Research On Longitudinal Vibration Characteristics Of Warship Shafting And Its Semi-active Control Technology Based On Magnetorheological Elastomers

The propeller is the main propulsion of ships and underwater vehicles, which works in spatially non-uniform wake field wherein axial pulse forces are produced and transferred to the hull through the propulsion shaft, thrust bearing, and bases. Consequently, axial vibration of shafting occurs and propagates through the supporting structure to the hull, where the vibration is radiated as structure-borne noise. It is the main source of ship radiated noises and should be paid more attention. Studying on the transmission law of propeller exciting forces and the internal vibration mechanism in propulsion shaft is very important to take the effective control measures and is the key to controlling the low frequency vibrations and noises of ship excited by stern. In this paper, the longitudinal vibration, transversal vibration, and the longitudinal and transversal coupling vibration of propulsion shaft are studied and the parameter affecting law is also investigated. The semi-active vibration control measures are proposed to control the longitudinal vibration of propulsion shaft. A semi-active dynamic vibration absorber(SDVA) using magneto-rheological elastomers(MREs) is designed to control the longitudinal vibration of shaft. Experimental research has been carried out to verify its frequency-shift property and vibration absorption capacity. The specific contents include:(1) The structure of propulsion shaft of submarine is introduced and simplified. The natural frequencies and model shapes of longitudinal and transversal vibrations of propulsion shaft are derived using the analytical method. The characteristics of longitudinal and transversal linear vibrations of propulsion shaft are analyzed. The dynamic equation of longitudinal and transversal coupling vibration of shafting is derived using the elastic mechanic method. According to the actual structure, the analysis model of propulsion shaft is built using FEM. The responses of longitudinal and transversal coupling vibration in time and frequency domains are calculated. The law of coupling vibration responses under different excitation force amplitudes, different excitation frequencies, and different damping in extreme load situation is also investigated. The results have certain guidance for the vibration and noise control of propulsion shaft.(2) A MRE-based dynamic vibration absorber connected to ship shafting in parallel is introduced. The mechanical system of ship shafting with absorber is set up using the modal interception and synthesis method, which the propulsion shaft is considered as a flexible continuous beam. The longitudinal vibration of propulsion shaft influenced by the parameters of dynamic vibration absorber is analyzed. The material characteristic of main preparation materials of MREs is introduced. The simple dipole model is introduced to study on the magneto-mechanical properties of MREs and the main factors affecting the magneto-induced shear modulus of MREs. Moreover, a macroscopic viscoelasticity model described the mechanical performance of MREs is proposed to analyze the mechanical properties of MREs depending on external magnetic field.(3) The preparation process of MREs is introduced. The MREs based on natural rubber(NR) are fabricated in this work. The modulus and loss factor have been evaluated by using a modified dynamic mechanical analyzer(DMA) and the test data has been fitted. A semi-active dynamic vibration absorber(SDVA) by making use of the variable shear modulus properties of MREs is designed to control the longitudinal vibration of shaft. The structure of SDVA is proposed and the theoretical simulation analyses have also been carried out to investigate the frequency-shift property of single one and the vibration absorption capacity of three ones in combined type.(4) Through experimental research of the MRE-based dynamic vibration absorber, the frequency-shift property of single one and the vibration absorption capacity of three ones in combined type have been verified. The effect of control current and temperature on the natural frequency of single MRE-based dynamic vibration absorber is studied. The research lays on the foundation for the application of the MRE-based dynamic vibration absorber in the warship.Based on the mechanism analyses, the parameter control measures of the ship shafting system are suggested to reduce the responses of longitudinal and transversal coupling vibration of propulsion shaft. An SDVA based on MREs are proposed to control the longitudinal vibration of propulsion shaft. The designed SDVA has been verified it has better performance in terms of frequency-shift property and vibration absorption capacity. The results are helpful to the design of underwater vehicles for lower noises.