Investigation On Vibro-acoustic Responses Of Shaft-hull Coupled System And Influence Of Hull Non-uniformity In Low And Middle Frequency Domain

Research and reduction of radiated sound of the submarine in low and middle frequency domains are beneficial for its safety and operation ability.Concentrating on the vibrations and sound radiations of the shaft-hull system,taking into account the system exhibits different forms of vibrations in different frequency domains,and considering the misalignment between centroid and geometric center in practical structure,eccentric beam model is modeled analytically in low frequency domain and shell model is analyzed numerically in middle frequency domain in this thesis.The effects of mass eccentricity,distribution of density and connections on the vibration and acoustic properties of the beam model were discussed.The influences of load non-uniformity,structure non-uniformity,distribution and connection of masses,number,distribution and stiffness of the connections between shaft and hull on the system responses are studied.The modes and vibration responses of the shaft-hull system with uniform and non-uniform hull were tested in air.A series of experiments about the vibrations and acoustics in water were conducted,which are associated with the shaft-hull system with normal foundation and lower foundation.The main works are listed as:(1)On the basis of the derived coupled flexural-longitudinal vibration equations of the eccentric beam,the single stepped and double stepped beam models are built using analytic and modified transfer matrix methods,respectively.The discipline of influence on the properties and responses of the beam caused by mass eccentricity was revealed.The effects of density distribution and connection condition on the vibrations and acoustics were studied.The results indicate that mass eccentricity will induce coupled flexural-longitudinal vibrations.It also can lower the flexural frequencies due to the increase on mass moment of inertia and raise the longitudinal frequencies owing to the strengthening on the axial stiffness.The further mass distributed from the center,the lower the frequencies of longitudinal modes,while thus non-uniformity can increase the frequencies of flexural modes.Meanwhile,it has influence on the modes of all form of modes.The amplitude of the vertical transmitted force depends on the distance from connection to excitation point in very low frequency domain,while it is controlled by stiffness of the connection in higher frequency domain.(2)The vibrational and acoustic model of the shaft-hull system under water is built by finite element method/boundary element method.The effects on vibrations and sound radiations of the hull caused by non-uniformity of loads,structures and masses are researched.Response properties of the system under excitations in three typical directions are provided,and effects of shaft and connection condition on the sound radiation of the system are analyzed at the same time.The results reveal that the bending modes can be excited under nonuniform axial excitation while the non-uniformity of structures can change the characteristics of the hull,i.e.,high circumferential wavenumber couples with low circumferential wavenumber in one mode.As a result,the radiated sound will be enhanced.The transmitted force will be dynamically amplified at the natural frequencies of the shaft,which lead to an increase on the sound power of the system.The sound can be depressed by adding an additional supporting in the vicinity of the main force transmission path to modify the loading condition.(3)After analysis on the coupled shaft-hull system,the vibrations and acoustic responses of the system in air/water are conducted experimentally.Meanwhile,the effects and mechanism of the foundation reduction acts on the responses are investigated and analyzed.The experimental and simulated results validate the rationality and accuracy of the simulation models.The corresponding frequencies of the modal shapes that the foundation swings in the axial direction rise in the lower-foundation model because the axial stiffness of the foundation is enlarged.Therefore,the acoustic response decreases slightly below this frequency.Aiming at investigating the vibrational and acoustic responses of the shafthull system in low and middle frequency domain,and exploring the effects of non-uniformity in the hull,a series of theoretical and experimental investigations are carried out in this thesis.Based on clarifying the influence of non-uniformities in excitation,structure,mass and shaft on the system,several passive control methods are proposed,which endows this thesis with great significance in engineering design of submarine.