Study On Vibro-acoustic Modeling And Characteristics Of Underwater Multi-layer Composite Plate And Shell Structures

With the development of modern industry and thevibration and noise reduction technology for ship,the multi-layer structures,which have t advantages of light weight,high strength,excellent vibration and noise reduction performance and strong designability,have been widely used in the design of surface ships and underwater vehicles.The study of its underwater vibration and acoustic radiation is of great significance to realize the acoustic stealth design of new ships and submarines.The acoustic and vibration characteristics of underwater multi-layer composite structures are very different from those in the air mainly due to the loading effect of the surrounding heavy fluid and the fluid loading effect has a great influence on the sound and vibration characteristics of the structure.Moreover,the research on the underwater acoustic and vibration characterristics of composite structures involves the theories of acoustics,material mechanics,mathematics and other disciplines,which brings great challenges to the underwater vibro-acoustic modeling of mulei-layer composite structures.Based on the background of vibration and acoustic radiation of multi-layer plate and shell structures in the design and application of submarines,a series of research work has been carried out on the establishment of vibro-acoustic coupling model and the analysis of vibro-acoustic characteristic parameters of multi-layer composite plate and shell structures:Inder to get rid of the limitations of modeling accuracy and complex boundary adaptability in the analysis of vibration and acoustic radiation of multi-layer damping plate structures,based on the first-order shear deformation theory and sawtooth lamination theory,the displacement functions of each layer of the composite plate structure are described respectively,and the displacement function of the sandwich damping layer is expressed as a linear combination of the displacement functions of the base layer and the constraint layer.The boundary spring penalty factor is introduced to simulate the complex boundary conditions of the composite damping plate structure,and the displacement tolerance function is constructed based on the improved Fourier spectrum function.Considering the strong coupling relationship between external fluid and composite damping plate structure,the Rayleigh integral equation is used and a unified model for underwater acoustic-vibration coupling analysis of multi-layer damping plate structure under complex boundary conditions is established by using energy variational principle.The accuracy and convergence of the model established in this paper are verified by numerical examples,and the effects of fluid medium,boundary conditions,core damping parameters and panel layer parameters on the vibration and acoustic radiation characteristics of composite damping plates are analyzed.Aiming at solving the external acoustic radiation problems of axisymmetric multi-layer composite shell structure immersed in an infinite fluid domain,the idea of spectral boundary element method is introduced to improve the traditional boundary element method and a generalized spectral-collocation boundary element method based on the integrity region of the surface of multi-layer composite shell structure is developed,which can effectively solve the problem of element matching of normal displacement variables and sound pressure variables in the work done.Based on the above method,an underwater acoustic-vibration coupling analysis model is established,which is suitable for multi-layer shell structures under arbitrary boundary conditions.Firstly,based on the generalized spectral method and the first-order shear deformation laminated shell theory,the complex elastic boundary conditions and elastic coupling simulation of substructure interface are realized by introducing penalty method and the vibration admissible functions of multi-layer shell structure satisfying arbitrary boundary conditions is derived.For the external acoustic radiation problems of axisymmetric multi-layer composite shell structure,the Helmholtz integral equation is transformed into the boundary integral equation.By using the method in this paper,the related variables of the boundary integral equation are expanded by higher order spectral function polynomials,and the sound field equation which does not depend on element division is obtained.Referring to the treatment of displacement admissible functions,in cylindrical coordinates,the acoustic variables and Green’s functions are expanded into the form of Fourier series and Chebyshev polynomials.In this case,the cost of numerically solving the acoustic problems is reduced.And then,based on Helmholtz boundary integral equation,the one-dimensional form sound field solution equation of multi-layer composite shell structure is obtained.For non-unique solution of characteristic frequency points and singular integral problems,the CHIEF point method and elliptic integral method are used to deal with the problem.Considering the far-field radiation condition of Sommerfeld at infinity and and the external fluid loading effect,a unified vibro-acoustic coupling model of multi-layer shells is established based on principle of virtual work.The accuracy and effectiveness of the proposed method are investigated through a series of numerical calculations and comparative verification.The natural vibration characteristics of underwater multi-layer shell structure are studied by using the generalized spectrum-collocation boundary element method which based on the intact region of structural surface established in this paper.The vibration and acoustic radiation response characteristics of underwater multi-layer shell structure under external harmonic excitation are further analyzed.The effects of boundary conditions,material parameters,laying scheme,excitation position,functional power-law index and material pore distribution on the acoustic and vibration characteristics of underwater multi-layer shell structure are discussed in detail.The proposed method is further extended to solve the vibration and acoustic radiation problem of underwater combined shell structures,and a vibro-acoustic prediction model is established for underwater combined composite shell structures.Consodering the shape characteristics of the underwater vehicle,taking conical-cylindrical-spherical shell and spherical-cylindrical-spherical shell as examples,the vibration and sound radiation characteristics of the combined shells are studied.According to the structural characteristics,the combined composite shells are decomposed into several substructures at interface.The elastic coupling simulation of substructure interface is realized by introducing the coupling spring penalty factor.At the same time,based on the principle of energy,considering the coupling effect between fluid and structure,the energy functional of combined composite shells is constructed,and the unified formula for acoustic vibration coupling of underwater combined composite shell structure is established.The accuracy and effectiveness of the proposed method for analyzing vibration and acoustic characteristics of underwater combined composite shell structure is verified by comparing the numerical examples with the results of literature and numerical software.On this basis,the vibro-acoustic characteristics of underwater multi-layer combined shell structure are carried out,and the mechanism of influence of geometric parameters,material layer structure,exciting force and functionally graded index and pore distribution on vibro-acoustic characteristics of underwater multi-layereded composite combined shell structure are studied in detail,which provides method reference and data support for the acoustic design of underwater multi-layer combined shell structure.