To evaluate theinfluence of dof on manoeuvring predictions by direct CFD zig-zag simulations

In this paper, direct CFD zig-zag simulations of 10°/10° and 20°/20° are performed in deep water condition under the consideration of different degrees of freedom, namely 3 DOF, 4 DOF, and 6 DOF, to evaluate their influence on manoeuvring predictions. A modern container ship KCS with a slightly simplified semi-balanced rudder is chosen as the benchmark model. All simulations are conducted in the numerical environment FINETM/Marine with the ISIS-CFD code as the flow solver. It solves impressible unsteady RANS equations in full hexahedral unstructured meshes and implicitly couples the flow field with motion equations of a rigid body in 6 DOF. Current direct manoeuvring simulations are achieved by means of the overlapping grid technique. To reduce the computational effort, propeller effect is modelled by a simple prescribed body force model. Trajectories are straightforward recorded without any further treatment to extract hydrodynamic derivatives. The prediction accuracy is evaluated by comparing derived parameters, i.e. overshoot angles and times, peak yaw rates, drift angles, etc. against experimental data. In conclusion, 4 DOF and 6 DOF concept present similar results for current ship type. The tiny changes in pitch and heave motion indicate they can be neglectable to simplify the complex mesh strategy. In addition, the large roll angle over zig-zag manoeuvres implies that 4 DOF concept should be more reasonable for container ships to obtain roll-motion-related data. Meanwhile, 3 DOF concept underestimates all overshoot angles in each simulation. This also highlights the reasonability of 4 DOF concept