Precise Modelling Of Floating Raft System And Design Of High-performance Raft

The structural noises from hosts and auxiliary machines are main noise sources of a submarine,which is working in low-speed cruising condition.The structural noise largely influences a submarine’s stealth performance.Floating raft system is widely used in submarines,because this system is effective in reducing vibration and structural noise of a hull.The prediction and control of vibro-acoustics performance are the core issues in designing a floating raft system.The rapid developments of the acoustic design of submarine put forward higher demands for the control of structural noise.For this purpose,the precise model of floating raft system and the more efficient technique for the vibration control are needed.The thesis is under the support of National Defense Science Foundation "Acoustic Optimization of Floating Raft System" and "Isolation Technique for the Vibro-acoustic Coupling of the Floating Raft System",National 973 sub-project "Design of Raft with High Vibration Isolation Performance",and National Natural Science Foundation.Main contents and results are as follows:1.Precise model of the floating raft system with attached equipment is established for the performance evaluation at design stage,and the coupled vibration problems caused by the attached equipment are discussed.FRFbased substructuring method is improved to solve this kind of model.Based on the theoretical modeling,the effects of multi-direction coupling between different Dofs on the vibro-acoustic performance of raft system are analyzed.The results reveal that the synthesis results will be underrated if the rotational Dofs are neglected in the process of system modelling,and the calculation errors will be amplified as the synthesis number increases.Coupling mechanism of the attached equipment is described to analyze the influence of the attached equipment on the transmissibility of raft system.The parameter analysis is performed to provide a quantitative understanding for the attached equipment.2.Based on the improved FRF-based substructuring method,the precise model of the floating raft system is established by considering the influence of the multi-direction transmission.The dynamic decoupling method is used to identify multi-directional impedance matrix of the rubber isolator.And the FRF matrix of each substructure is derived based on both experiment and simulation data.Based on that,frequency response matrix of the system is derived using the FRF-based substructuring method.The experimental studies on the floating raft system are conducted to get the vibration responses under the excitation of an electromagnetic shaker.The experimental results are consistent with the synthesis results,verifying the accuracy of precise model.3.To enhance the isolation performance of the floating raft system,band gap property of the periodic structures is introduced into the design of the raft.The method,combining both Bloch theorem and finite element method,is developed to study the dispersion characteristic of the trichiral/hexachiral structure and to obtain the distribution of the band gap.The results of parametric analysis expose the trend of the band gap distribution.Based on that,the chiral structure is adopted as the raft in the vibration isolation system.According to the investigation of the influences of the chiral raft,it can be seen that the system radiation is suppressed in the region of band gap.On the basis of chiral lattice,we propose a hybrid structure,which is constituted by several layers of chiral cell with different topology.Stop band of the hybrid structure superposes the band gap achieved by each layer,which provides a great potential for the realization of vibration suppression over a broad frequency range.An optimization scheme for the chiral raft is developed with the aim of minimizing the radiated sound power in the target band.Then the design plans of the periodic structure raft in the isolation system are summarized.4.Based on the simulation results,the chiral raft system for the actual hull is established and the experiments are conducted.Frequency response characteristics of the chiral raft are measured to verify raft’s inherent isolation performance.The results show that the chiral raft can suppress the vibration transmission in the middle and high frequency.Then the experiments of the raft system are performed under different exciting conditions.Comparisons between the results measured from the chiral raft system and frame raft system prove the superior isolation performance of chiral raft.The thesis has conducted a series of studies in both theoretical and experimental for the modelling and efficient vibration control of floating raft system.A method is presented to obtain the coupling vibration transmissibility,and a design framework of high-performance chiral raft is proposed.The results provide references for the design of the floating raft system,and show great engineering value and military significance.