| Abstract | One of the main design requirements of an escort tug is that it must operate at speeds up to 10 knots, with a yaw angle between 35 and 55 degrees. In this condition the hull generates a large hydrodynamic force, which is used to turn a disabled tanker. A significant feature of an escort tug design is the large, low aspect ratio fin at the bow, which is not common on other types of ship. This fin generates 50 percent of the total hydrodynamic force at the operating yaw angles, and so it is expected that it will have a large effect on the flow patterns around the tug. Preliminary experiments were carried out to develop some of the techniques necessary for obtaining reliable results from PIV measurements for a hull with a yaw angle in a towing tank (Molyneux & Xu, 2005). This work included the development of a prototype seeding system and finding the most suitable orientation of the laser sheet relative to the direction of motion of the hull. Some preliminary predictions of the flow patterns for a yaw angle of 45 degrees, with and without the fin were made using Fluent, a commercial computational fluid dynamics program (Molyneux, 2005) and the results of these simulations were used to plan the experiments described in this report. The preliminary CFD simulations had shown that the effect of the fin was most visible on the flow patterns under the hull, on the downstream side of the centreline. This region of the flow should contain a large vortex formed by the fin. In order to visualize this large vortex, the laser plane for the PIV system needed to be oriented across the direction of the undisturbed flow. This report describes model experiments carried out to measure the flow patterns around a scale model of an escort tug using Particle Image Velocimetry (PIV). The PIV system is described in detail elsewhere (Molyneux, 2006). This report describes the experiment methods and presents the results. Some discussion on the results is given and recommendations for improvements to the experiment techniques are made. |
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