Numerical Study On The Viscous Flow And Hydrodynamic Forces On A Manoeuvring Ship In Restricted Waters

Manoeuvrability is one of the most important hydrodynamic performances of ships, which has a close relationship with navigation safety. In the last two decades, with the development of modern ships towards higher speed, large-sized and specialized ones, ship manoeuvring is becoming more and more difficult, and the results of marine accident would be more serious once it happens. Especially with the ship size becoming lager, waters such as the water area near bank, the estuary region, the gulf and the inland channel are becoming shallower and narrower relatively, which are the so-called restricted waters. Compared with the case of unrestricted waters, ships navigating in restricted waters face more obstacles, which increases the possibility of collision and grounding accidents. This requires ships to possess better manoeuvring performance. On the other hand, the shallow bottom, the bank and other obstacles make the hydrodynamic forces and moments acting on the hull more complicated, which makes the ship manoeuvring more difficult. Therefore, it is urgent to study the ship manoeuvrability in restricted waters. At present, this research is becoming one of the hot topics in the field of ship hydrodynamics.During the last decade, with the rapid development of computer science and technology, the performance of computer has been improved greatly. This has provided the necessary hardware condition for the development and practical application of Computational Fluid Dynamics (CFD) in ship hydrodynamics, and made it possible to simulate the complicated flow around a manoeuvring ship and to calculate the hydrodynamic forces on the ship. In this thesis, a general purpose CFD code, FLUENT, and its UDF (User Defined Functions) are used for simulating the viscous flows around a manoeuvring ship in restricted waters and calculating the hydrodynamic forces acting on the hull by solving the Reynolds-Averaged Navier-Stokes (RANS) equations. In the process of numerical calculation of the hydrodynamic forces, suitable turbulence model and high qualified grid are the keys to a successful simulation. The numerical results of viscous hydrodynamic forces on a ship in restricted waters obtained with five two-equation turbulence models, which are widely used in computation of ship viscous hydrodynamic forces, are compared with measurements to determine the suitable model. Structured grids, unstructured grids and hybrid grids with the layer near the boundary are adopted to divide the computational region, and computation of the hydrodynamic forces is conducted with these three grids. The accuracy of the numerical results with these grids is analyzed by comparing the numerical results with experimental ones. By comprehensively considering both the computational efficiency and the time consumed in the grids generation, it is found by numerical experiments that the hybrid grids with the layer near the boundary is a convenient and effective one. For some real ships with different ship form, suggestions are put forward for a fast and efficient grid generation of high-qualified hybrid grids.For some typical ship manoeuvring motions in restricted waters, numerical study on simulation of the viscous flow and calculation of the hydrodynamic forces is carried out by solving the RANS equations: (1) Numerical study on the viscous flow and the hydrodynamic forces on a ship in oblique motion in shallow water. The numerical study is carried out for a KVLCC2 tanker. Firstly, for the ship in oblique motion in deep water, determination of the computational domain, check of the grid dependence and selection of the turbulence model in numerical simulation are investigated. The effectivity of the numerical method is validated by comparing the numerical results with experimental data published in literature and the experimental results obtained by oblique towing test in the towing tank of Shanghai Jiao Tong University. Then the numerical method is applied to predict the resistance, the lateral force and yaw moment on the ship at different drift angles in shallow water with different water depths, and the influences of the water depth and drift angle on the hydrodynamic forces are analyzed.(2) Numerical study on the viscous flow and the hydrodynamic forces on a ship in turning motion in shallow water. The numerical study is carried out for the Esso Osaka tanker. By solving the control equations in the rotating coordinate system, numerical simulation of the viscous flow is conducted for the ship in turning motion at different water depths. Firstly, for the ship in turning motion in deep water, computations are performed and the numerical results of the hydrodynamic forces are compared with measurements to verify the numerical method. Then the viscous flow and the hydrodynamic forces are calculated for the ship model at seven water depths. The influence of the water depth on the hydrodynamic forces acting on the ship in turning motion is obtained.(3) Numerical study on the viscous flow and the hydrodynamic forces on a ship sailing along a bank. The numerical study is carried out for a Series 60 ship with Cb=0.6. Firstly, grid generation of computational domain is investigated for this ship form with thin bow and stern and a fat middle ship. An efficient grid generation of high-qualified hybrid grids with the layer near the boundary is proposed. Then the numerical study is conducted for the viscous flow and the hydrodynamic forces acting on the ship sailing at different water depths and different distances to the bank. By analyzing the numerical results, the changing tendence of the hydrodynamic forces with the water depth and the distance to the bank is obtained.(4) Numerical study on the viscous flow and the hydrodynamic forces acting on a ship undergoing unsteady lateral berthing motion in shallow water. By solving the unsteady RANS equations, numerical simulation of the transient viscous flow around a ship in unsteady lateral berthing motion is conducted, and the time history of the viscous force on the hull during the berthing process is predicted. Firstly, for a standard Wigley mathematical ship form, key techniques in numerical simulation of the transient viscous flow around a ship in unsteady lateral berthing motion, such as time step, pressure-velocity coupling algorithm and the turbulance model suitable for simulation, are investigated. The numerical results of the lateral forces on the Wigley model in unsteady lateral berthing motion at different water depths are compared with experimental data to verify the numerical method. Then for a real ship form, KVLCC2 tanker, numerical simulation of the viscous flow around the ship in unsteady lateral berthing motion is conducted, and the time histories of the hydrodynamic force and moment on the ship are predicted.In this thesis, numerical method for viscous flow by solving RANS equations is applied to simulate the viscous flow around ships in some typical manoeuvring motions in restricted waters. The characteristics of the viscous flow around the manoeuvring ships are captured. By analyzing the flow information, one can get deep insight into the changing mechanism of the hydrodynamic forces on a manoeuvring ship in restricted waters. Moreover, the numeircal method can accurately predict the viscous forces on a manoeuvring ship in restricted waters, demonstrate the changing tendence of the hydrodynamic forces with the factors including the water depth and the distance to bank, as well as the transient characteristics of the hydrodynamic forces. This is theoretically significant and practically useful for research on ship manoeuvring and control in restricted waters.