Investigation On The Virtual Experiments By RANS Method And Analysis Technologies For Ship Resistance And Seakeeping

Ship resistance and seakeeping performance have always been a great concern for ship designers and users.There are three kinds of methods to evaluate the performance of a ship: model experimental,theoretical and numerical methods.Model experiments are the most reliable at present.However,the experimental cost is high,and only limited fluid field information can be obtained.The model scale,the disturbance of the flow field and the precision level of apparatus have influences on experimental data.Theoretical methods are limited to the structure with the simple geometric shape.For numerical methods,boundary element method based on potential theory has a high efficiency in solving hydrodynamic problems but neglects the fluid viscosity.In recent years,with the development of computer science and technology,computational fluid dynamics(CFD)method that solves Reynolds-averaged Navier-Stokes(RANS)equation has been developed rapidly.CFD method is more efficient than model experiments,also considers fluid viscosity.Therefore,it has become an indispensable tool to solve hydrodynamic problems in naval architecture and ocean engineering.In the world,many major maritime research institutes have regarded CFD as their Facilities & Tools.And a number of scientific research institutes work together to develop the virtual tanks or numerical tanks,such as the VIRTUE(The Virtual Tank Utility in Europe),and China Numerical Tank.Virtual tank for ship performance is popular and hot,and further research on the virtual experiments of ship performance and analysis technology is of great significance for improving ship design.The recent researches on the virtual tank for ship performance are reviewed.And it is found that a good progress has been made in the study on the prediction of ship performance with virtual experiments.However,the application range of virtual tank needs to be widened and the mechanisms of ship resistance and seakeeping performance need to be studied further,such as(1)CFD can be regarded as a reliable tool to determine the resistance of ships.However,in comparison to model scale experiments,some deviation in resistance of model ship may still occur.(2)It is known that the environment in the ocean is complicated,the study on the motion response and added resistance of ship in waves is generally for the ship in head waves,while the study on the ship in oblique waves is few.In fact,the short waves are more often encountered for a large ship.Most papers aimed at validating the numerical method of prediction of wave added resistance by comparing with experimental results,less on the mechanism of wave added resistance,especially for shortwave added resistance.(3)Many numerical studies are carried out on free roll decay of a ship.It is common to assume a damping model,then the damping coefficients are obtained by energy method from the data of free roll decay.But the assumption of roll damping model in advance undoubtedly adds the constraints to the system,which shows obvious limitation.(4)Parametric rolling is one of the most dangerous phenomena which could lead to serious accidents due to significant roll.Parametric rolling of a ship is a strong nonlinear phenomenon,heavily influenced by the fluid viscosity.The corresponding research and published papers on parametric rolling based on RANS method which considering viscosity effect are relatively few.Accordingly,the modeling and implementation of the virtual tank for the ship,and the corresponding analysis technology are studied using a commercial software STAR CCM+ in this paper.The main works and innovations are summarized as follows:(1)The numerical simulations of flow around ships are carried out and the resistances are calculated for ships at various speeds based on RANS method.For ships at both medium speed and high speed,Volume Of Fluid(VOF)is adopted to capture the interface,and attitude effect is considered using overset mesh technology.The numerical results of flow around ships(DTMB 5512,KVLCC2,KCS,trimaran)at both medium speed and high speed are verified and validated.For the ship at low speed,wave resistance is small,and the main component is the viscous resistance,and the resistance is less affected by the ship attitude,so numerical simulation of flow around the ship under double model condition without considering the influence of ship attitude is carried out,and wake measurement method is introduced to calculate viscous resistance.The numerical resistance is verified and validated.In addition,influences of speed and model scale on resistance are studied.All the above studies give a better understanding of the resistance mechanism.(2)There are wave-making zone and damping zones around the boundaries of numerical wave tank,wave-making zone at the inlet and damping zone at the outlet.The strategies of temporal-spatial discretization with a trade-off between the efficiency and accuracy are presented.In the virtual tank,the simulations are carried out for KVLCC2 in head waves with different wavelengths.The numerical motion and added resistance of the ship show good agreement with experimental ones.Further,the radiation and diffraction components of wave added resistance are given.Similarly,the numerical simulation of the motion response of S175 in oblique waves are carried out,and corresponding wave added resistance is calculated out.When a ship encounters shortwave,the motion response is so small that the ship model can be fixed to calculate the shortwave added resistance(diffraction resistance).Through the research on influences of wavelength and incident angle on wave added resistance,the knowledge of wave added resistance is improved.(3)The roll damping is one of the key factors for evaluating the ship rolling in waves accurately.Numerical simulation of free roll decay is carried out based on RANS method with the help of overset mesh technology,in which,VOF is adopted to capture the free surface.And numerical results are verified and validated.In practice,the roll damping is generally obtained through the energy method from the data of free roll decay.In the present paper,the non-parametric identification method is introduced.Both Tikhonov's regularization and Markov Chain Monte Carlo(MCMC)are adopted to solve the Volterra integral equation of the first kind which is ill-posed.The roll damping and nonlinear restoring force are then identified.Compared with the traditional methods,it doesn't require the damping model in advance for the non-parametric identification method,which avoids too much assumption.The reconstructed roll motion based on the damping and nonlinear restoring force derived from non-parametric identification method show good agreement with CFD result.Moreover,the influences of several factors on roll damping and nonlinear restoring force,such as bilge keel width,speed,initial roll angle,model scale,and restricted water,are studied.(4)It is found that there are few published papers on the numerical simulation of parametric rolling based on CFD.Actually,parametric rolling with large amplitude is a strong nonlinear phenomenon,which is greatly influenced by fluid viscosity.In the present paper,two methods are adopted to simulate parametric rolling.One is a hybrid numerical prediction method combining potential theory and CFD method.The other is the numerical simulation based on RANS directly,in which,two new types of initial disturbance excitation are tried.As for the CFD method,the viscosity effect is considered,and VOF is used to capture the free surface,and the restoring force is calculated according to the instantaneous area of the wetted surface.Therefore,the CFD method can not only predict the developing stage of parametric rolling but also describe the developed stage.It shows obvious advantages in both improving the precision of the calculated results and understanding the mechanism of the physcial problems.Furthermore,the effects of initial conditions,ship speed,wave steepness,transverse moment of inertia and vertical height of gravity center on parametric rolling are studied.Pictures about roll amplitude of parametric rolling varying with speed,wave steepness,transverse moment of inertia and vertical position of the center of gravity are given,which is important for the design,operation and off-loading of a ship.