Acoustic Modeling, Optimization And Contrl Of Floating Raft System

Floating raft system is the key technology to isolate vibration of hosts and auxiliary machines, and reduce the structural noise of ships and submarines effectively. For the acoustic design of quiet submarines, quantitative prediction, control treatment selection and optimization of machinery noise are important factors urgently needed to find a new breakthrough in the performance of submarines. Under the support of National Defense Science Foundation for the projects"Acoustic Optimization of Floating Raft Sytem"and"Research on New Type of Active Vibration Isolators", the support of National 973 subproject"Design of Raft with High Vibration Isolation Performance", the paper is devoted to modeling the vibration transmission of floating raft system and resultant acoustic radiation, providing design guidelines and abatement techniques for floating raft system to achieve an optimized noise level.In chapter 1, the detailed literature reviews on complex flexible isolation system and the vibro-acoustic modeling of complex underwater cylindrical structures are summarized, both in the aspects of research objects and the modeling approaches. Some potential modeling techniques are pointed out. The vibration control means, including passive, active vibration control and broadband control by periodic structures are reviewed. Also the optimization techniques are reviewed. The difficulties about application of these methods to floating raft system are pointed out.In chapter 2, a general method for the integrated vibration transmission and acoustic modeling of floating raft system is presented. The coupled finite element/boundary element method (FE/BEM) is employed to study the vibro-acoustic behavior of the fluid-loaded base-cylindrical shell substructure. The modeling of vibration transmission from the vibrating machinery to the base structure is based on the Frequency Response Function-based (FRF-based) substructuring method. The effect of coupling between substructures on the vibration transmission of floating raft system is investigated. The influence of fluid-loading, the wave effects in the vibration isolators, the parameters of base-cylindrical shell substructure on the vibro-acoustic behavior of the floating raft system is investigated in detail. The acoustic design guidelines of base-cylindrical shell are summarized. Based on the modeling results, the vibration isolation measures, such as weighted mean-square force and velocity transmissibilities at base, power flow transmissibility, mean-square velocities and radiated power transmissibilities on surface of cylindrical shell and mean-square pressure transmissibility at far-field are advanced. The relationship between the performance indices of vibration isolation and those of acoustic radiation is studied. The equivalent substitute for measuring the performance of suppressing sound radiation in the far-field is advanced.In chapter 3, for suppressing the effects of resonances of foundation that significantly increase the force transmissibility and degrade the isolation performance, periodic structure, a passive band-filter vibration isolation device is developed by using its band gaps, in which waves are effectively attenuated. A curved beam periodic structure possesses of broad low frequency band gap has been designed by using the curved beam, which has the characteristic of coupled radial-tangential stiffness and has the potential of conversion of waves. The multi-layer curved beam periodic structure is composed of rigid plates and curved beams. Based on Flugge's theory, the six-order coupled differential equations are derived to describe the in-plane motion of curved beam, the dispersion relationships and the six wave components that propagate in curved beam are obtained. The wave approach is employed to study the wave propagation, reflection and transmission in coupled curved beam and semi-infinite straight beam, the semi-infinite curved beam with multiple vibration isolation masses, the coupled masses and finite curved beams. The simulation results demonstrate that, for a given incident extensional or flexural wave, the reflected and transmitted waves contain both kinds of waves. In order to simplify the design process of curved beam periodic structure, the equivalent m-k model is developed. The transfer matrice invariant method and the wave vector approach are employed to analyze the band gap and the driving-point and transfer mobilities of curved beam periodic structure, respectively. The simulation results verify the feasibility of obtaining broad low frequency band by using the curved beam. The transmission matrix method is used to study the dynamics of curved beam periodic structure in one DOF system, two DOFs system with flexible plate as foundations and the floating raft system. Numerical simulation results have demonstrated that by use of periodic structure, the vibration transmission at resonances of bases and isolators is reduced, and the radiation of the foundation plate/shell at resonances is suppressed. Finally, the design guidelines of designing curved beam periodic structure in a given floating raft system are summarized.In chapter 4, to improve the low frequency vibration isolation performance of the vibration source-mounts-receiver (SMR) system and suppress the resultant sound radiation, the multi-channel active vibration isolation is employed. Numerical simulation of vibration control of a submerged base-cylindrical shell substructure with active vibration isolation from a vibration flexible plate is presented. The adaptive