Escort tug hydrodynamics: analysis of flow around ships at large yaw angles

Molyneux, William David (2006) Escort tug hydrodynamics: analysis of flow around ships at large yaw angles. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The ship hydrodynamics literature contains little information on a hull at a yaw angle greater than that commonly encountered in ship manoeuvring. New ship types, such as escort tugs with very low aspect ratio fin keels, will benefit from an analysis of the hydrodynamics that occurs at yaw angles between 20 and 45 degrees. Computational Fluid Dynamics (CFD) is an important technique that could be used for this analysis, but the accuracy of predicted forces and flow patterns at high yaw angles was unknown prior to this research. A new data set of three-dimensional flow vectors in planes around an escort tug model was obtained using Particle Image Velocimetry (PIV). These data were used to validate the flow vectors predicted by a commercial CFD code. As part of the validation study, a method for numerically analyzing the difference between measured and predicted flow vectors was developed. The method was used to evaluate CFD predictions of flow patterns around a conventional hull (at 10 and 35 degrees yaw) against published experimental results. The type of computational mesh was found to affect the accuracy of the forces predicted for a hull with a yaw angle, but different hull types needed different meshing approaches. The forces at 10 degrees yaw for a typical high-speed merchant ship were predicted to within 5% of experimental results using an unstructured tetrahedral mesh, whereas a structured hexahedral mesh gave force predictions accurate to within 15%. For the escort tug model, which was a wider and shallower hull shape, the situation was reversed, and the structured mesh gave force predictions accurate to within 5% of the experimental data up to 40 degrees yaw. There was no noticeable difference in the predicted flow patterns between meshing approaches for the tug model. Mean flow vector magnitudes were within 10% of measured values. As a result of this research PIV has been developed into a practical technique for measurements around a hull with a yaw angle and CFD has been shown to give insights into the flow around an escort tug and its appendages, within a specified level of accuracy.

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