Study On Multi-degree Of Freedom Coupled Motion And Surf-riding/Broaching Instability Of Trimaran

Trimaran has many advantages,compared with monohull,such as higher speed,better stability,larger deck area and so on.It has been widely concerned by scholars from all over the world and developed rapidly in recent years.At present,trimaran has been widely used in military and civil fields in many countries.At present,there are still some deficiencies and gaps in the researches about trimaran at home and abroad,which need to be further studied,such as the unique hull configuration of trimaran brings about the problems of multi-degree-of freedom coupled motion,motion stability in waves,and instability of the second generation of intact stability.Therefore,it is of great theoretical significance and engineering application value to study the multi-degree-of-freedom motion coupling problem of trimaran and the surfriding/broaching-to in waves,so as to deeply grasp the hydrodynamic characteristics of trimaran,safe navigation and improve the seaworthiness of the ship.Based on the theory of viscous fluid mechanics,this paper uses open sourced CFD code OpenFOAM to simulate the multi-DOF motion of a trimaran sailing in various wave direction.On this basis,the multi-DOF coupled motions and motion stability changes of trimaran are studied.The suitable numerical simulation method and calculation parameters are given for different multi-degree-of-freedom motion problems,and the numerical simulation method is verified by comparing with experimental results.Aiming at the coupling problem of large roll and pitch motion of ships,the characteristics of coupled motion of trimaran in waves were analyzed,and it was found that the coupled motion of trimaran in waves can be divided into reciprocating torsional motion,spiral torsional motion and pitch motion with little influence of roll motion.The motion and force analysis of reciprocating and spiral torsional motions with more obvious coupling characteristics are carried out,and the occurrence conditions of different types of torsional motions are discussed.The variation of trimaran’s torsional motion with wave direction,wave steepness and encounter frequency is studied.It is found that the torsional motion is most obviously affected by encounter frequency.The results show that the dangerous reciprocating torsional motion of trimaran is more likely to occur when the trimaran is sailing in bow skew waves with high encounter frequency.In order to solve the problem of the stability change of the roll and pitch motion of a trimaran during navigation,the roll and pitch motion are analyzed by using nonlinear dynamics method.Based on the comparison of the phase plan and Poincare section of the motion response,the periodical variation characteristics of the trimaran’s roll and pitch motion in various wave directions are studied.From the point of view of motion stability,the variation laws of trimaran’s roll and pitch motion during the course of heading wave,following wave,beam wave,oblique heading wave and oblique stern wave are discussed respectively.According to the numerical simulation results of trimaran’s motion responses with different side hull layouts,the influence of side hull position change on the coupled motion characteristics and motion stability of trimaran is studied.Three DOF motion model,five DOF motion model and six DOF motion model were used to simulate the multi-DOF motion of trimaran,and the characteristics of multi-DOF coupled motion of trimaran were studied through comparative analysis.The periodical variation of velocity caused by wave effects during navigation and its effect on yawing motion are studied.When the speed of a trimaran is affected by waves and loses its steady periodic variation,the ship will have a heading angle deflection.This phenomenon is further studied,and the variation of trimaran’s heave,pitch and roll motion after the waves affect the ship speed and yawing is discussed.The results show that the variation of yaw motion caused by waves is accompanied by the obvious increase of heave,pitch and roll motion,which leads to the dangerous navigation of ships.This paper studies the surf-riding/broaching instability of trimaran in following waves and oblique stern waves.A 6-DOF free sailing model is used to simulate the navigation of trimaran in following waves and oblique stern waves.According to the variation of time history of motion responses during navigation,the phenomena of surf-riding and broaching-to instability of trimaran ship are studied.And the occurrence conditions of surf-riding/broaching in following wave and oblique stern waves are given respectively.The influence of ship speed on surf-riding/broaching is analyzed,and the variation of different motions caused by wave riding/yaw throw is given.The results show that surf-riding/broaching phenomenon is more likely to occur when a trimaran is sailing at high speed,and within the range of wave velocity,broaching phenomenon is more likely to occur with the increase of wave velocity.In this paper,the multiple degrees of freedom coupled motion and stability changes of trimaran are studied.The results further reveal the hydrodynamic characteristics of trimaran,and provide important theoretical support for the navigation safety and further development of trimaran.