Research On The CFD-based Modeling Of 4-DoF MMG Model And Maneuverability Prediction For A Twin-propeller Twin-rudder Ship

Ship maneuverability is one of the important hydrodynamic performances involving safety and efficiency of navigation.With the development in the tonnage of ships,there are some problems appear during ship maneuvers,which have aroused the attention of researchers worldwide.On one hand,for the container ship and RORO ship with small initial metacentric height,as well as the high-speed surface combatant such as destroyer and frigate,obvious ship heeling usually can be observed during the tight maneuvers.Remarkable ship heeling may lead to the shift of cargo and reduce the stability of ship.It may result in the capsizing of ship under the action of environmental factors,such as wind,waves,current,and so on.On the other hand,for the ship under the maneuvering condition,the change of ship motion attitude may lead to the increase of resistance and the speed drop,resulting in the increase of propeller loads and the reduction of propulsive efficiency.In particular,for the twin-propeller twin-rudder ship the loads of two propellers exhibit asymmetrical behavior,which is remarkable for high-speed surface combatants.The phenomenon of increase in propeller loads and asymmetrical behavior of the propeller loads would result in an unbalanced stress on the shaft connection units,which would cause the failure of shafting and even reduce the service life of main engine.The phenomenon of ship heeling,increasing load of main engine,and asymmetrical behavior of propeller loads for the twin-propeller twin-rudder ship during maneuvers may threat the safety of ship navigation and main engine operation.Hence,it is necessary to study the above issues in-depth,in order to establish a corresponding assessment methodology of security and then to take the contermeasures.Ship heeling and the change in propeller loads during maneuvers are closely related to the hydrodynamic characteristics of ship,involving the hull-propeller-rudder interaction etc.under the maneuvering condition.These issues are highly complicated,and it is hard to adopt the traditional experimental approach for in-depth research due to the limitation of technology and equipment,economic cost,and so on.With the development of computer technology and numerical method,Computational Fluid Dynamics(CFD)is more and more widely applied in the field of ship and ocean engineering,providing a powerful tool for the investigation of complicated ship flow.Based on the CFD method,the hydrodynamic characteristic of twin-propeller twin-rudder ship under the maneuvering condition is studied in this doctoral dissertation.The four degrees of freedom(4-Do F)ship maneuvering mathematical model for the twin-propeller twin-rudder ship is established and is applied to prediction of the rolling-coupled ship maneuvers and the changed propeller loads during maneuvers.Aiming at the prediction of 4-Do F ship maneuver and propeller loads of the twin-propeller twin-rudder ship,MMG model being capable of considering the interaction among hull,propeller,and rudder is selected to describe the ship maneuvering motion.Through the analysis and improvement of the existing mathematical model,the improved form of MMG model suitable for describing the maneuvering characteristics of twin-propeller twin-rudder ship reasonably is obtained.Taking a typical twin-propeller twin-rudder ship model,ONR Tumblehome(ONRT)model,as the study subject,a series of ship model tests are simulated by using CFD method,and the hydrodynamic characteristics and the flow field of hull,propeller,and rudder under the maneuvering condition are studied.The concrete contents are as follows:(1)The research on the hydrodynamic performance of hull for the twin-propeller twin-rudder ship under the maneuvering condition is carried out by using RANS method.The static drift test,circular motion test(CMT),and static heel test are simulated with the ONRT model without propeller or rudder,and the computed hydrodynamic forces and moments are validated by comparing with the experimental data in literature.Based on the analysis of the wave pattern and the flow field around the hull under different working conditions,the mechanism of hydrodynamic forces acting on the hull under the maneuvering condition is explored.Through the regression of computed forces and moments under different working conditions,the hydrodynamic derivatives related to sway velocity,yaw rate,and heeling angle are obtained,which are in good consistence with the derivatives derived from the experimental data.In addition,the free roll decay tests are simulated with the ONRT model without propeller or rudder,and the obtained time history of roll decay is in good agreement with the experimental data in literature.Based on the time histories of roll decay and flow field around the ship model,the effects of bilge keels and shaft brackets on the ship rolling are discussed.The computed time histories of roll moment,roll angle,roll angular velocity,and roll angular acceleration are regressed based on the 1-Do F equation of roll motion,and the added moment of inertia and roll damping coefficient are obtained.