Research On Longitudinal Stability Optimization And Control Of Multihull

With the deepening of marine development,traditional monohulls cannot fully meet the requirements of modern maritime development.Therefore,more and more new type of ship has emerged,attracting a large number of experts and scholars from various countries to study them.Multihull belongs to new high-tech ships,they have gradually developed in the field of naval architecture and ocean engineering according to their high mobility and strong carrying capacity.However,the particularity of the multihull structure makes it produce a large heave and pitch motion when sailing in the sea,resulting in hull damage and seasickness of crew.In view of the above problems,the longitudinal stability optimization and control of multihull is studied in this thesis.The main research contents are as follows:1.Research on longitudinal motion calculation of trimaran.Heave and pitch motions of trimaran are studied using Computational Fluid Dynamics(CFD)simulation,the mathematical model of trimaran motion,and model test.CFD simulations need to establish a threedimensional geometric model of the trimaran and import the geometric model into the CFD simulation environment for simulating the heave and pitch motion of the trimaran under different sea conditions and speeds.The method of mathematical model adopts three steps to calculate the longitudinal motion of the trimaran.Firstly,the standard wave spectrum is selected as the wave excitation to obtain the wave force and moment acting on the trimaran;secondly,the wave force and moment function are obtained by identification method;thirdly,the hydrodynamic coefficient of the trimaran longitudinal motion is calculated by CFD simulation.Combined with the above three points,the mathematical model of trimaran longitudinal motion is established to calculate the heave and pitch motion of the trimaran.Model test is to drag the trimaran model into a towing tank and collect its heave and pitch motion data.The calculation results of CFD simulation and mathematical model are compared with the model test results,and the results show that both CFD simulation and mathematical model can effectively calculate the heave and pitch motions of the trimaran.2.Research on longitudinal stability appendages of the trimaran.T-foil and flap are selected as the longitudinal stability appendages through structure characteristics analysis of the trimaran,which is respectively installed on the bow and stern.Firstly,the resultant force of the T-foil and flap is made equal to the wave force on the trimaran under high sea conditions.Combined with the width and profile outline of the bow and stern,the structure and size parameters of the T-foil and flap are designed.Then,the specific installation position of the longitudinal stability appendage needs to be determined.The flap is installed on the stern edge of the trimaran.By establishing the evaluation function of the T-foil installation position,taking the resistance,heave,and pitch motion as the optimization indexes,the motion simulation of the trimaran under multi-working conditions is carried out in CFD software,and the data sets of trimaran sailing resistance,heave and pitch motion under different vertical and longitudinal installation positions of T-foil are obtained.The data sets are substituted into the evaluation function for obtaining the best installation position of the T-foil.Finally,the motion simulation of the trimaran without appendages and with longitudinal stability appendages is carried out in CFD software,the numerical comparison curves of heave and pitch motion are obtained,and the model test is performed simultaneously.Through the two-way iterative verification of simulation and model test,it is proved that the designed appendages can restrain the heave and pitch motion of the trimaran.3.Research on the control strategy of trimaran longitudinal motion.Firstly,the mathematical model of the trimaran with appendages and the necessity of decoupled appendages are analyzed.The decoupled Proportion Integration Differentiation(PID)control strategy is proposed,and the effectiveness of the decoupled PID control strategy for motion reduction is proved through simulation.Secondly,to overcome the limitation of high differential order in the decoupled PID control strategy,a hybrid decoupled control strategy is proposed.The force and moment generated by the appendages are used to reduce the wave force and moment,and the motion control is carried out by combining the heave displacement and pitch angle displacement with S-plane controller.The effectiveness of the hybrid control strategy is proved through simulation.Finally,to better restrain the trimaran motion,an improved method of the hybrid decoupled control strategy is proposed,which replaces the displacement feedback with the velocity feedback.Simulation results show the superiority of the improved hybrid decoupled control strategy in motion reduction.4.Construction of trimaran longitudinal stabilization control system and model test.The longitudinal stability control system of trimaran includes a hardware system,actuator,and control strategy.The hardware system includes sensors,upper computer,lower computer,and the circuit board design;the actuator includes the hydraulic cylinder and the longitudinal stability appendages;the control strategy includes the hybrid decoupled control strategy and the improved hybrid decoupled control strategy.Under typical working conditions,model tests of the trimaran without appendages,with passive and controlled appendages are carried out respectively.The motion data is processed and analyzed,and the results show that the effect of controlled appendages on reducing heave and pitch motion of the trimaran is stronger than that of passive appendages,and the effect of the improved hybrid decoupled control strategy is better than that of the hybrid decoupled control strategy.5.Research on extended application of longitudinal stability control method.Extending the longitudinal stability control method of trimaran to a catamaran,the longitudinal motion of the catamaran is studied by CFD simulation,the longitudinal stability appendages of the catamaran are designed and installed,and the decoupled control method of the catamaran with appendages is performed.Model tests of the catamaran are carried out,and the motion statistical analysis of the catamaran without appendages,with passive and controlled appendages are conducted.The results show that the heave and pitch motion of the catamaran can be effectively suppressed after the longitudinal stability appendages are installed,and the suppression effect of the controlled appendage on the motion is better than that of the passive appendages,which proves the feasibility of extending the application of the trimaran longitudinal stability research method.It has guiding significance and practical value for investigating the longitudinal stability of multihull real ships.