| Hydrodynamic calculation of ships advancing in waves is substance of seakeeping analysis and basis of ship design and optimization.With the variety of new ships and the improvement of their performance index,the demands for the development of ship hydrodynamics study are increasing.Despite that the evolution of computer technology encourages the application of computational dynamic fluids(CFD)method in the study of ship hydrodynamics,the obstacle brought by long time consumption limits its wide use in this area.Thus,the numerical methods based on three dimensional potential flow theory remain the major measures in present study.Traditional computation models with linearity,micro amplitude,frequency domain characteristics are becoming harder and harder to meet the demands of research and engineering practice,and the new computation models with nonlinearity,large amplitude,time domain characteristics are becoming the direction of studies.Recently,researchers at home and abroad mostly consider micro amplitude motions of ships in linear waves in studying ship hydrodynamics within field of potential flow theory and consider less nonlinerity in comptation models,for which,the study of large amplitude motions of ships in nonlinear waves and related physical phenomena is not sufficient.The same problem exists in most of commercial softwares based on traditional methods.Thus,how to establish a method with adequate accuracy,high efficiency and wide applicability for ship hydrodynamics analysis is the key point of seakeeping analysis,as well as,the major content of present thesis.In time-domain hydrodynamic calculation of ship with forward speed,boundary element method(BEM)incorporated with Green's 2nd theorem is adopted.With respect to the difference of Green's function,the methods in calculation are classified into 3 types:Rankine panel method(RPM)(or simple Green's function method),time domain free surface Green's function method(FSGM),multi-domain hybrid Green's function method(or multi-domain method).In Rankine panel method,the expression of Green's function is simple and the computation is stable.However,the amount of panels under discretization is huge,the truncation on free surface is required and far field radiation condition needs to be appended.In free-surface Green's function method,only ship body surface is required to be divided into panels to allocate sources and sinks.However,the expression of free surface Green's function is complex,the calculation of it is hard and time-consuming,Moreover,computation of Green's funciton nearby the free surface sometimes trend to oscillation divergent.Multi-domain method is proposed to inherit the advantages of RPM and FSGM.In this method,imaginary vertical control surfaces are used to divide the computation domain.In the inner part the Rankine panel method is applied,in the outer domain the free-surface Green's function method is used,on the control surface,the velocity potential and its normal derivatives are kept continuous,the velocity potentials in the inner and outer domain are obtained from joint solution.The Multi-domain method shows potential both in research and engineering practice and represents the development trend of numerical methods based on potential flow theory.Many problems remain in present study with multi-domain methods after investigations and these problems are summarized as following: 1)the nonlinearity components in inner domain are not sufficiently considered;2)the method used in computation of time-domain free-surface Green's function in outer domain is rough;3)the nonlinearity of incident wave itself is not sufficiently considered.Pointing to these 3 problems,in present thesis new Multi-domain methods are proposed to study computation of large amplitude nonlinear motions of ships with forward speed in waves and related applications.Firstly,in present thesis the improvements of inner domain of multi-domain method are applied and a new method named MDHOBEM?IW is proposed to take effect of coupled item of incident wave into consideration.In this method,theoretically,based on the assumption that the velocity potentials and induced wave elevations of steady ship wave and incident wave are in zero order,the velocity potentials and induced wave elevation of unsteady disturbance are in first order,we re-derived nonlinear dynamic free surface boundary condition,kinematic free surface boundary condition and body surface boundary condition.Together with governing equation,the definition solution problem under nonlinear boundary conditions is established.Compared with tranditionnal multi-domain method(MDHOBEM),this new method considers more of the effect of incident wave to radiation and diffraction probelm.Numerically,the discrete boundary integral equations(BIEs)are obtained incorporated with higher-order boundary element method(HOBEM).The unsteady disturbance velocity potential and hydrodynamics are then evaluated by solving BIEs,and substituting them into time domain motion equations,the time series of six degree motions are eventually obtained.5 ships with different shape characteristics(Wigley III,S175,S60,KCS and 50500T)are chosen as computation models.The motion responses of them in waves are calculated by MDHOBEM?IW and results are compared with experiment data,which validates the convergence and accuracy of present method.The parametric roll phenomenon of C11 ship in regular/irregular waves is studied by MDHOBME?IW.Compared with MDHOBEM,which only considered the effect of steady ship wave,MDHOBEM?IW shows advantages in predicting large amplitude ship motions in linear waves.Secondly,for the calculation of the outer domain,the improvement is made paralleled with inner domain.A new multi-domain method called MDHOBEM?VI is proposed,which is based on vertical integral form of Green's function.In this method,during the process of conducting the integral of Green's function on the control surface,the vertical integral is ahead of schedule,thus new form kernel function of panel integral is obtained.The 4-th order ordinary differential equation and initial conditions it satisfies are then derived,too.The kernel function in form of vertical integral has better convergence than original kernel function in point source form,numerical results validate that the stability of the kernel function in form of vertical integral is better,too.In addition,vertical integral method reduces the computation amount of memorial part in the outer domain,shortens computation time in calculation,especially for long time simulation.3 ships with different shape characteristics(Wigley III,S175 and 50500T)are chosen as computation models in study of their motion responses and added wave resistances.By comparison and analysis,conclusions are made that MDHOBEM?VI improves efficiency and stability of calculation while at the same time keeps the accuracy.Lastly,based on the improvement of inner and outer domain,a new multi-domain method called MDHOBEM?HOS is proposed with incident wave generated by higher order spectrum method(HOS).Compared with former study,this method not only considers the “wave-body” interaction,but also takes the evolution of waves themselves,namely the “wave-wave” interaction,and their nonlinearity into account.Results of numerical simulation show that the wave fields of waves generated by HOS method contain more nonlinearity than that of linear waves.3 ships with different shape characteristics(S175,S60 and one tumblehome form ship)are chosen as computation models in studying ship motion response in nonlinear waves by MDHOBEM?HOS,results of which validate better performance of MDHOBEM?HOS in capturing nonlinear phenomena and indicate the advantage of this method in predicting ship motions in dangerous and extreme sea conditions(such as parametric roll).In summary,based on three dimensional potential flow theory,improvements of multi-domain method are made in the three aspects of inner,outer and wave excitations in present thesis.Different levels of numerical model for ship hydrodynamics calculation are proposed,a set methods for studying large amplitude motions of ships advancing in nonlinear waves and its related application with adequate accuracy,high efficiency,high stability and wide applicability are developed,which would play an important role in research and engineering practice. |
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