The Vibro-acoustic Model And Characteristical Study Of Submerged Or Floated Cylindrical Shells With Multiple Boundary Constraints Including Free Surface

In the research of vibro-acoustic problem of fluid field of the cylindrical shell-fluid coupling system,the field is usually considered to be infinite.However,in practical engineering problem,some classical acoustic boundary conditions such as free surface objectively are pretty common.In addition,the boundary conditions have influence on the vibro-acoustic performance of the coupled system in some degree.At present,the numerical methods like,finite element method and boundary element method,were mainly adopted to these academic problems both domestically and broadly.Furthermore,the theoretical analysis and mechanism study on the vibro-acoustic behavior of the shell-liquid coupling system bounded by free surface can be hardly found.Especially,few approaches are effective on the partial coupling problem and the combined boundaries in the research of the cylindrical shell-flow coupled system.To better understand these scenarios,some analytical or semi-analytical methods are presented in this thesis.Also,the theoretical analysis on the vibro-acoustic characteristics of a cylindrical shell are systematically carried out in semi-infinite or confined domains,submerged or floating states,external or internal fluid fields,respectively.To begin with,a two-dimensional vibro-acoustic model of the cylindrical shell-fluid partial coupling system(the axis of the cylindrical shell is parallel to the water surface)is established and a new theoretical solution is brought up with in this paper.The core thought of the solution is to build up the sound pressure and shell displacement function based on two different coordinate systems,respectively.Then,the Galerkin method is adopted to deal with the velocity continuity condition at the interface between the shell surface and the sound field.In the end,the comparison of the results with the simulations' shows that the present method is more accurate and wider applicable.The present method also provides the important technical and the theoretical basis for further application in the related problems(e.g.floating state and wharf mooring state etc.).Additionally,a two-dimensional vibro-acoustic model of the cylindrical shell with combined boundaries,in which the acoustic boundary conditions consist of a free surface and a vertical rigid surface,is established.By adopting the virtual source method,three virtual sources are introduced to deal with the boundary constraints.Whereafter,the governing equation can be solved theoretically by transferring the three virtual source coordinates to the real source coordinate with the Graf addition theorem.The accuracy of the approach is verified by comparing with the simulation results of finite element software Comsol.Moreover,the present method lays a solid theoretical foundation for the more complex combined boundaries later.Next,the vibration and far-field acoustic radiation of a finite cylindrical shell at finite submerged depth(submerged state)are studied in the next section.In order to improve the calculation efficiency with the ensuring of calculation accuracy,two sets of the physical models in two steps are introduced to solve the problem in this paper.In the first step,the acoustic barrier model is adopted to calculate the vibration response.And in the second one,the far-field sound pressure is calculated by adopting the acoustic barrier column model combined with the stationary phase method.Compared with the simulation results,the accuracy of the present method is verified with much higher computational efficiency than FEM and BEM.Besides,the free surface influence on the natural frequencies,the mode shapes,the directivity and the fluctuation of the far-field sound pressure is analyzed,the multiple scattering effect caused by the free surface is discussed,and the approximate method for calculating the far-field acoustic radiation of an inclined cylindrical shell is presented.Furthermore,the vibration and far-field acoustic radiation characteristics of a finite cylindrical shell in floating state are further studied by adopting two sets of physical models combined with the critical technique of solving the partial coupling problem.Compared with the simulation results of FEM and BEM,the accuracy of the present method is verified.Based on these analysis,the improved studies on the free vibration characteristics of a cylindrical shell partially coupled with internal fluid or with fluid fields on both sides are carried out in this paper.Finally,the vibro-acoustic characteristics on a finite cylindrical shell moored at wharf are studied by assembling the combined boundaries approaches and the critical partial coupling technique.Besides,the influences of the internal fluid field and the different types of seabed sound absorption boundary on the free vibration characteristics of the wharf mooring cylindrical shell are discussed.In this thesis,the theoretical methods for solving the vibro-acoustic problem of a cylindrical shell coupled with fluid field under various working conditions are presented from the shallow to the deep water and two technical difficulties have been overcome successfully.In addition,the vibro-acoustic performances of the cylindrical shell under different working conditions are systematically studied,which enriched the theoretical system of a cylindrical shell coupled with fluid field.In the future,based on the vibrational and acoustical data of a cylindrical shell with acoustic boundary constraints,the fast acoustic prediction under a large submerged deep environment will be realized.