The performance and cavitation characteristics of marine propellers and hydrofoils are strongly affected by tip vortex behavior. A number of previous computational studies have been done on tip vortices, both in aerodynamic and marine applications. The focus, however, has primarily been on validating methods for prediction and advancing the understanding of tip-vortex formation in general, rather than showing effects of tip modifications on tip vortices. Studies of the most relevance to the current work include computational studies by Dacles-Mariani et al. (1995) and Hsiao and Pauley (1998, 1999). Dacles-Mariani et al. carried out interactively a computational and experimental study of the wingtip vortex in the near field using a full Navier-Stokes simulation, accompanied with the Baldwin-Barth turbulence model. Although they showed improvement over numerical results obtained by previous researchers, the tip vortex strength was underpredicted. Hsiao and Pauley (1998) studied the steady-state tip vortex flow over a finite-span hydrofoil, also using the Baldwin-Barth turbulence model. They were able to achieve good agreement in pressure distribution and oil flow pattern with experimental data and accurately predict vertical and axial velocities of the tip vortex core within the near-field region. Far downstream, however, the computed flow field was overly diffused within the tip vortex core. Hsiao and Pauley (1999) also carried out a computational study of the tip vortex flow generated by a marine propeller. The general characteristics of the flow were well predicted but the vortex core was again overly diffused. In this study, a computational comparison of the performance of rounded tip and ducted tip hydrofoils has been performed, with the long-term goal of improving marine propeller performance by optimizing duct geometry. A ducted tip hydrofoil/propeller is one in which flow-through ducts, aligned approximately with the hydrofoil/blade chord, are affixed at the hydrofoil/blade tips. The ducted tip geometry for a hydrofoil was first proposed by Green et. al (1988). Water and wind tunnel tests have shown that the flow-through ducts suppress the tip vortex roll-up, thus resulting in a substantial delay in the onset of tip vortex cavitation (Green and Duan, 1995). This comes with little change in the lift to drag ratio. The ducted tip has also been studied on a propeller. Sea trials on a ducted tip propeller, and a conventional one of the same diameter, conducted by Hordnes and Green, (1998) showed that the cavitation inception index could be reduced by approximately 50% by installing the ducted tips. This came without efficiency loss. The efficiency of the ducted tip propeller is in fact up to 6% higher than the efficiency of the conventional propeller. In the present study, steady flow over rounded and ducted tip hydrofoils has been studied computationally. The aim of the study was to expand our knowledge and understanding of the flow around a duct attached to the tip of a hydrofoil and thus provide a good basis for computational optimization of a ducted tip propeller blade.
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