Study On Dynamic Behavior And Acoustic Radiation For The Coupling Structure Of Cylindrical Shells With Complex Boundary Conditions

Establishing the internal relationship among the property of vibration source,the vibration characteristics of flexible base structure,the transmission paths of vibration energy and the structural acoustic radiation,is the necessary way to reveal the vibro-acoustic and control mechanism.Also,it is the important theoretical basis to realize the effective vibration isolation and low noise design of structural systems.Doubtlessly,the accurately modeling for the structural dynamic system and its acoustic radiation problem is the key to deeply carry out the theoretical study.Considering the various types of excitation forces,boundary conditions of flexible base structure,coupling conditions of substructures,arrangement forms of isolators and their locations,which are often encountered in practical engineering applications,an analytical model for the isolation and acoustic radiation system mounting on an elastic plate-cylindrical shell structure is established in this thesis.For the factors mentioned above,the research in this paper will be of great theoretical significance and practical application value.According to the problem of dynamic characteristics and acoustic radiation for the coupling structural system of cylindrical shells with complex boundary conditions,the work of this dissertation is presented as follow:The methods existed in the literature for solving the vibration problems of circular cylindrical shells are usually limited by some difficulties,especially the puzzles of boundary conditions,for instance,even only considering the classical homogeneous boundary conditions,one will have a total of 136 different combinations.In view of these limitations,an analytical model for the vibration of circular cylindrical shells with elastically restrained edges,which can be adapted to various boundary conditions,is established in this thesis.Considering fully the coupling relationship of the flexural waves,torsional waves and logitudinal waves in cylindrical shells,the three displacement functions are generally represented by a set of improved two-dimensional Fourier series,and sought in a strong form by letting the solution exactly satisfy both the governing differential equations and the boundary conditions on a point-wise basis,without any simplified and approximate treatments.And finally,the solution is obtained by means of the Rayleigh-Ritz method.The proposal method can be used to model arbitrary boundary conditions by simply adjusting the stiffness of supported springs on boundaries,without need of making any corresponding modifications to the solution algorithms and implementation procedures as typically required in other techniques.Additionally,one of important advantages of this approach is that this modeling method is convenient to analyze the impact of boundary conditions on the dynamic characteristics of cylindrical shells.To validate the correctness and superiority of the method presented in this thesis,some results are compared with those found in the literature,and with some results calculated using the Finite Element Method.An analytical model for the vibration of an elastic plate-cylindrical shell structure with complex boundary conditions and coupling conditions is constructed,in which the six displacements for the plate and shell structures are represented simultaneously by the composite Fourier series method.The boundary conditions and coupling conditions are modeled respectively.By setting four types of uniform coupling springs along their conjunctions,the mechanical coupling effects for the plate-shell structure are completely considered.Ultimately,the Hamilton's principle is employed to derive the characteristic equation for the structural system.The effectiveness of the current method is verified by contrast with the results calculated using the Finite Element Method.On this basis,the forced response and the transmission characteristics of vibratory power flow for this structural system are analyzed.According to both the information of force and vibration response,the distributions and transmission paths for the vibratory energy in the plate-cylindrical shell structure are clarified by employing the structural intensity technology.The physical essence of the power transmission from the elastic plate to the cylindrical shell is revealed by deeply investigating the influences of various coupling conditions,positions and other structural parameters on the power transmission characteristics.An approach of combining the rigid body theory and improved Fourier series method is proposed to model the single stage and two-stages isolation systems based on an elastic plate-cylindrical shell structure,and the isolation performance is investigated.Considering the practical problems of the machinery system design in the field of marine engineering,the sensitivity analysis for various structural parameters in this isolation system is implemented.In addition,the influences of the boundary conditions of flexible base structure,coupling conditions of substructures,types of excitation forces and locations of isolators on the power transmission in this isolation system are investigated comprehensively.Taking the minimization of the power transmission into the flexible base structure from the vibratory machinery as the optimum control strategy,the active control simulation study is executed by the feed-forward control method,and the active isolation performance under different excitation forces or moments are compared with those of passive pattern.The acoustic radiation model of the isolation system based on a flexible plate-shell coupled structure is established.Through the derivation for the acoustic radiation modes of cylindrical shells,the mathematical and physical relationship between the vibration on the structural surface and the acoustic field in space can be obtained,then the characteristic research and mechanism exploration on the acoustic radiation of the coupled structural system are performed from the perspective of acoustic radiation modes.After that the impacts of the types of excitations,the positions,number and forms of actuators on the acoustic radiation of the entire structural system through the cylindrical shell are investigated.Furthermore,the corresponding control strategies are established by taking the radiated acoustic power as the evaluation objective for the isolation and acoustic performances of the entire structural system.Thus,it provide a new way and an effective technical mean to the design of isolation and acoustics for the flexible isolation system.Finally,the corresponding experimental setups are built up,then the experimental study on the dynamic characteristics of circular cylindrical shells and plate-shell coupled structure is performed.The modeling method proposed in this thesis is verified powerfully by the measured results,and some special experimental phenomena as well as various reasons for experimental errors are discussed in detail.Consequently,the experimental research is viewed as an important supplement to the research contents of this dissertation,and it is beneficial to profoundly understand the complicated dynamic characteristics of cylindrical shells and their coupling structures.