Study On Fluid-structure Interaction Mechanism And Algorithm Of Propellers And Hydrofoils

The study on hydroelastic problems of propellers and hydrofoils is of great importance for both the vibration/noise reduction of ships and the development in the hydroelasticity theory of ships.This thesis focuses on the fluid-structure interaction analysis of elastic propellers and hydrofoils.Two-way strongly coupled fluid-structure interaction models for elastic propellers and hydrofoils are established.The three-dimensional panel method combined with the finite element method is employed to study the unsteady hydrodynamic performance of elastic propellers and hydrofoils.The important parameters which affect the added mass and damping matrices of the elastic hydrofoils and propellers are examined.Dynamic strains and bearing forces of the elastic propellers are calculated.In addition,the added mass and damping coefficients of a rotating marine propeller vibrating in a six-degree motion are analyzed by considering the relationship between the added matrices of the rotating marine propeller due to local vibrations of the propeller blades.The unsteady bearing forces due to propellers vibrating in the six-degree motion are also investigated.The primary contributions of this thesis are as follows:(1)Based on the non-penetration condition on the fluid-structure interface,numerical models are established for two-way strongly coupled fluid-structure interaction analysis of underwater propellers/hydrofoils.The hydrodynamic force acting on the structures can be represented by Fr+Fv.Here,Fr contains the hydrodynamic forces due to the rigid propeller rotating in the nonuniform flow(rigid hydrofoil advancing in the nonuniform flow),which is the driving force of the fluid-structure system.Fv represents the hydrodynamic forces due to elastic propeller/hydrofoil vibrating in the uniform flow,resulting in flow induced added mass and damping matrices.These flow induced matrices may be combined with the structural mass and damping matrices to achieve the decoupling of the two-way fluid-structure interaction problem.In this thesis,a three-dimensional panel method in time and frequency domains for fluid combined with the finite element method for the structures is developed.The proposed method can solve the full nonlinear Bernoulli formula to determine the driving force of the fluid-structure coupling system directly.The application of the pure frequency-dependent panel method coupled with the finite element method is not straightforward to deal with the nonlinear terms of the nonlinear Bernoulli formula.In addition,the present method overcomes the disadvantages of the pure time-dependent panel method coupled with the finite element method.The proposed method provides great physical insights into the effect of the added matrices explicitly and the computational cost required by the present method is considerable smaller than that of the coupled time-dependent panel/finite element method.The proposed numerical method is employed to study the dynamic strains and bearing forces of the elastic propellers.It is found that the elasticity of the propellers and the added-damping due to the fluid are crucial importance in the design of propeller-shaft-ship system with lower vibration and noise level.It is also observed that the non-penetration conditions may be imposed on the undeformed surface of the propellers to predict the unsteady bearing forces in the spatially non-uniform inflow.(2)The added mass and damping matrices for the elastic hydrofoils moving in water are investigated based on the two-way fluid-structure interaction models of elastic hydrofoils.It is assumed that the hydrofoils vibrate in uniform flow.The frequency-dependent panel method and finite element method is employed in the present analysis.The most important parameters affecting the added mass and damping matrices of the elastic hydrofoils are determined,and the effect of different boundary conditions on those added matrices are also analyzed.If the geometry of the wake sheet is unchangeable and no cavitation exist in the inviscid fluid,the following conclusions can be drawn from present analyses:1)the stationary flow is sufficient for determining the wet modes of the hydrofoils;2)in the case of relatively small reduced frequency fvc/V(fv,c and V are the excitation frequency,the chord length and axial inflow velocity,respectively),the added damping of the fluid can significantly affect the structural responses of the hydrofoils and reduce the amplitude of the response near resonance;3)the attack angle has insignificant effect on the added mass and damping matrices;4)the non-penetration condition imposed on the fluid-structure interface of the hydrofoils is equivalent to the case imposed on the undeformed surface of the hydrofoils.(3)Numerical method for computing the added mass and damping coefficients due to the vibration of a rigid propeller with six degrees of freedom is developed by considering the relationship between the added matrices of the rotating elastic propeller due to local vibrations of propeller blades.The effects of key parameters and different boundary conditions on the added mass and damping coefficients are investigated.The results indicate that:1)the ratio fv/fa can significantly affect the added mass and damping,which plays an important role in the effect of advance ratio on the added mass and damping of the propeller.The effect of advance ratio on the added mass and damping is much more significant at lower ratio fv/fa(fv/f?1);2)the non-penetration boundary conditions can be imposed on the undeformed blade surface for analyzing the unsteady bearing force due to the longitudinal vibration of the shaft,while for predicting the unsteady bearing force due to the lateral vibration of the shaft,the boundary conditions should be imposed on the deformed surface of the blade.Furthermore,the discrepancy of the bearing forces due to the lateral vibration computed based on the two types of boundary conditions become notable when the ratio is relatively small(fv/fa ?1);3)the absolute values of all coefficients in the added mass matrix decrease as the ratio fv/fa is increased,and the absolute values of the coefficients in the added damping matrix increase with an increase in the advance ratio.In addition,the unsteady axial bearing forces induced by the axial vibration of the rigid propeller is found to be negligible compared to the bearing force generated by the elastic propeller rotating in the non-uniform inflow.(4)Three-dimensional panel methods in frequency domain combined with the finite element method are employed to establish the two-way fluid-structure interaction model of elastic propellers.The propeller is assumed to vibrate in the uniform flow.Physical insight into the added mass and damping of a rotating elastic marine propeller is considered.The important parameters and different boundary conditions which may affect the added mass and damping matrices of marine propellers are analyzed.Under the assumptions of unchangeable geometry of the wake sheet and inviscid flow without cavitation,the results show that fv/fa may affect the added mass and the effect of advance ratio on the wet modes due to the fact that fv/fa affect the applicability of different boundary conditions.In addition,fv/fa can significantly affect the added damping,and the effect of advance ratio on the added damping also depends upon fv/fa.The following conclusions can be drawn from the analysis:1)in the case of fv/fa>1,the non-penetration boundary conditions may be imposed on the undeformed surface of the propeller in order to calculate the added mass and damping matrices;the effect of the advance ratio on the added mass and damping matrices can be neglected;2)at a lower ratio fv/fa(fv/fa ?1),the non-penetration boundary conditions should be imposed on the deformed blade surface rather than on the undeformed one for predicting the added mass matrix.However,the non-penetration boundary condition can be applied on the undeformed surface of the propeller blade for calculating the added damping matrix.In addition,the natural frequencies of the wet modes of the propellers decrease as the ratio fv/fa decreases.It is also observed that the effect of the advance radio on the added matrices should not be neglected for lower ratio fv/fa(fv/fa ?1).As the advance ratio is increased,the wet mode frequencies of the propeller will increase and the effect of the added damping will be less significant;3)the added damping should not be neglected regardless of the value of the ratio fv/fa.The amplitude of the unsteady bearing force can be significantly reduced by the added damping.The reduction of the amplitude of the unsteady bearing force achieves its maximum in the case of fv/fa=1.