| Concealment is the basic guarantee for underwater warships to exert their maneuvering assault function.Its concealment mainly highlights the performance of acoustic stealth.The performance of acoustic stealth of underwater warships directly affects their survivability and combat effectiveness.Mechanical noise is one of the main noise sources of underwater warships.On the one hand,the mechanical noise is generated by the vibration isolation system such as support,base and floating raft.On the other hand,mechanical equipment as sound source radiates air noise directly into the ship and transmits it through the hull structure to cause underwater radiation noise.Previous research on mechanical noise mainly focused on the structural vibration and acoustic radiation characteristics caused by mechanical equipment as a force excitation source,while ignoring relationship between the air noise and underwater radiation noise.Because of the high density of mechanical equipment and the narrow cabin space,the intensity of cabin air noise is greatly enhanced,and then part of the air noise will cause underwater radiated noise through the hull structure.Because transmission noises contain a lot of noise information about the operation of mechanical equipment inside the ship and the vibration information of the hull structure itself,when the submarine is in a low-speed navigation or concealed state,the underwater transmission noise caused by the cabin air noise is closely related to the acoustic stealth of the ship itself.With the development and application of quiet electromechanical equipment and vibration isolation technology,the problem of underwater radiated noise caused by cabin air noise has become more and more prominent,which can not be ignored in underwater ship acoustic design.Aiming at the problem of underwater radiated noise caused by underwater ship cabin noise,this paper studies the noise transmission characteristics of underwater cylindrical shells between interior field and exterior field.By combining with the spatial distribution characteristics of the sound field inside the shells and the characteristics of the structural vibration response,the noise transmission model of the shell structure between interior field and exterior field is established in different frequency range.On the basis of the noise transmission model,the distribution characteristics of sound field inside and outside the cabin structure under force source excitation,sound source excitation and their mixed excitation are studied.In addition,the directivity of large mechanical equipment as volume source and the influence of surface acoustic scattering are calculated and analyzed,and then the contribution of different noise sources to the sound field inside and outside the shell are compared.The theoretical model of acoustic transmission of the shell in irregular interior space is further established,and the acoustic transmission characteristics of the shell with a large number of mechanical equipment occupying the interior space of the cabin are carried out.The influence of the size of the equipment occupying the interior space on the noise transmission of the interior and exterior sound fields is also analyzed.Finally,an experiment was carried out to verify the reliability of the main conclusions of the vibro-acoustic characteristics of the shell under complex excitations.Based on the measurement of the sound field inside the cabin model,the radiated noise was predicted to verify the validity of the noise transmission model.The research content of this paper is as follows:1.Research on noise transfer characteristics inside and outside cylindrical shells:internal sound field characteristics and structural acoustic response are two important aspects of noise transfer inside and outside cylindrical shells.For the characteristics of sound field in cylindrical shell,firstly,the normal mode density of sound field in shell structure is studied.According to the modal density function,the internal sound field characteristics in different frequency range are analyzed by the normal mode theory and the diffusion sound field theory,respectively.Detailedly,the normal mode theory is suitable at the low frequencies,and the diffusion sound field theory can be used to analyze the internal sound field in the medium and high frequency range.For the vibration response of shell,based on Donnell's thin shell vibration equation and Helmholz's equation,the vibration equation is deduced.The limitation of the theory of thin shell with increasing frequency is expounded,and then the plate approximation model of shell structure is given.Finally,the noise transmission model is built according to the ring frequency of the shell structure,and the noise transmission characteristics of the shell structure in different frequency range are analyzed.Under the ring frequency,the coupling relationship between the cavity modes and the vibration modes dominates the sound transmission characteristics.And the sound insulation quality of the shell structure plays a major role in the sound transmission characteristics over the ring frequency.2.The influence of different excitations on the interior/exterior sound field of the shell:In this paper,the internal noise sources of naval ship structure are divided into force source excitation and sound source excitation,which are loaded on the inner surface of the shell structure in the form of local force and distributed force,respectively.The contribution of different excitations to the interior/exterior sound field are discussed.It is found that the vibration response of the shell mainly reflects the vibration of the shell under the force excitation,while the change of sound field in the shell structure is more sensitive to the source excitation.The excitation sources are classified and studied in detail,such as linear force source,surface force source,dipole sound source and quadrupole sound source.Aiming at large-scale mechanical equipment,a sound transmission model with spatial distribution and directivity under volume source excitation is proposed,and the effects of source directivity and surface acoustic scattering on sound transmission are revealed.3.Influences of irregular interior space on the interior/exterior sound field of the shell:On the basis of Donnell's thin shell theory model and integrated modal method,a sound transmission model with irregular internal structure is proposed.The effect of the occupancy of the machinery on the coupling relationship between the cavity modes and the vibration modes of the shell is theoretically analyzed.By changing the volume of the equipment,the influence of space occupation on the interior/exterior sound field of the shell is studied.The relationship between the the mechanical equipment occupation and the directivity of underwater radiated noise of the cylindrical shell is discussed.4.The relevant experimental research:In order to verify the validity of the noise transfer model inside and outside the shell,an experimental study on the prediction of the radiated noise outside the cabin model was carried out.The air noise and underwater noise of the cabin model are caused by the vibration of the single and combined excitation shell by the exciter and the speaker respectively.The microphone near the inner wall of the cabin and the hydrophone near the outer wall are used to verify the validity of the noise transfer model inside and outside the cabin model structure.Further,the relationship between the internal and external noise of the shell structure under complex excitation is studied by using the acceleration sensor on the inner wall of the cabin,and the main conclusions of the contribution of different excitation modes to the internal and external noise of the shell structure are verified.The results of cabin test have certain guiding significance and engineering application value for acoustic transmission prediction of underwater vehicles. |
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