(2)The research on the hydrodynamic performance of propeller for the twin-propeller twin-rudder ship under the maneuvering condition is carried out by using RANS method.The open water test of ONRT propeller under the oblique inflow condition is simulated,and the mechanism of the change in hydrodynamic characteristic of propeller affected by the lateral inflow is analyzed based on the obtained flow field around the propeller.Further,in order to explore the influence of ship wake on the characteristic of propeller under the maneuvering condition,the self-propulsion test,static drift test,and CMT are simulated with the fully appended ONRT model.Through the analysis of the nominal wake and flow field around the propeller under the maneuvering condition,the mechanism of ship wake affecting the hydrodynamic characteristic of propeller is explored.Modeling for the hydrodynamic characteristic of propeller under the maneuvering condition is investigated based on the numerical results: The propeller thrust and torque under the maneuvering condition are described by the model of effective wake fraction.The applicability of the existing models of effective wake fraction is discussed by regression analysis,and the reasonable model of effective wake fraction for the twin-propeller twin-rudder ship under the maneuvering condition is established.In addition,the parameters in the propeller side force model are obtained by the linear regression of the relation between propeller side force and ship motion.(3)The research on the hydrodynamic performance of rudder for the twin-propeller twin-rudder ship under the maneuvering condition is carried out by using RANS method.The rudder force test,static drift test,and CMT are simulated with the fully appended ONRT model,and the hydrodynamic performance of rudder under different ship motions and propeller loads are obtained.Based on the flow field around the stern and the pressure distribution on the rudder surface,the effects of ship wake and propeller slipstream on the rudder performance under the maneuvering condition are analysed,and special attention is paid to the difference of flow-straightening effect at port and starbord sides of the twin-propeller twin-rudder ship.The numerical results are regressed based on the prescribed rudder force model for twin-propeller twin-rudder ship,and the corresponding parameters are obtained,which are in good agreement with the experimental data in literature.The 4-Do F MMG model for the twin-propeller twin-rudder ship is established based on the CFD-based numerical simulation of a series of model tests.By using the fourth-order Runge-Kutta algorithm to solve the model,the turning circle tests and zigzag tests of ONRT model are simulated.The obtained trajectories and the time histories of kinematics parameters,including speed,drift angle,yaw rate,roll angle,and so on,are compared with the data of free-running model tests in literature,which shows a good consistency.Furthermore,considering the close relation between the change in propeller loads and the characteristic of main engine during ship maneuvers,the mathematical model of main engine is coupled with the established MMG model,and the numerical simulations of free-running model tests considering the hull-propeller-engine interaction are realized.Through the numerical simulation of turning circle test and zigzag tests under three control strategies of main engine,i.e.,constant propeller revolutions,constant torque,and constant power,the influence of the control strategy of main engine on the change of portside and starboard propeller loads during ship maneuvers and ship maneuvering motion are explored.Through the present study,a deeper understanding of the characteristic of flow field around the ship and the hull-propeller-rudder interaction mechanism for the twin-propeller twin-rudder ship during maneuvers is achieved,and the effectiveness of the CFD-system based method in the prediction of 4-Do F maneuvers for the twin-propeller twin-rudder ship is verified.It provides technical support for the safety evaluation of ship maneuverability and stability in ship design stage.Furthermore,based on the simulation of free-running model tests considering the hull-propeller-engine interaction,the variation and distribution of the propeller loads for the twin-propeller twin-rudder ship during maneuvers are clarified,which provides a reference for the design and selection of the main engine,and is beneficial to improve the overall design level of ship-engine-propeller system.