Numerical investigation of ship responses in head sea

Abstract

This thesis aims to provide a numerical framework to investigate ship motions in head train waves with sufficient accuracy and least computational and financial effort. OpenFOAM CFD software is utilized along with the waves2Foam package to assess the problem. Due to the complexity of the problem three steps are taken to reach the goal.

The first step was a systematic grid refinement study of wave propagation in a two-dimensional wave flume tank. An example ocean wave was defined using a grid with a specific number of divisions over the wave height and with a specific aspect ratio. The cells in the directions of wave height and wavelength are refined four times, each time with factor of square root of two. Two courant numbers were considered, resulting in a total of 50 cases. The results show no adverse effect with long cells meaning higher edge length in the wave-propagation direction. It was found that tall cells (high y-to-x aspect ratio) caused ripples in the free-surface. It was found that the number of cells over the wave height has the most impact on the computed wave amplitude. It was also found that the phase shift is a function of the time step and the number of cells in the wavelength direction.

In the second step, forces on a cylinder in a three-dimensional domain were assessed considering two grids. Comparing the results with experimental data shows an excellent agreement for both cases.

The third step was computing ship motions with a constant forward speed in head waves. SST turbulence model is utilized to deal with turbulence features of the problem. The ship is set to pitch and heave freely, but other motion components were fixed. Due to the symmetric nature of the problem, only a half-domain is simulated to reduce the computational effort. The results were compared to experimental data, and favorable accuracy is obtained.

At the final stage, the ship model was enlarged with a factor of 5 and assessed with a comparable set-up and mesh to model scale simulation. The result shows higher values of pitch motions. Some possible reasons for these discrepancies between different model scales are discussed.