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The study of ship motions in regular waves using a mesh-free numerical method

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Cartwright, BK (2012) The study of ship motions in regular waves using a mesh-free numerical method. Research Master thesis, University of Tasmania.

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Abstract

Mesh-free methods are becoming popular in the maritime engineering fields for their ability
to handle non-benign fluid flows. Predictions of ship motions made using mesh-free methods
need to be validated for benign conditions, such as regular waves, before progressing to nonbenign
conditions. This thesis aims to validate the response of a ship in regular waves by the
Smoothed Particle Hydrodynamics (SPH) mesh-free method.
Specifically, the SPH technique uses a set of interpolation points, designated SPH particles,
located at nodes that track the centre of discrete fluid volumes with time. As part of this
research a set of simple rules was established to locate the free surface of the fluid based on
the location of the SPH particles. These simple rules were then used to validate the
hydrostatics of a ship floating in the fluid, identifying the vertical location of the water line to
0.22% of the Design Water Line length.
The propagation of regular waves in SPH has historically been problematic, resulting in
diminishing wave height with propagation distance. In this study, non-diminishing deepwater
regular waves were generated in a shallow tank by moving segments of the floor in
prescribed orbital motions, a technique developed by the researcher and hereinafter called the
moving-floor technique. The resulting waves showed no discernible loss in wave height with
propagation distance, and were computationally more efficient than modelling a full-depth
tank. The resulting surface profiles of the waves were within ± 5% of the theoretical values,
while the velocity and pressure profiles were within ± 10%.
The pitch and heave transfer functions for a round bilge high speed displacement hull form at
Froude numbers of 0.25 and 0.5 were predicted using waves in SPH developed by the
moving-floor technique. These predictions were compared to transfer functions obtained
from experiments in a towing tank. The results obtained using SPH generally underpredicted
the experimental results by about 10%, but by as much as 50% at peaks or at high
frequencies where the responses were small. Reasons for the under-prediction by the SPH
technique are discussed in this thesis.
The outcomes of this research demonstrate that with some refinement, the SPH technique
should be capable of accurately predicting the motions of a ship in regular waves. It is hoped
this work will serve as a stepping stone to exploit the flexibility of the SPH technique to
analyse any shape hull, to be applied to non-linear waves, and to be coupled with a structural
solver.

Item Type: Thesis (Research Master)
Keywords: ship motions, regular waves, smothed particle hydrodynamics, SPH, mesh-free
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Date Deposited: 13 Jan 2013 23:39
Last Modified: 11 Mar 2016 05:52
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