multi-channel control based on the filtered-x least mean squares (FxLMS) algorithm is used in the active vibration isolation to reduce the acceleration responses of all the connecting points on the bases. The sound radiation with/without control is studied by the dynamic model of the base-cylindrical shell substructure. The relationship between active vibration isolation and the resultant sound radiation is investigated in the time-domain. Numerical results have demonstrated that suppression of vibration on the bases leads to attenuation of sound radiation in the far-field induced by the radial displacement dominant mode of the shell. The performance of control is influenced by the coupling between control channels and related to the structural vibration modes. An experimental system, including a vibration plate, four active vibration isolators and a fluid-loaded plate is established to investigate the role of the proposed active vibration isolation in suppressing vibration transmission as well as underwater sound radiation. The exciting frequency is chosen to be nearly equal to a natural frequency of the coupled system to control the resonance of the fluid-loaded plate. The measured results show that the proposed active isolation is effective.In chapter 5, by using the FRF-based substructuring formulation, the expressions of design sensitivity analysis (SA) in terms of the partial derivatives of the physical parameters as well as FRFs of substructures are developed. In order to overcome the shortcomings of SA that may be trapped in a local optimum, the hybrid genetic algorithm (HGA) involving the FRF-based substructuring SA and the genetic algorithm (GA) is developed. To improve the performance at low frequencies, the proposed optimal schemes are employed to determine the optimal stiffness of the upper and lower isolators under five different objective functions concerning both vibration transmission and acoustic radiation. The influence of different objective functions, fluid-loading, the optimized frequency band and the source properties on the optimized results is studied. The optimized results demonstrate that some of the objective functions are equivalent to each other and can be replaced by each other. This investigation reveals that the optimized results for reducing vibration transmission to the base is not necessarily the optimized results for suppressing the sound radiation to the far-field. Thus, it should take into account of fluid-structure interaction and take comprehensive consideration of different vibration or acoustic objectives in the design stage. The numerical results also show that HGA possesses the merits of local search ability of SA and the strong global search ability of GA. Compared with SA and GA, however, HGA requires more evaluation of objective functions in each iteration step and as a result is not computationally efficient.In chapter 6, to improve the modeling accuracy of FRF-based substructuring method by using actual measured data, effects of various commonly encountered errors and noise of test data on the modeling results are investigated; propagation of uncertainties of FRFs through FRF-based substructuring is quantified based on an analytical approximation using static moment method; and filters constructed by the SVD approach and piecewise linear interpolation method are used to condition the FRF datas. An experiment to validate the effectiveness of the developed FRF-based substructuring method in modeling isolation system composed of flexible raft and base-cylindrical shell connected through multiple isolators is carried out. The experimental results demonstrate that it should take into account of the wave effects of isolators and the coupling between different degrees of freedom (DOFs). The applicability of FRF-based substructuring method in modeling vibration transmission in floating raft system is verified and the modeling errors are analysed. The experiment of detecting the band gap of designed curved beam periodic structure by measuring driving-point and transfer mobilities is carried out. The experimental results verify the accurancy of the proposed methods in dynamic modeling. The comparative trials of one DOF system, two DOFs system with flexible plate as foundations, a single-layer vibration isolation system connected through multiple isolators and the floating raft system with/without curved beam periodic structure are carried out. The experimental simulation results have demonstrated that by use of periodic structure, the vibration transmission at resonances of bases is reduced and the radiation of the foundation plate/shell at resonances is suppressed. The suppression is related to the structural vibration modes and dependent on the tested points and the frequencies. The modeling results also show that the performance of periodic structure is influenced by the mass ratio of the masses itself to the loaded masses. The vibration transmission in periodic structure is determined by the impedance loading on the input/output end. For the single-layer vibration isolation system connected through multiple isolators with proposed curved beam periodic structure, the experimental results show that responses on both the bases and the surface of the cylindrical shell are attenuated in the band gap of periodic structure. An average attenuation of 10~15dB on the bases and 4~5dB on the shell are obtained. Thus curved beam periodic structure offers a new means for controlling vibration tranmission in broad frequency band.Finally, the research and the contributions are summarized and some problems to be further studied are pointed out.