Study On Mechanism Of Longitudinal Stability And Twice Porpoising Of A Planing Trimaran

As a kind of fast and convenient waterborne vehicle,the pursuit of higher design speed and good navigation stability has always been an important goal in the design of planing boats.While the longitudinal instability of planing hull is one of the main limiting factors of design speed.In this paper,based on the combination of "Model Test&Numerical Simulation",two key issues of longitudinal motion stability of a planing trimaran with a tunnel under the effect of hydro-aerodynamic lift are studied:1)the mechanism and influence of the tunnel on keeping longitudinal stability of the hull during steady direct navigation state,and the destabilizing critical states are focused;2)the mechanism and influence of the tunnel on instability performance during porpoising process,the speed range and vibration amplitude of each stage are also focused.Among them,the proposal of the phenomenon of twice porpoising,the establishment of the index to comprehensively characterize the longitudinal stability based on the limit cycles,and the consideration of the longitudinal stability of the planing trimaran is positive to achieve the "faster and more stable" of the high-speed ships in the future.Firstly,the planing trimaran model towing test in calm water is performed,and study the stability of the direct navigation steady speed range and the porpoising speed range under a series of the longitudinal position of gravity center and moment of inertia.Based on the test results,the special phenomena of "Second Stability" and "Second Porpoising" of planing trimaran in the test are summarized and analyzed.At the same time,the effects of the longitudinal position of the gravity center and inertia moment on the steady state speed range,multiple porpoising speed range and the amplitude and period of porpoising vibration are summarized.The quantitative analysis of the influence of the moment of inertia on the longitudinal stability was completed for the first time.Second,based on the "STAR-CCM+" software,using the finite volume method,the SST k-ω turbulence model is used to solve the RANS equation,the VOF method is used to simulate the two-phase flow of water and air,and the overset grid is used to simulate the heave and pitch motion of the high-speed planing trimaran.By analyzing the errors of the test results,the reliability verification of the numerical scheme is carried out.The comparison between the numerical simulation results and the towing tests shows that the established numerical scheme is suitable for the simulation of the direct navigation of the planing trimaran.At the same time,compared with the experimental images,the numerical scheme can capture the details of the flow field around the planing trimaran and the vibration of the porpoising process.Thirdly,a monohull planing ship that same as the main parameters of the planing trimaran is designed,and the influence of the sailing attitude and dynamic behavior of the hull after the introduction of the tunnel is compared and analyzed.Starting from the influence of the tunnel on the wave-making flow of the main hull,the distribution changes of the forces(moments)affected by the tunnel on each planing surface of the planing trimaran in the stable sailing state are discussed.The introduction of the tunnel makes the force center move backward in the high speed section,and the stabilizing moment arm increases,so as to maintain a considerable stabilizing moment.This improves the longitudinal motion stability of the planing trimaran,and extends the stable speed range for direct navigation.Fourthly,the mechanism of twice porpoising of the planing hull is analyzed,and the vibration amplitude,frequency characteristics in the range of longitudinal instability speed are compared and analyzed.The spatial position difference of the tunnel vibration process and the aerodynamic lift effect of the tunnel to suppress the heave and pitch motion and to improve the vibration amplitude and frequency of the planing trimaran are discussed.Based on the theory of self-excited vibration,the law of the phase difference of the vibration between the heave and pitch motion with the porpoising process of the planing trimaran is analyzed.Based on the theory of limit cycles,the criterion index of longitudinal stability at high speed was established.And it was discovered that when the phase difference approaches π,the coupled energy input of heave and pitch motion decreased,the vibration of the porpoising process of the planing trimaran weakened,and the navigation state is close to a direct sailing and stable.Lastly,the hydrodynamic performance of different tunnel parameters of planing trimaran is systematically compared.And the effects of tunnel parameters of planing trimaran on the resistance,navigation state,limit speed of stable direct navigation,the oscillation characteristics of the first porpoising process,secondary stability speed interval and the influence of vibration characteristics of second porpoising process are all analyzed.And based on the numerical calculation results,the empirical formulas of the limit cycle stability index on the parameters such as moment of inertia and ship type are obtained,which not only provides the reference to the design of the planing trimaran with the goal of optimizing the longitudinal motion stability of the craft,but also provides an important guiding significance for further raising the upper limit of ship speed.