Research On The Nonlinear Ship Hydroelasticity Based On CFD-FEM Two-way Coupling Method

With the increase of ship size,the stiffness of the hull structure becomes smaller.This means that the frequency of wave excitation tends to be closer to the natural frequency of hull vibration,which in turn makes the hydroelastic responses more significant.Compared with the potential flow based solvers,the CFD（Computational Fluid Dynamics）technology,which is based on solving RANS equation,can easily take all these non-linear factors into account.By further coupling with Finite Element Method,the CFD-FEM method can also consider the influence of the structure deformation on the flow field.The CFD-FEM method has become another important means to study the ship hydroelasticity besides the model test.Firstly,based on commercial software STARCCM⁺and ABAQUS,the two way coupling method based on CFD-FEM is established in this paper.The accuracy of the numerical algorithm is verified by comparing the experimental and numerical results in the literature.On the other hand,it shows that the two-way coupling method based on CFD-FEM is closer to the experimental value than the traditional hydroelastic method based on potential flow theory.On the basis of verifying the reliability of the numerical method,some factors affecting the nonlinear hydroelastic response of ship hull are further studied.Firstly,the effects of wavelength-shiplength ratio（λ/L）,wave frequency-natural frequency ratio（ω_e/ω₂）and wave steepness（H/λ）on the hydroelastic response of ship hull without forward speed in head sea conditions are calculated.It is found thatλ/L has the most significant effect on the vertical bending moment.For example,whenω_e/ω₂=1,the amplitude of the non-dimensional vertical bending moment of section4 corresponding toλ/L=1.07 is 2 times of the value underλ/L=0.6.Moreover,the effects fromω_e/ω₂ and H/λon the hydroelasticity will behave differently under differentλ/L.On this basis,the influence of ship speed on the response of hydroelastic calculation model is calculated.It is found that when the natural frequency of the hull girder is high,although the speed will significantly increase the amplitude of the rigid body motion of the hull,the influence on the vertical bending moment of the section near the midship is not significant.Finally,the flow field pressure and the vertical displacement of the hull girder with the same stiffness as the test model are modally analyzed.It is found that the first-order mode shape of flow field pressure is mainly influenced by the of ship’s rigid body motion,while the higher-order is affected by the elastic deformation of ship girder The pressure modes of the flow field are completely different under head and following sea.Meanwhile,the main rigid modes of the hull response are also different.