Research On Sound Radiation From The Flexible Base Of A Vibration Isolation System And Its Active Control

Ship hull noise induced by the vibration of power plants is a major noise source of ships.It is prone to produce vibration and sound radiation of the hull because the vibration of power plants reveals strong line spectrum characteristics and its response confined at low to mid frequency range which enlarge the coincidence of excitation and hull response frequency range.Vibration isolation is a major and effective technique to attenuate vibration to the base structure,the implementation of active vibration isolation could even improve the vibration isolation performance at low frequency range.Based on the requirement of quietness,comfort of ships and the stealth of submarines,subjects of the prediction of hull noise induced by the power plant,and moreover,the design and optimize of passive/active vibration isolation obtain widely research focus.Although the main concern is sound radiation from the hull structure,the coupling of power plants,vibration isolation structure and flexible base brings about complicated dynamic behavior.An universal flexible base vibration isolation coupling model was established in this these to discuss the mechanism of power transmission from power plant to hull structure.From the viewpoint of sound radiation modes analyse the mechanism of vibration energy to acoustic energy.The mechanism analysis of both vibration and sound radiation allow further discussion on the active vibration isolation technique oriented to minimized structure-borne sound.This main content of this thesis are listed as follow:The mobility/impedance analysis method was used in this thesis to establish an semi-analytical model of a flexible base vibration isolation system.Without lose of generality,the possible types of sub-structure of the vibration isolation system are considered and their impedance/mobility are given.To be more specific,in the frame of flexible base rigid equipment(FBRE)modeling method,the rigid body motion theory,multi-degree-of-freedom isolator,and energy method are available to obtain the response of the whole structure.Based on the vibration modal shapes of the coupled structure,emphasis were put on the coupling between substructures and the distribution of energy alone the system.Furthermore,the power transmission path from power plant to the base could be analysed.Experiments were conducted to verify the accuracy of modeling.Considering the shape characteristics of hull structure,the analytical form of radiation impedance matrix for both panel and cylindrical structure were derived.By introduced the concept of sound radiation modes,the sound radiation characteristics were analysed for the two types.Especially,the effect of depth on the radiation modes and radiation efficiencies were discussed.The analysis on sound radiation modes provided a effective tool to comprehend the conversion of vibration power to acoustic power by examine the coincide degree of vibration modal shapes and sound radiation modal shapes.Besides,the effect of boundary conditions on the sound radiation is discussed.Based on the obtained vibration and sound radiation transfer function,the feasibility and effectiveness of active vibration isolation oriented to minimum structure-borne sound were discussed.Different cost functions of feed-forward control were chosen and their control performance were compared and evaluated.The main evaluation criterion was set as sound radiation power,transmitted power,control force amplitude and sound pressure directivity were also considered to assess the effectiveness and the practical applicability.Moreover,different control configuration are considered.The mechanisms of effectiveness and control failures were analysed.At the end,the sound-structure interactions was analysed.A method based on sound reactance matrix to estimate the natural frequencies of 'wet' structure modes was presented.The theoretical sound reactance matrices were derived for both a baffled plate facing a layer of water and a baffled cylindrical shell facing infinite water.The effect of depth on the added mass to the plate,the control performance of feed-forward control and the gain of feedback control were examined.For the cylindrical shell,the distribution of added mass alone its surface explains the effect of added mass to different vibration modes.