The Vibro-Acoustic Characteristics of Cylindrical Shells in the State of Immersion and Floating
|School||Huazhong University of Science and Technology|
|Course||Design and manufacture of ships and marine structures|
|Keywords||Structural-fluid coupling system Fourier transformation Semi-submerged Partially immersed Free surface Rigid wall Input power flow Far field sound pressure Stationary phase method Image method|
The work of the thesis is a part of the research project “Characteristics of VibrationalPower Flow in Submerged Coupled Cylindrical shell”, which is supported by the NationalNatural Science Foundation of China (Contract Number:40976058). Based on the wavepropagation approach, the vibro-acoustic characteristics of the cylindrical shells in thefollowing cases are studied: the semi-submerged cylindrical shell, the partially immersedcylindrical shells, the cylindrical shells which is submerged in the fluid with finite depthfrom the free surface or with finite distance form the rigid wall and the cylindrical shellssubmerged in the fluid with the boundary of the free surface and the rigid wall.The studies which are related to our work are introduced briefly. The main contentsinclude the outlines of the studies about the vibro-acoustic characteristics of cylindricalshell in the free space, the summary of the methods exployed in the sound radiationanalysis, the overview of the studies of the vibro-acoustic characteristics of cylindricalshell in half space and the introduction of the image method applied in acoustics.The vibro-acoustic characteristic of the semi-submerged cylindrical shell is studied.The domain of the fluid can be seen as semi-infinite. The expression can be expanded intoseries which can satisfy the boundary condition of the free surface in this case. Thestructure-fluid coupling equation can be obtained applying the boundary condition of theinterface of the structure and fluid. Introducing the stationary phase method, theapproximate expression of the far field sound pressure is obtained and the results arecompared with those in infinite fluid field.The coupling of the circumferential mode of the semi-submerged cylindrical shellmakes the impedance matrix very complicate which leads to the low solution efficiency ofthe coupling equation. A method of diagonal decoupling is employed here to solve theproblem. Using the method, the non-diagonal elements of the impedance matrix areassumed to be zero and the equation can be decoupled. The far field sound pressure can becalculated more efficient and the validity is proved by comparing the results with those inliterature. Based on the method, the input power flow is studied. This method alsofacilitates the study of more complicate cylindrical shells partially immersed. The characteristics of the sound radiation and vibrational power flow of the partiallysubmerged cylindrical shell under a harmonic excitation are studied. The approximateacoustic boundary of the free surface is used to solve the fluid domain. The structure-fluidcoupling equation is established based on the Flügge and Helmholtz theories. The far-fieldsound pressure is calculated and compared with that in infinite field.The far-field sound radiation of a submerged cylindrical shell with finite depth fromthe free surface is studied. The image method is applied to satisfy the boundary conditionof the free surface. Based on the Flügge shell theory and the Helmholtz equation, thestructure-acoustic coupling equation is established. The expression of the far-field soundpressure is obtained using the stationary phase method. In order to evaluate the effect ofthe depth from the free surface, the results of the submerged cylindrical shell with finitedepth from the free surface are compared to those of the submerged cylindrical shell in theinfinite fluid. The characteristics of the far-field sound pressure with the change of thedepth are investigated. It is found that the depth has important influence on the far-fieldsound pressure radiated from the submerged cylindrical shell due to the presence of freesurface.In practice, when the submarine runs near the seabed or the submarine is tested in thepresence of dock, the fluid domain can be seen as semi-infinite and the seabed and thedock can be processed as rigid wall The far-field acoustic radiation of a submergedcylindrical shell which is located at a finite distance from the rigid wall is studied. Basedon the image method, Flügge shell theory and the Helmholtz equation, thestructure-acoustic coupling equation is established and the expression of the far-fieldsound pressure is obtained. In order to evaluate the effect of the rigid wall, the results arecompared to those of the submerged cylindrical shell in presence of free surface.The far field sound pressure of the cylindrical shells with the boundary of thecombination of free surface and rigid wall is finally studied. The free surface and rigidwall is assumed to be perpendicular. It is assumed that the sound wave is reflected onlyonce by the boundary that is the reflected wave from the rigid wall can not be reflectedagain by the free surface and vice versa. Based on the assumption, the expression of thefarfiled sound pressure radiated by the cylindrical shell is otained. The expressions of the far field sound pressure are obtained while the cylindricalshells are located in the fluid with various boundary conditions. The vibro-acousitccharacteristics of the cylindrical shell in different immersion states are studied in our workwhich can serve as the theoretical basis for the establishment of the sound radiationprediction of the cylindrical shells in different immersion states.