Time Domain Hybrid Green Function Method And Prediction Of Ship Motions In Waves

With the development of navy and ocean engineering, new requirements on predicting motion and hydrodynamic performances for ship and ocean structure in waves have been raised. Frequency domain method is widely applied but only efficient to treat steady state problems, for complex condition such as non-linear and transient problem, it is more appropriate to use time domain method. So a reasonable and practical approach of prediction for ship motion in time domain is valuable and significant.In this thesis, a time domain hybrid Green function method is developed by means of following steps: The fluid filed is decomposed into inner and outer domain by an imaginary control surface; the Rankine source is applied in inner domain, while the transient Green function is used in outer domain. The initial boundary value problems are solved by the continuity condition on control surface, and the simulating for motions of floating body in oblique waves is presented. The discretization parameters of numerical calculation are discussed by unitary paramater, and the reasonable ranges of the parameters are proposed by considering both accuracy and efficiency. A cylindrical platform and LNG ship are studied and the comparison between numerical results and experimental data are in good agreement, which shows the present method is suitable for solving the divergence problem due to ship with flare.Based on the assumption that ship’s wave making can be neglected and a new approach for free surface boundary condition, the time domain hybrid Green function method is applied on predicting hydrodynamic performance and motion of ship with forward speed. A Wigley hull ship and a S175 containership advancing in waves are simulated and analyzed respectively, and the comparison between numerical results and experimental data shows good agreement, which proves that the present method is also applicable for ship with forward speed. The character and scope of application for both time domain and zero-frequency method which account the forward speed effects are also discussed.Furthermore the time domain hybrid Green function method is successfully extended to simulating for the coupled motions of two ships arranged side by side in waves. By use of wave-interaction theory for multiple bodies, the radiation and diffraction problem of two ships advancing in waves are solved, and then the time histeries of couple motions of two ships are obtained. A Wigley hull ship and a rectangular barge arranged side by side in beam waves are investigated, the hydrodynamic interactions between them are accounted for. The coupled motions between two ships advancing in waves are also simulated. Good agreement is obtained between numerical results and experimental data for all case studies and proper resonant phenomena is also showed, which indicates the current method is correct and available.Based on the generation for transient wet ship hull panels, the nonlinear simulation of ship motion is established by adopting time domain hybrid Green function method. The Froude-Krylov force and restoring force are calculated by nonlinear theory on transient wet surface, while the radiation and diffraction problem are still solved by linear theory. The method is verified by using a Wigley hull ship and a S175 containership. The comparison between numerical results and experimental data shows that nonlinear results are better than linear ones, which proves the advantage of time domain theory.