Calculation of Unsteady Hydrodynamic Forces on a Maneuvering Ship

The ability of ships to keep or change the speed, heading and position is the main research topic of ship maneuverability. Maneuverability is one of the most important navigational performances, which is closely related to navigation safety and efficiency. In order to ensure the safe navigation, it is necessary to predict ship maneuvering performances.One of the important methods to predict ship maneuverability is the computer simulation method with mathematical model (i.e., the equations of ship maneuvering motion). To use this method, it is necessary to determine the hydrodynamic derivatives in the equations of ship maneuvering motion. Currently, the captive model test is recognized as the most reliable one among the methods to determine the hydrodynamic derivatives. In the captive model test, the ship model is forced to undergo special motion in the model basin by some mechanism. During the test, the hydrodynamic forces on the ship model are measured and the hydrodynamic derivatives are obtained by analyzing the test data. Once the mathematical model of ship maneuvering motion is established, it is solved by computer to simulate various kinds of ship maneuvering motion, from which the characteristic parameters of ship maneuverability can be obtained. Among the captive model tests, Planar Motion Mechanism (PMM) test is the effective one with comprehensive function and can measure various velocity derivatives, angular velocity derivatives, acceleration derivatives and the coupling derivatives. But the model test has its own deficiencies: high costs, long test period, inconvenient to design optimizing maneuverability by changing hull form, unsuitable to predict the maneuvering performance under different afloat and loading conditions, and also the scale effect cannot be eliminated.Along with the rapid development of computer science and technology, Computational Fluid Dynamics (CFD) technology has made it possible to solve the problems of ship maneuverability. When using the CFD technology to simulate the flow field of captive model test and to determine the hydrodynamic forces on the maneuvering ship model, it can overcome the shortcomings of the ship model test such as high costs and long test period, and it is also convenient to design optimizing maneuverability by correcting hull form and maneuvering devices. And when using the CFD technology to simulate the flow field around the full scale ship, the scale effect can be avoided.In this thesis, CFD method is applied to numerically simulate the viscous flow field around the captive model in PMM test, and to calculate the hydrodynamic forces and hydrodynamic derivatives on the model, including pure sway motion and pure yaw motion. Considering the complexity of the problem, the effects of the free surface elevation, appendages and changes of ship afloat condition are ignored in the calculation. The KVLCC1 tanker is taken as the research object, for which the viscous flow field around the hull undergoing pure sway motion in deep and shallow water and undergoing pure yaw motion in shallow water is simulated, and the sway forces and yaw moments acting on the hull are calculated, the influences of speed and oscillation amplitude and frequency on hydrodynamics are analyzed, the characteristics of the unsteady flow field around the hull undergoing maneuvering motion are also analyzed and discussed. The proposed numerical method is verified by comparing the numerical results of sway forces and yaw moments with available experimental results. Based on the numerical results of sway forces and yaw moments, linear hydrodynamic derivatives of ship maneuvering are determined.