Active Control Of Sound Radiated From A Submerged Finite Cylindrical Shell

The low-frequency line spectrum noise is an important component of self-noise in underwater vehicles, and may be effective features for enemy’s underwater acoustic detecing system, which can result in a significant impact on the safety and attack ability for underwater vehicles. The line spectrum noise of the underwater vehicle is closely related with the vibration characteristics of the dynamical system, transmission and rotational systems, which cannot be effectively controlled by passive methods, whereas active methods can be used to control it. In this dissertation, the underwater vehicle is simplified as a finite cylindrical shell, the active control of noise radiated from such a structure is investigated based on the analysis of the low frequency vibro-acoustic characteristics. The main work of this thesis is summarized as follows.(1) The low frequency vibro-acoustic characteristics of a finite cylindrical shell are analyzed. Aiming a finite cylindrical shell with simple support boundary, the vibro-acoustic theoretical model is established using Flugge’s thin shell theory. The low-frequency vibro-acoustic characteristics of the shells in different fluids are analyzed, and a computational method of the acoustic radiation modes is given. Finally, the relationship between the vibration modes and the acoustic radiation modes at low frequencies in different fluids is investigated, and the dominant acoustic radiation modes corresponding to different fluids are determined.(2) Active control of sound radiated from a submerged finite cylindrical shell based on force sources is investigated. Firstly, considering the fluid-structure coupling interaction, the analytic expression of the optimal complex intensity of secondary forces is derived. When comparing the control effects with single control force and two control forces, it is found that the radiation sound of the submerged finite cylindrical shell can be controlled by arranging exciters on the shells to generate control force, optimizing the location and number of the exciters, and manipulating the amplitude and phase of the control forces. In addition, the radiated sound can be attenuated through controlling the dominant acoustic radiation modes at low frequencies. From the directionality of the total sound pressures and the modal sound pressures before and after active control, the control mechanisms are different for different secondary control force locations at the same frequency, thus the control effects are not the same. When aiming to minimize the total sound power, the physical mechanism of active control lies in reducing the amplitudes of the dominant acoustic radiation modes corresponding to structural vibration modes, and restricting the increase of the amplitudes of other acoustic radiation modes at the same time, which ensures effective control of the sound radiated from structural vibration.(3) Active control of sound radiated from a submerged finite cylindrical shell based on sound sources is investigated. Firstly, the mutual radiation impedance between the primary cylindrical shell and secondary sound sources is calculated, the influence of mutual radiation impedance on low-frequency radiation sound power is discussed. Then considering the fluid-structure interaction effects and using quadratic optimal theory, the optimal secondary source strength is derived targeting at the minimization of the radiated sound power. The influences of the distance between the primary cylindrical shells and secondary sources and the position of the secondary excitation on control effects are discussed. Moreover, the spatial distribution of far-field sound pressure is observed and compared before and after control. Finally, when local control is applied, the sound pressure at the positions of the error sensors are observably reduced when aiming to minimize the sound pressure mean square and an averge reduction of about10dB can be achieved.(4) Near-field and virtual error sensing for active control of sound radiated from a submerged finite cylindrical shell are proposed. Using the discrete structural acoustical sensors and virtual sensors, the surface vibration signal or near-field sound pressure of the structure can be obtained through the accelerometer mounted on the structural surface or hydrophone. Then the sound pressure at specific locations is estimated using the sensor transfer functions. Active control can be achieved by targeting at minimizing the estimated pressure. The results show that similar effects are acquired by the minimization of estimated far-field pressure and direct minimization of far field sound pressure.(5) The experimental study on active control of sound radiation from a submerged finite cylindrical shell is carried out using secondary force sources and sound sources. The effects of active noise control, convergence of the algorithm, stability of the adaptive controller are verified, and the influence of the frequency components and variation of the primary noise, the configuration of the secondary actuators and error sensors on the control effect is also analyzed.