International Journal of Small Craft Technology
ESCORT TUG AT LARGE YAW ANGLE: COMPARISON OF CFD PREDICTIONS WITH EXPERIMENTAL DATA
D Molyneux, Institute for Ocean Technology, Canada N Bose, Australian Maritime College Launceston, Australia SUMMARY
Escort tugs operate at high yaw angles in order to produce forces to steer and stop the vessel they are escorting in an emergency. In this paper, RANS predictions of forces and flow patterns around the hull of an escort tug model are compared with experimental data. Two alternative meshing strategies were used, one using tetrahedral elements with triangular faces and one using hexahedral elements with quadrilateral faces.
Experiments were carried out with and without the low aspect ratio fin that is typical of many escort tugs. Lift and drag forces were measured experimentally for yaw angles from 15 to 45 degrees. Flow measurements around the tug at 45 degrees yaw were obtained using a stereoscopic particle image velocimetry (PIV) system.
The results from each CFD simulation were compared to the measured flow patterns using a numerical procedure that led to a quantitative measure of the accuracy of the predicted results. The analysis of the flow patterns indicated that the main features of the flow were predicted, and that on average, the predicted velocity magnitudes were within 10% of the measured values. Neither mesh approach had a significant effect on the accuracy of the flow pattern predictions. The hexahedral mesh gave more accurate force predictions that the tetrahedral mesh. Forces were predicted by the CFD code with this mesh to within 5 % of the experimentally obtained values.
1. INTRODUCTION
Classical ship hydrodynamics focuses on ships moving forward in a straight line, or turning slowly under the action of a foil like rudder in calm water. These are generally considered to be the design conditions, and the ‘off-design’ conditions, where these assumptions are no longer valid have seldom been studied. An escort tug is a vessel where ‘off-design’ hydrodynamics are an essential part of the ship’s operational profile (Allan & Molyneux, 2004). In this situation, the tug’s hull and propulsion system are positioned to create a hydrodynamic force, which is used to bring a loaded oil tanker under control in an emergency. The tug is attached to a towline at the stern of the tanker, and by using vectored thrust, it is held at a yaw angle of approximately 45 degrees. The maximum practical speed of operation for escort tugs is about 10 knots. The designs of escort tugs to date have been developed from practical experience and model experiments to measure lift and drag forces. A full understanding of the flow has not been developed, and it is unlikely that escort tugs can be developed to their full potential without this knowledge.
One method of trying to understand the flow around a hull with a large angle of attack (yaw angle) is to use computational
fluid dynamics (CFD). The basic
equations of fluid motion can be combined with the hull geometry and some assumptions about the turbulence in the flow to give mathematical predictions of the pressure on the hull surface and the flow vectors within the fluid. Very little numerical analysis has been carried out on the
hydrodynamics of hull shapes designed to operate at large yaw angles, and so the accuracy of CFD in these situations is unknown.
An earlier study of the ability of a commercial Reynolds Averaged Navier-Stokes (RANS) CFD code to predict flow patterns around a Series 60 CB=0.6 hull with yaw (Molyneux and Bose, 2007) concluded that there was very little difference in the predicted flow patterns between an unstructured mesh made from tetrahedral elements and a structured mesh made from hexahedral elements, when each was compared with experiment data. The Series 60 hull was not designed for large angles of attack to the flow and there was no force
data
available for the hull above 10 degrees of yaw, so the comparison was incomplete.
It was recommended (Molyneux & Bose, 2007) that the conclusions on the best meshing strategy for the Series 60 hull should be checked using hull forms designed to operate at yaw angles over 30 degrees. This paper extends the comparison of forces and flow patterns calculated using tetrahedral
and hexahedral CFD
meshing strategies to a hull shape designed specifically to operate in ‘off-design’ conditions given by yaw angles up to 45 degrees. Some conclusions are made on the effectiveness of commercial RANS based CFD codes within the design process for ship hulls that are required to operate at large yaw angles.
©2008: Royal Institution of Naval Architects
B-41
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