Theoretical And Experimental Study On Influences Of Asymmetric Substructures On The Vibro-Acoustic Signatures Of Submerged Propeller-Shafting-Hull

Acoustical optimization design of a submarine hull is an effective way to improve the acoustic stealth performance of the submarine.Extensive efforts have been dedicated to the effects of the sub-structures in the hull on the vibroacoustic characteristics of the submerged submarine.However,attentions are mainly focused on the influences of such symmetric sub-structures as ring stiffeners,bulkheads,and so on.The effects of such asymmetric structures on board as the thrust bearing base,on the acoustic radiation from the propellershafting-hull have seldom been discussed,and the related mechanisms resulting in the effects have not been clearly revealed.The capability of these sub-structures to significantly change the vibro-acoustic responses of the submarine,however,has been proved.Thus,this thesis is focused on the influences of the asymmetrical and non-uniform sub-structures of the hull on the underwater vibro-acoustic characteristics of the propeller-shafting-hull coupled system under propeller excitation.In the analysis,analytical methods,semi-analytical methods,numerical methods and experiments are applied.One-dimensional vibration models like straight beams,curved beams,vibroacoustic models of the beam-coupled shell systems and simplified submarine models with more complex sub-structures are developed.Based on the method used and models established,the effects of these complex asymmetric substructures and the corresponding physical mechanisms are systematically investigated.The organization of the thesis is as follows:Firstly,in order to reveal the mechanisms,the hull is decomposed into onedimensional structures in the axial and circumferential directions,respectively.In the axial direction,based on vibrational principle,three models are derived,including a straight beam with abrupt quality change in the cross-section,a straight beam with eccentrically concentrated mass elements and a doublebeam system with an eccentric beam.Then,the wave-number expansion method is further employed to examine the effects of the wave coupling caused by asymmetric structures and the shear wave energy localization mechanism induced by the concentrating elements.In the circumferential direction,using the generalized shell theory and the Hamilton's principle,a curved beam model,with longitudinal-shear-bending coupling and eccentrically located elements considered,is developed.The coupling characteristics of the circumferential modes caused by the non-axisymmetric structure are analytically studied,and the mathematical and physical mechanisms of the wavenumber coupling characteristics induced by the asymmetric structures and the effects of the coupling on the system responses are revealed.Secondly,with a modified variational method,a semi-analytical vibration model of a shafting-asymmetrical hull is formulated.Combined with the physical mechanisms obtained from preceding studies of the one-dimensional structures,the influences of the asymmetric substructures in the hull on the vibration responses of both the hull and shafting-hull coupled system have been discussed.Moreover,the reasons leading to the effects are revealed by analyzing the energy input for the circumferential wavenumbers and the contributions of the wavenumber coupling.By coupling the semi-analytical model with the acoustic boundary element method,a vibro-acoustic model for the submerged hull is developed and the circumferential and axial modal radiation efficiencies are then formulated,so that the relationship between the vibrational energy for each wavenumber component and the far-field sound power radiated by the hull is established.Then,based on the physical insights into the coupling contributions of the circumferential wavenumbers and of the axial structural modes shown in the previous sections,a research method in the wavenumber-frequency domain based on numerical or experimental resulted is developed.The method is suitable for analyzing the influences of more complex asymmetric structures on the underwater vibro-acoustic responses of the propeller-shafting-hull coupled system.A large-scale model simplified from a submarine is employed as an example and the changes in the sound radiation of the coupled system resulting from typical asymmetric substructures such as the thrust bearing foundation are examined.The mechanisms for the changes are revealed with the developed wavenumber domain analysis.Finally,in order to validate the developed methods and the corresponding mechanism revealed with the theoretical studies,a large-scale underwater vibration and sound radiation test is conducted.For compare purpose,three models,namely the standard modes,the symmetric foundation model and the asymmetric hull model are employed in the test.Through the comparison between the measured sound pressures from the three models,the effects the corresponding asymmetric substructures on the sound radiation of the coupled are presented.The velocity responses of the wet surface in the wavenumberfrequency domain obtained from the test and numerical calculation are presented and compared with each other.Combined with a comparison between the structural modes obtained from the test and the wavenumber domain responses calculated by the presented research method,the developed research method in the wavenumber-frequency domain is proved to be accurate and reliable to analyze the influences of the asymmetric substructures.Thus,the corresponding conclusions drawn in the previous sections are validated.In this paper,a series of theoretical and experimental studies are systematically carried out to investigate the influences of the asymmetric structures inside the hull on the vibration and sound radiation characteristics of the hull excited by the propeller forces in the medium-frequency and lowfrequency ranges.The influence laws of the asymmetric structures and relevant mechanisms are revealed.Above all,the discussion and the revealed mechanisms in this thesis are of great significance in science and engineering.