Modeling And Active Control Of Coupled Vibration Of A Shaft-hull System

Stealth performance of underwater vehicles is always one of the key points of low noise equipment research. In the past few decades, lots of investigations were dedicated to mechanisms and control strategies of structural vibration and acoustic radiation. The former focuses on vibration and acoustic characteristics of composite structures like beams, plates and shells, and the latter focuses on control principles as well as realization. Shaft-hull coupled vibration induced by the propeller in the wake field is a major part of structural vibration at low frequency. While passive vibration isolation has limited performance at low frequency, vibration is hard to suppress in this bandwidth. On the basis of current research and with the aim to suppress vibration induced by the propeller thrust force, this thesis discusses some basic problems and discloses some vibration principles.Considering interaction between shaft and hull, this thesis had established a synthesized model of these substructures, analyzed natural characteristics of coupled system and influence of parameter variation to the whole system (such as stiffness of the thrust bearing), compared commons and differences of vibration under propeller disturbance excited-force and excited-control force. Through properly controlling vibration transmission at source, control strategy from above is applied on shaft to attenuate hull vibration. The conducted investigations in this thesis are listed as follows.In Chapter 2, established coupled vibration model of shaft-hull system with FRFs synthesis method, analysis method and finite element method. Analysis method is employed to compute the natural characteristics of three-dimension vibration of shaft which has a mass in the end, FEM is employed to study vibration of stiffened shell, from these processes FRF has been obtained. Based on this, frequency description model of system is gained with linking units through FRFs synthesis method, the model is verified by FEM. System vibration under propeller thrust force is discussed, FRF figures and mode shapes, etc, are obtained.In chapter 3, based on model which is established in chapter2, effects of parameters variation brought to system are analyzed, system vibration between control force excited at different locations and propeller disturbance excited are compared, dynamical characteristics under lateral disturbance force excited is discussed. The result shows that both stiffness of longitudinal bar and thrust bearing have magnificent influence to longitudinal vibration of shaft, the frequency would increase along with stiffness growth, but it will keep unchanged when the stiffness is large enough. At the same condition, longitudinal vibration frequency of shaft is determined by the relative small stiffness. With respect to control force location, result shows that it applied on shaft is a possible way while foundation is not an ideal location in that there is a significant insensitive area in its FRF. Analysis about vibration under lateral force excited shows that there are more peak values in longitudinal vibration under lateral excited than longitudinal excited, while lateral vibration under lateral excited has nothing different between longitudinal excited but has bigger magnitude. Otherwise, longitudinal vibration frequency of shell under lateral excited is sensitive to support stiffness, when this stiffness become bigger, longitudinal vibration frequency of shell would quickly increase to the same value of under longitudinal excited.In chapter 4, aiming to control the vibration induced by propeller thrust force, a new strategy has been applied which bring control force on shaft to cancel propeller thrust force to attenuate shaft longitudinal vibration, in turn to suppress hull vibration. Configuration like adaptive filter and NLMS algorithm are employed. Simulation of control shows that this method could effectively control system vibration, and required control forces are different when thrust bearing stiffness variation.In chapter 5, an experimental model of shaft-hull system is built. Shell and foundation testing modal, experimental FRF under different circumstance as well as active vibration control are analyzed by measurement and control. Experimental results show that shaft longitudinal vibration frequency would increase under shaft rotating speed growth with unchangeable static thrust. Adaptive method can be applied to control vibration transmission of this system. Therefore, experimental results have proved the theoretical analyses.