Research On Dynamic Characteristics Of Power System Based On Substructure Receptance Method

Beam,circular plate,and cylinder shell are the typical types of structures used in aeronautic,aerospace,and ship engineering.The vibration characteristics of those finite structures with elastic boundary conditions and complex excitations are analyzed with the calculation method of equivalent mechanical receptance proposed in this thesis.The dynamic system of an unmanned underwater vehicle is taken as the study object,and an equivalent modeling method is proposed for the vibration analysis of various complex structures,such as engine,auxiliary machine,and ring isolator.Finally,a parametric model of the dynamic system is established,with which the effects of each exciting force and major parameters of each sub-structure are carefully analyzed,and the effects of clasp boundary condition and fluid load are also examined.Thus,by using equivalent mechanical receptance,a theoretical modeling method for vibro-acoustic analysis of complex structures is established,which can provide useful guidance in the design of noise-and-vibration reduction of practical complex structures.The main achievements and innovation points are as follows:1.The mechanical receptance of finite structure with elastic boundary conditions and complex excitations are examined.Compared with the existing theories,the supplementary function of improved Fourier series are described by a general form of trigonometric function.The effect of boundary condition on the vibration characteristics is studied in the aspect of modal frequency.With low-frequency vibration data,the parameter of elastic boundary stiffness is successfully retrieved by choosing appropriate vibration displacement function.According to the energy conservation law and orthogonality of trigonometric functions,the explicit expression of equivalent mechanical mobility are obtained,which makes a certain degree of simplification in solving the inconvenient formulations caused by complex excitation.The proposed method lays the foundation on and provides guidance for the analysis of complexly-coupled vibration system.2.An equivalent modeling method is proposed to analyze the vibration characteristics of some major structures,such as engine,auxiliary machine,and ring isolator,which makes it possible for complex structures to conduct parametric modeling and also expands the applications of mechanical receptance method in the practical engineering.The engine is divided into four sub-structures.External exciting forces and internal coupling forces are expanded to a series of line forces with cosine distribution along the circumferential direction,so as to unify the form of boundary and forces in vibration equation.This method provides important reference for equivalent modeling of complex structures,such as wobble-plate engines.For the auxiliary machineries,the modes has sparse distribution with very small number of modes located in the frequency band of interest.Based on this feature,parametric equivalent modeling method for auxiliary machineries is proposed using the first mode,which provides a simple but efficient way to examine the vibration characteristics of complex and compact structures.Besides,an equivalent modeling method is also proposed for the analysis of complex vibration isolators according to the Four-parameter model of ring isolator.In this model,the equivalent stiffness is obtained by the vibration transmitting rate calculated from the power device to cabin shell,which simplifies the analysis model of dynamic system.3.A parametric analysis model of torpedo dynamic system,including main engine,auxiliary machineries,ring isolators,and cabin shells,is established by using sub-structure receptance method.Also studied is the effect of clasp boundary condition and fluid load.Preliminary results show that the clasp has great effect on vibration characteristic of cabin shell in the low frequency,and the boundary condition should be taken into account while predicting sound radiation from platform test data.Based on the above results,an accurate vibro-acoustics prediction model is further established by using vibration data of the excitation source or cabin shell,which provides an accurate and efficient method for vibro-acoustics analysis.4.Based on the parametric model of the dynamic system,the following engineering instructive regularities are concluded:(1)Axial force(applied on cylinder block)and roller force make major contribution to the vibration response of cabin shell,and their transmission path should be fully retained in dynamic system model.(2)The vibration energy transferred from seawater pipe to cabin shell cannot be ignored,and the effect of seawater pipe on the low-frequency vibration is significant.Thus,the seawater pipe should be retained in dynamic system model.(3)Lateral force plays a major role in the structure vibration of cabin shell,and the vibration energy transferred from auxiliary machinery to cabin shell can be significantly reduced either by using low-density material or by decreasing the installation radius.(4)Rationally increasing the stiffness of ring isolator can reduce the low-frequency vibration response of cabin shell,but it has no significant effect on the high-frequency vibration.(5) Increasing the loss factor of cabin shell has little effect on the low-frequency vibration,but it is useful to reduce the high-frequency vibration of cabin shell.(6)Compared with existing measures of vibration-and-noise reduction,rationally designing the structural parameter of engine,such as increasing the radius of swash plate box,decreasing the length of swash plate box,and increasing the thickness of swash plate box,can reduce the vibration response of cabin shell significantly.