The Research On Vibration Isolation Characteristic Of Floating Raft System With Dynamic Vibration Absorbers

The floating raft system is a novel isolation device for vibration control, which has now started to be widely used for vibration isolation of power plants on ships. The system can depress the mechanical vibratory energy transmission from the power plant to the ship hull; reduce the self-noise and radiation noise of the ship structures, meanwhile, effectively isolate the sudden shocks from the bottom of ship. The theoretical and experimental study on floating raft systems has drawn much attention in recent years. However, conventional floating raft system is limited to passive isolation, and it has some diadvantages in practical application. First, in the case that the ship is voyaging in low speed, the excitation frequency is equal to or lower than the natural frequency of the floating raft system, which will cause the system fail to work properly. Second, the system performance deviates its best state if the vibration source is changed to variable-frequency excitation due to the changing of voyaging ship speed, which will increase the vibration level on the base and radiated noise from the hull.This dissertation is a part of an engineering project item that is performed in the author's research group. It mainly focuses on the application of dynamic vibration absorbers (DVAs) on floating raft system by analyzing the power flow characteristics of the floating raft system with DVAs on it.The practical floating raft system usually consistes of box girder structures, which is complicated and difficult for analysis. Therefore, the first part of the dissertation discusses the feasibility of applying the DVAs on plate raft system, and provides theoretical foundation for the application of DVA on the practical complex floating raft system. The mathematical models of the complex coupled system with/without the DVAs are implemented by assembling the mobility matrices of the subsystems. Then the power flow transmission characteristics of the coupled system with/without the DVAs are investigated to evaluate and compare the isolation performance from the view of vibration energy transmission. The results show that DVAs, Semi-Active DVAs in particular, can significantly improve the isolation performance of the floating raft system. The results also give some valuable inspirations. Installing the DVAs opposite to the lower isolators will make the vibration attenuation effect more remarkable. For the passive DVA, when excitation frequency is equal to its resonant frequenc, it works properly; otherwise, it barely attenuate the vibration even exacerbate the vibration of the raft. The group consistes of DVAs with different resonant frequencies significantly broadens the frequency bandwidth of effective vibration control. Semi-Active DVAs have a wider frequency bandwidth of effect than passive DVAs. The system performance is sensitive to structural parameters of vibration absorbers such as damp and mass ratio. For the DVA that satisfies the system stability and other requirements, the one with smaller damp and larger mass ratio can significantly improve the performance of the floating raft system.Investigating the power flow in floating raft system based on the synthesis of mobility matrixes is the main method in this research area. However, the previous work mainly confines to simple structures, such as beam or plate, whose mobility matrix can be obtained by analysis. In addition, this qualitative method cannot satisfy the analysis requirements of practical problems. In the thesis, a FEM analysis method for investigating the characteristic of the vibratory power transmission of the complex floating raft system is proposed for the practical floating raft system.The method employs the fore-treatment module of FEM software to generate the structural model of complex floating raft system. In this dissertation, the model buildings of floating raft, vibration isolator and base are described. The influence of the element type and size on the analyzed results is discussed. The complex stiffness method is proposed to simulate the impedance of the vibration isolators in the finite element models, and the stiffness and damp coefficient varying with frequency are converted based on the principle from the velocity impedance measured in the testing. The proper frequency domain for the constant parameter model is also discussed. As for the basement, either the full-scale modeling or simple modeling based on complex stiffness method is applicable in this analysis method. By using the harmonic response analysis technique, the response of the linear system under multiple harmonic excitations in same frequency or different frequencies can be determined. Thus, the desired vectors required to calculate the power transmission could be obtained.The theme of this thesis is to do elementary research on the application of DVAs on floating raft system by theoretical, numerical and experiment method, so it is necessary to design an experimental platform. The platform should not only have the dynamic characteristics of the practical complex floating raft system, but also a small mass convenient for control by small effort. The interior damping of the raft dissipates the transmission energy from the vibrating source, which is one reason that the floating raft system has an outstanding performance in vibration isolation. Therefore, increasing the interior damping of the raft by some effective ways, such as using joint bolt connection or attaching rubber material to the raft, will improve the performance of the floating raft system and effectively depress the power flow transmission. A model floating raft system with a novel structural style is designed and realized for research purpose. The raft of the system is consisted of two parts called upper raft and lower raft connected with joint bolt. There are also some blocks, which can be steel or rubber material, between these two parts. More importantly, this combined type floating raft system provides enough space for installing the DVAs. Both the analysis and experimental results show that the model has a good performance of vibration isolation.In order to get the dynamic characteristics of the floating raft system at low frequency stage, some experiments are carried out on the model floating raft system with rigid foundation and elastic foundation. These experiments include measuring the structural frequency response function (FRF) and testing the vibration isolation performance of the system. By using the measured frequency response function (FRF) data to update analytical models, it provides some guidance for the parameters setting of the DVAs and a reliable analytical model for further analysis on the application of DVAs on the complex floating raft system. Based on the above work, the thesis studied the characteristics of the complex floating raft with DVAs on it combined with the experiment on damping capacity of the DVAs on the model floating raft system. This study gives estimation and prediction of the damping capacity of the DVAs under multiple operating modes. Experimental results coincide with the analyzed results, which demonstrate the reliability of the proposed method.