| As an indispensable part of ships, seawater pipe system plays an important part in ships, such as engine cooling systems, weight compensation and CO2 absorption. It serves as the ―vas system of ships‖ and sustains the life of ship. However, as the seawater pipe system works, undesired vibration and noise are produced. Such vibration and noise are concentrated in the low-frequency range. Low-frequency vibration and noise can be carried to a district far away with little attenuation, and they characterize an apparent spectrum in the frequency range. Moreover, the low-frequency sound and vibration are always co-coupled when they are transmitting in the seawater pipe system, thus accelerating the difficulty of sound and vibration control. Hence, the sound and vibration control problem for the seawater pipe system becomes an urgently problem during the design and fabricate of new ships.Recently, the development of Phononic Crystals(PCs) in the area of condensed matter physics brings a totally new idea to the sound and vibration control for the seawater pipe system in ships. The characteristics of band gaps(BGs) in PCs can be utilized to manipulate the wave propagation in such a periodic structure which is constructed according to the PCs theory, thereby providing a possible way to solve the current problem in seawater pipe systems.This dissertation introduces the PCs theory into the structure design for seawater pipe systems. By constructing the pipe wall to be axially periodic structure, the pipe system owns the classical character of BGs, thus the elastic waves propagating the seawater pipe can be controlled and improved. Such a periodic structure for seawater pipe is named as periodic pipe. The main findings and inclusions of this dissertation are given as follows:1. Develops the calculation method to describe the propagation of sound and vibration in periodic pipes. Based on the cylindrical shell theory, a semi-analytical finite element(FE) method is deduced for the calculation of structural BGs and vibration of various circumferential modes. Under the one-way and two-way fluid-structure interaction condition, the FE model is established to carry out the fluid-induced vibration investigation. Further, a transfer matrix(TM) method is developed for the investigation of acoustic states of a fluid-filled pipe with various anechoic instruments, as well as a FE model of acoustic medium coupled with the elastic pipe structure. The TM method and the FE model can well predict the noise propagation inside the periodic pipe system.2. Studies the band gap properties of structural waves of periodic pipes and the fluid-induced vibration of the system. Enlightened by the Bragg scattering(BS) mechanism and locally resonant(LR) mechanism of PCs, several periodic structures for the seawater pipe is constructed, the and the formation mechanism and the parameter modulating law of elastic BGs are illuminated. Moreover, the effects of fluid velocity and boundary condition on the BGs are analyzed, following with the exploration of fluid-induced vibration characteristics for periodic pipes. In the present work, the BG formation mechanism of different circumferential modes and the possible instability condition of periodic pipe are discovered for the first time. The low-frequency super-wide BGs of a BS type coupled with a LR type is firstly obtained in such a fluid-structure interaction system as pipe system. Also, to the best of the author’s knowledge, the control effects of periodic pipes on fluid-induced vibration under the one-way and two-way coupling condition are never be studied before.3. Analyzes the influence of several parameters on the band gap of acoustic wave and reveals the formation mechanism for acoustic bands. By installing the expanded muffler with/without extended neck and the Helmholtz muffler into the seawater pipe system periodically, the BS- and LR- acoustic BGs are obtained, making the propagation of acoustic wave in the system manipulable. Results show that: the BGs formatted by the expanded muffler array obey the BS mechanism, thus their central frequencies satisfy the Bragg condition, and the contour of the maximum sound insulation within the BGs displays the same law as that of a single muffler; the Helmholtz muffler array in the pipe system actually serve as the acoustic LR-type periodic structure, thus giving rise to low-frequency LR BGs and realizing ―small size controlling big wavelengths‖ in the acoustics area; Synthesizing the expand cavity and Helmholtz resonant cavity into one new muffler and placing it periodically in the pipe system, a super-wide BG with strong attenuation coefficient in the low frequency could be produced, as well as several broad BGs in higher frequency domain.4. Proposes a periodic structure for controlling both the sound and vibration in seawater pipes. Based on the LR mechanism and the Bragg scattering mechanism of PCs, a periodic pipe within which both the sound and the vibration transmitting in the system can be controlled is design, realizing utilizing one periodic structure obtaining the acoustic and structural BGs simultaneously. Such a structure is named as sound and vibration co-control(SVCC) periodic pipe. Further, the law of parameter influence on the BGs and the formation mechanism of acoustic-structural BGs are explored. The construction of such a SVCC periodic pipe is a primarily work in this area, as well as the revealment of formation mechanism of acoustic BG in the Bragg periodic pipe structure of which its basic cell is consisted of a nonmetal material tube connected with a steel tube(or with a muffler placed in the middle of the nonmetal tube).5. Validates the suppression abilities of the periodic pipes. A pipe experimental system is built and the sound insulation and vibration transmission test experiment of the SVCC periodic pipes are carried out. Experimental results strongly improve the control capability of sound and vibration in either a SVCC pipe constructed based on the BS mechanism or a SVCC pipe constructed based on the LR mechanism.In conclusion, the research on the propagation properties of elastic waves in periodic pipes provides a new technology for sound and vibration of seawater pipe systems. Such a technology may solve the problem of sound and vibration suppression in seawater pipes of ships. |
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