Research On Viscosity-equivalent Reduced Order Method Of Ship’s Roll Damping And Motion Response

Roll problem is a key problem in ship seakeeping.With the development of research level,the method of calculating roll has been developed from ship model test to numerical simulation.In the actual process of ship roll calculation,there still exists the contradiction between the calculation accuracy and efficiency.The commonly used potential flow theory is fast and efficient,but it is difficult to simulate the viscous effect.The single constant damping coefficient is used to correct the damping,and the calculation accuracy is slightly poor.Viscous flow theory has high calculation accuracy,but high calculation cost and slow calculation speed.Therefore,a Viscosity-Equivalent Reduced Order Method(VEROM)for rolling motion is proposed in this paper,so as to improve the prediction accuracy and speed of ship seakeeping.In this paper,a method to obtain the nonlinear damping characteristics of ship’s roll is proposed.Based on a forced amplification motion,the viscous model with solving the Navier-Stokes equation and the non-viscous model with solving the Euler equation were used to carry out the simulation,and the hydrodynamic moment under different periods was predicted.The damping coefficient was acquired by using the energy-equivalent method,and two KC number-damping coefficient curves were obtained.The characteristics of viscous damping effect of ship’s roll varying with KC number were gained by the difference between the curves.Secondly,a Viscosity-Equivalent Reducted Order Method based of ship’s rolling motion is proposed.This method is based on the finite volume method to solve the Euler equation.The viscosity of the fluid is ignored,and the mesh of the non-viscous model is used to reduce the number of mesh and improve the computational efficiency.At the same time,based on the principle of viscous equivalence,the roll damping is modified in real time to improve the accuracy and achieve the balance between the calculation efficiency and the accuracy.To verify the effectiveness of the method,the VEROM model and viscous model are used to predict the rolling response of the typical two-dimensional central hull.The results show that the accuracy of VEROM model is close to that of viscous model.However,the computational efficiency is far more than viscous model.And the prediction result of VEROM model is close to the test result,which meets the precision requirement of engineering application.Thirdly,a typical DTMB5415 standard ship model test was carried out to verify the accuracy of VEROM method in predicting the response of typical 3D ship rolling motion.In this part,the natural period of the ship model and the corresponding rolling RAO curve were measured by rolling free attenuation test and rolling response test under regular wave in the towing tank.The VEROM model is used to simulate the test condition.Compared with the test results,the VEROM prediction error is less than 10% in the wave energy concentration area.Finally,in order to predict the ship’s motion response under irregular wave environment in actual sea condition,the Linear superposition method is used to simulate the Ochi-Hubble six-parameter spectrum.By using the VEROM model and the viscous model,the spectral analysis method is used to process the time-domain calculation results,and the significant value is obtained.The compared results show that the error of the VEROM model is less than 10%,and the computational efficiency is 20 times higher than that of the viscous model,which verifies the effectiveness of the proposed method.