| When a ship advancing in the seaway,ship wave and ship motion in waves are basics of speedability and seakeeping analysis.Studies of ship wave and ship motion are also significant for hull form design and optimization.Development of the analysis of ship hydrodynamics are currently in the trend from linear frequency domain to nonlinear time domain,and from single to comprehensive ship navigation performance.With the development of computer and computational science,computational fluid dynamics(CFD)are adopted for simulation of ship wave and ship motion in waves more and more Frequently.While the method is quite time-consuming for solving problem with free surface waves and does not meet the requirement of high efficiency of performance prediction in engineering design.Therefore,common commercial software used in early design stage of ship and ocean engineering is developed with potential method,such as WAMIT,SESAM,AQWA,HYDROSTAR,and others.Deficiencies of the commercial software are that most kinds are based on linear theory,so they are often invalid for nonlinear problem and relatively weak for conditions with forward speed.So time domain analysis is carried out for ship wave and motion in waves of navigating ship in the present paper.This work also attempts to incorporate problems of ship wave and wave-ship interactions in hydrodynamic analysis.The corresponding numerical program is developed.By studying on linear and nonlinear ship wave,ship motion in waves,hydrodynamic loads and second order wave force,improvements of conventional numerical methods for ship hydrodynamics analysis are expected to enhance numerical accuracy and stability of time domain hydrodynamics analysis of ship with forward speed,and to broaden the scope of applications of potential flow method in ship hydrodynamic performance prediction.Green's function methods are the most common technique used for potential flow computations.The free surface Green's function(FSG)method is developed in early stages.Because of the highly oscillatory nature of the FSG near free surface,numerical evaluation of the FSG is quite difficult for problem of navigating ship and the computation is apt to divergence for ships with large flare.Rankine panel method is unconditionally stable,although the discretize quantity is large.Since the multi-threads algorithm is widely used various numerical computations at present,Rankine panel method incorporated with multi-threads parallel computing technique is quite efficient.Besides,in the analysis of time domain ship motions with forward speed,ship wave effects on ship motion are conveniently taken into consideration by using Rankine panel method,which can overcome the limitation of Neumann-Kelvin linearized free surface condition used in FSG method.Consequently,speed effects are considered more precisely and accuracy of computations of ship motion and loads are improved.In Rankine panel method,panel discretization is need for infinite free surface theoretically.Therefore,a multi-domain high order boundary element method for nonlinear time domain ship motions is proposed.Rankine panel method with nonlinear time domain free surface condition is applied in inner domain.By introducing transition function,time domain FSG method can be directed used in outer domain.As a result,radiation is analytically satisfied and computation is stable.In addition,discretized quantity decreases dramatically as size of inner domain reduces.Ship wave is investigated in the first place.The present numerical method has the following improvements: A combined nonlinear free surface condition is adopted;Sinkage and trim is determined by nonlinear iteration,and the ship attitude is taken into consideration in resistance computation;HOBEM is used to solve boundary integral equation and a parallel computation is carried out.Numerical studied are conducted for 7 types of hulls.Physical problems of ship waves of various hulls,as well as sinkage and trim effects on wave drag are detailedly investigated.Followed by comparative studies of linear and nonlinear ship waves.Take into consideration of ship wave effects in the derivation the initial boundary value problem of wave-ship interactions with forward speed.Then the numerical program for solving time domain ship motions by Rankine panel method incorporated with HOBEM is developed.Four types of hulls are taken as numerical example.Hydrodynamic loads and ship motions are computed by the present numerical program.Numerical results indicate the present method by considering ship wave effects has higher accuracy than the Neumann-Kelvin linearized method which is frequently-used in previous research.Comparing to double-body flow linearized method,motions of realistic ships in relatively high speed and of full-formed ships can be improved.In the developed multi-domain high order boundary element method(MDHOBEM),the ship wave linearized boundary conditions are imposed in inner domain for the first time.Nonlinear wave force and nonlinear restoring force are taken into time domain ship motion computations.Numerical investigations are carried out for 4 types of hulls.It demonstrates the MDHOBEM is more accurate than classic BEMs.Size of inner domain can be restricted to a quite small value in the present computation.Consequently,it reduces more than 40% discretized quantities comparing to Rankine panel method.The above theory is applied to analize time domain second order wave force and added wave resistance,as well as analysis of hydrodynamic interactions of side by side arranged vessels in close proximity.The near field integral equation is derived by perturbation method.Physical meanings of different components of second order force are detailedly analyzed.Results indicate the coupled terms of first order force,motion,velocity and free surface elevation are main contributions.Three dimensional energy radiated formula based on time domain results is also derived for evaluation of added wave resistance.For various hulls,proportions of components of added wave resistance are investigated by near field method.Besides,comparative studies are carried out for the present near field method and energy radiated method,as well as other numerical methods.For the analysis of multi-body hydrodynamic interactions,numerical studies are carried out for a simplified LNG-FPSO model in zero speed case,and two ships advancing on parallel course in close proximity.Hydrodynamic coefficients,wave loads and motions,as well as variation of motions with lateral distance and forward speed are discussed in detailed.Numerical methods proposed in the present paper effectively improved accuracy of time domain hydrodynamic analysis of ships with forward speed.Nonlinear effects of the change of boundary,coupling of flow and wave force are considered as well,so to fill up domestic research.The present work is of both theoretical and engineering significance for comprehensive hydrodynamics analysis of voyaging ships. |
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