Prediction Of Ship Longitudinal Motion Response Based On Viscosity-equivalent Reduced Order Method

Seakeeping is an important index to measure whether the ship performance is superior or not.In the initial design stage of the ship,it is particularly important to establish a method that can accurately and quickly predict the seakeeping of the ship.At present,numerical prediction methods of seakeeping mainly include potential flow theory and viscous flow theory.The potential flow theory is the most important prediction method in the initial design stage of ships because of its fast calculation speed.However,the potential flow theory ignores the viscosity of the fluid and has limitations in dealing with the problems of high speed and strong nonlinearity.Viscous flow theory has high accuracy in seakeeping and can deal with strongly nonlinear problems.However,viscous flow theory has high requirements for computer and low computational efficiency.In order to ensure the accuracy of seakeeping calculation and improve the efficiency of seakeeping prediction,numerical method and experimental method are used to study the prediction method of seakeeping.Firstly,a prediction method of ship seakeeping based on modified Euler Overlay Method is proposed.Compared with the Euler Overlay Method,the modified Euler Overlay Method solves the inviscid turbulence model and sets the whole zone as the forcing zone,which can improve the calculation efficiency of seakeeping.Then,the modified Euler Overlay Method is used to verify the regular wave making,and the effects of time step,time discretization scheme,the number of refinement layers of free surface grid and the number of internal iterations on the wave making accuracy are analyzed.Considering the nonlinear factors,the seakeeping performance of Wigley III standard model under regular waves is predicted,and the calculated results are compared with the experimental results.The results of this method are compared with those of viscous flow method under irregular wave conditions.The results show that the method can accurately and quickly calculate the ship motion response under regular and irregular waves.However,this method has some limitations.First,when using the modified Euler Overlay Method to make waves,the wave will drift upward;Secondly,when viscous correction of potential flow is carried out,the damping coefficient should be selected artificially,and the subjectivity of damping correction is too large.In order to overcome the shortcomings of the above methods,a prediction method of ship longitudinal motion response based on viscosity-equivalent reduced order method(VEROM)is proposed.In the numerical wave making simulation,the wave height attenuation is corrected by increasing the wave height input at the wave making entrance,so as to avoid the wave drift phenomenon caused by setting the whole zone as the forcing zone.Based on the energy dissipation equivalence,the viscous damping effect of ship with different motion responses is obtained by forced amplification motion.The DTMB5415 ship model is taken as the research object to predict the motion response of the ship in regular waves.The towing tank model test is carried out to verify the accuracy of the numerical method.Then,DTMB5415 is simulated in irregular waves by viscous flow method,and the simulation results are compared with those of VEROM.It is found that VEROM can also be used to calculate the endurance of irregular waves.The results show that the prediction method of ship longitudinal motion response based on viscosity-equivalent reduced order method can effectively improve the calculation efficiency on the premise of ensuring the prediction accuracy.