Study On Fluid-Structure Interaction Dynamics Of Propeller-shaft Systems And Longitudinal Exciting Force Characteristics

Excitation forces are inevitably generated when the marine propeller works,and they can be categorized into two forms.One form is the primary excitation force of the propeller induced by the nonuniform inflow that is attributed to the hull stern surrounding flow,shaft tilting and ocean turbulence.The other form is the secondary excitation force of the propeller caused by the vibration of the shaft system.The total excitation force of the propeller is the combination of the above-mentioned two forms.The propeller excitation force can cause the vibrations of propeller blades as well as the shaft system,and even the hull.Therefore,studing on Fluid Structure Interaction(FSI)dynamic characteristics of fluid,propeller and shaft system;revealing the law that how fluid,propeller and shaft system and their coupled factor influence the excitation force;and further investigating the relationship of the primary and secondary excitation forces play crucial roles in engineering.Currently,literatures on this critical issue are mostly focused on specific aspects or a certain detail.For example,some researchers only study the FSI characteristics of the propeller or only study the vibration characteristics of the shaft system.Few literatures tackle this issue from global perspective that fully reveal the coupled characteristics among fluid,propeller and shaft;and further study the longitudinal excitation force transmitted to the hull and its impact factors under the complex coupled factors mentioned above.So in this paper,the bidirection FSI dynamic model of fluid-elastic propeller-shaft system is built.Then dynamic characteristics of FSI of propeller and shaft system are studied.Furthermore,the relationship between the primary excitation force and the secondary excitation force is analyzed.In addition,the transmissibility characteristics and enhancement effect of the longitudinal excitation force due to the shaft system are investigated.Then,the impacts and evolutional laws of the longitudinal excitation force that generated from the elastic effect of the blade,longitudinal vibration of the shaft system and the dynamic match characteristics of the propeller and the shaft system are ultimately revealed.Lastly,to minimize the longitudinal excitation force transmitted to the hull as the optimization target,and starting from the mass and frequency ratios of propeller and shaft,the optimized design method of the propeller-shaft system is preliminarily discussed.The study of this paper can be manifested into several aspects as below.1.With Boundary Element Method(BEM),and based on the theories of rotor dynamics,the forecast model and its calculation method of propeller excitation force under nonuniform flow and vibration of the shaft system simultaneously are proposed.Then,considering the coupled effect of fluid,elastic propeller and shaft system,using theories of structural dynamics,the bidirection FSI dynamic model of fluid-propeller-shaft system is built with Finite Element Method(FEM)and BEM.At last,compared with the previous experiments and calculation results from business softwares,the correctness of the FSI dynamic model and the efficacy of the program are validated.2.Based on FSI dynamic model,the vibrational characteristics of propeller blades and the shaft system are comparatively studied with and without the shaft system,under unidirection and bidirection FSI.Under nonuniform flow,the characteristics of the propeller longitudinal excitation force is comparatively studied using three models,i.e.the rigid propeller,the unidirection or bidirection FSI of elastic propeller,from which the elastic effect of the propeller to the characteristics of the longitudinal excitation force is then analyzed.The results show that when the deformation is large,or in the inflow there are high frequency components whose magnitude is large too,the bidirection FSI model should be applied,otherwise errors can be introduced.Compared with the rigid propeller,the longitudinal excitation force of the elastic propeller can be effectively magnified near the natural frequencies of the blades and also can be reduced in some frequency range.The excitation force can be optimized in the working rotation speed range with proper skew angle.3.Based on FSI dynamic model,the relationship between the primary and the secondary excitation force of the propeller is studied with numerical calculation and theoretical deduction.The results show that the longitudinal secondary excitation force is directly proportional to the magnitude of the longitudinal vibration of the shaft system,and is also directly proportional to the square of the frequency of the longitudinal vibration.With a series calculation of the propellers of different radii,blades,pitch ratios and area ratios,the estimation formula of the second order excitation force is fitted.4.Based on FSI dynamic model,the impact of coupled effect between the propeller and the shaft system on the coupled dynamic characteristics of elastic propeller-shaft system and their umbrella modals are studied.The results show that the larger the coupled effect is,the larger degree of the couple umbrella modal will increase.When the coupled effect is small,the influence of the shaft system to the propeller is like fixed support,and the opposite influence can be seen as appendage mass effect.Through the further deduction,the degree of coupled effect between the propeller and the shaft system is direct proportional to the number of blade Z and the mass ratio μ from the propeller to the shaft system,and is inverse proportional to the frequency ratio λ from the first order natural frequency of the propeller to the shaft system.5.Based on FSI dynamic model,the effect of the shaft system on the characteristics of the propeller longitudinal excitation force is then analyzed.The results show that the shaft has the transmissibility characteristics and enhancement effect for the longitudinal excitation force.The transmissibility characteristics mean that due to the shaft,the longitudinal excitation force will be magnified at the first order longitudinal modal of the the shaft system and the first order coupled longitudinal modal of the propeller-shaft system(coupled umbrella modal),and will not be magnified at the first order bending modal of blades(non-umbrella modal).The enhancement effect means that when the excitation frequency is below the first order longitudinal modal of the shaft system,the phase difference between the second order and the first order excitation force is always less than 90o.Therefore,the shaft has enhanced effect to the total propeller excitation force.6.The modal distribution law of the propeller-shaft system is studied by simplifying the system as a series of lumped mass-spring model.Compared with the results of FEM model,lumped parameters model can effectively simulate the first order longitudinal coupled modals of propeller-shaft system.Afterwards,the analytical expressions of the transmissibility of the longitudinal excitation force of this lumped parameters model are deducted,which theoretically explains the reason why the excitation force that transmit to the hull will only be magnified at the first order longitudinal modal of the shaft system and the first order coupled umbrella modal.Finally,based on the lumped mass model,the design of the propeller-shaft system is optimized by respectively analyzed the main parameters of the propeller and shaft(mass ratio,stiffness ratio,frequency ratio).The optimizational results show that the smaller the mass ratio,the larger the frequency ratio is,the better the longitudinal excitation force transmit.7.The metal and plastic propellers with same geometry are respectively manufactured,and then their dry and wet modals(coupled with shaft)are respectively measured on the propeller-shaft vibration experimental platform in the gravity water tunnel.The nonuniform flow is built by the wake screen and its nonuniformity is then measured by Laser Doppler Velocimetry(LDV).The test results show that the nonuniform flow works well.Then,the characteristic of the longitudinal excitation force of the metal and plastic propeller are respectively tested when the advance ratio is constant.Lastly,the relationship between the longitudinal vibration of the shaft system and its inducing secondary excitation force of the propeller is tested.All the experiment results vaildate the efficiacy of the FSI dynamic model or the conclusion of this paper.The study of this paper is instructive to the design and analysis of the propeller-shaft system.