is obtained by energy-based method.The damping loss factor will be implemented into the following vibro-acoustic model,forming a foundation for verifying the accuracy of the model.(2)Based on Kirchhoff thin plate theory,a 2.5D finite element(2.5D FE)model of plates or shells is developed by using the principle of minimum potential energy.In the model,the in-plane motion is considered.Based on three-dimensional elastic theory,by utilizing weak integrals of the dynamic equilibrium equations governing vibration of a continuous solid,and by combining the Galerkin method,a 2.5D FE model of the continuous solid is provided.Similar to the derivation process of the continuous solid,the Helmholtz equation of acoustic waves is used to establish a 2.5D FE model of a finite fluid.Based on the acoustic Rayleigh integral,a 2.5D boundary element(2.5D BE)model of a semi-infinite fluid is established.In the model,only vibration on the boundary of the fluid wetting the structure is required,which in contrast to the model generated from the full-space or half-space Green function,can effectively avoid errors in boundary truncations on baffles and the additional computation costs brought by the added virtual boundary on the back side of the structure.By implementing interface conditions of a solid(i.e.plates/shells or continuous solids)and a fluid(i.e.finite or semi-infinite fluids)into the boundary integrals of their corresponding weak integral forms,a coupling 2.5D FE-BE model of a composite structure formed of solids and fluids is formulated.Accuracy of the coupling model is verified by applying it to an infinitely long plate and by comparing the sound radiation and STL with those predicted with analytical methods and commercial software.Then the coupling model is used to study the vibro-acoustics of an aluminium extrusion from a high-speed train body.The effects of boundary condition and damping on the STL of the extrusion are systematically analysed,showing a deep insights of mechanism into the STL dips or sound radiation peaks.The influence of excitation location on the sound radiation of the extrusion is researched in detail,which indicates the results can be explained in wavenumber domain and the contribution of modal waves propagating in the extrusion can be categorized.(3)Based on the Biot’s poro-elastic theory,which is in expression of skeleton displacement and fluid in-pore pressure,a 2.5D FE model of a porous material is established.In the derivation process,the equivalent weak integrals of equations governing vibration of the skeleton and fluid(hereinafter referred to as two phases)and the Galerkin method are used.In the model,the elasticity,mass and damping of the skeleton are considered and the potential and inertial coupling between the two phases are included,while these factors can not be reflected in the empirical and equivalent fluid models.In addition,the viscous damping and thermal conductivity between the two phases are accounted.The accuracy of the 2.5D FE model is demonstrated by comparing the predicted surface normal impedance of a porous layer to results from other literatures.(4)Combining the boundary conditions at the interface of a porous medium,a solid and a fluid with the boundary integrals of the equivalent weak integral forms corresponding to each media,a coupling 2.5D FE-BE model for a structure consisting of the above media is derived.The coupling model is compared with the transfer matrix method(TMM),by applying them to single-and double-plate structures treated with porous medium layers.In calculation,phenomena of the skeleton resonance of a porous material layer and the mass-spring-mass resonance of the structure are revealed.These phenomena can be used to explain some STL dips and the significant STL improvement of a structure after resonance,and are useful for the STL explanation when the coupling model is applied to complex composite structures.Then the coupling model is used to investigate the effects of porous layer configurations on the vibro-acoustics of a composite structure of a high-speed train body.Through analysis,effective vibration and noise reduction measures are recommended and a new floor composite structure with a better STL performance is proposed.(5)A computationally efficient model for the vibro-acoustic prediction of a rib-stiffened panel with layers of acoustic treatment is developed.In the model,the rib-stiffened panel is baffled and assumed to be infinitely long;the panel and the fluid domain containing the incident and reflected sound waves are modelled using the 2.5D FE-BE.The acoustic treatment layer is assumed to be in-plane infinite and laterally extended to the baffles;the acoustic treatment layer and the fluid domain containing the transmitted sound waves are dealt with,approximately,using the TMM.The coupling of TMM and 2.5D FE-BE is formulated by the interface conditions of traction balance and displacement continuity,forming a vibro-acoustic model of the whole structure.It is shown that the model proposed can give results matching well with the full 2.5D FE-BE model,especially at high-frequencies while reducing calculation time by about a factor of three.The model is then utilized to conduct a STL optimization for the acoustic treatment of an aluminium extrusion from a high-speed train carriage.