A Strategy Research Of Ship Forced Rolling Virtual Test In Calm Water Based On The CFD Method

The motion of ship on sea can be solved by establishing differential equations of rolling motion.Therefore,it is necessary to determine the coefficients in the differential equation.The key to predict rolling motion characteristics accurately is also the most difficult work.In the past,the damping prediction method based on potential flow theory only considers the damping of wave damping in the five types of rolling damping components.If the potential flow theory is not corrected,the accuracy of roll damping prediction is not engineering practicability.In this paper,two-dimensional and three-dimensional numerical simulations of forced-rolling virtual experiments in calm water were conducted respectively based on the CFD method with commercial CFD solver,and the CFD simulation strategy for accurately predicting the damping coefficient was developed.At the same time,the author also studied the uncertainty of the rolling damping forecasting by CFD method in the two-dimensional simulation and the physical phenomenon of large-amplitude roll in the three-dimensional simulation.The calculation time of two-dimensional simulation is short,therefore more suitable for fast prediction of hull section roll damping or study of other methods at zero shipping speed.In the two-dimensional CFD simulations of forced rolling tests,the author describe in detail the process of using a commercial CFD software FLUENT to conduct the simulation of forced rolling test based on the sliding mesh technique and studied setting strategies of the computational domains,grids,boundary conditions,time steps,size of first layer grid and their sensitivity and convergence analysis.The process of obtaining the damping coefficient by the energy method was described in detail,and the comparison with the model test results was made.The results agree well with each other and the uncertainty of the CFD forecast damping coefficient is studied.If a ship has a shipping speed,its rolling damping depends on the shipping speed,so it is necessary to study the 3D CFD simulation strategy of forced rolling test.The 3D CFD simulations of virtual test for forced rolling based on the sliding mesh technique were conducted.By comparing the CFD forecast results with the existing test results,the two results are in good agreement.In addition,it is also found that the damping moment under the equivalent linear damping coefficient is different from the actual damping moment at the large roll angle.It often appears large-amplitude rolling movement in the extreme sea conditions,large-amplitude rolling often lead to the bilge keel running out of water.By using CFD simulation of large rolling movement,it can be found that the phenomenon of bilge keel running out of the water lead to drastic reduction of the bilge keel pressure damping.It also shows the phenomenon that bilge keel re-entry into water mixing bubble,which is completely different from the situation when bilge keels is completely immersed.In addition,the CFD technology was also used in this paper to calculate the pressure load of a cylinder with a single bilge keel during the process of forced rolling.It is found that the roll damping coefficient is related with the pressure load of bilge keel.As the increase of the width of bilge keel,the pressure load of bilge keel increases,and then the roll damping coefficient increases.It is also found that the profile of bilge keel effect the pressure load of bilge keel and roll damping coefficient.The plat bilge keel has the best anti-rolling effect compared with wedge section or other complex shape section.