Generating Pre-swirl Using Mesh Deformation

The Orca-class vessels of the Royal Canadian Navy are equipped with an open shaft arrangement that consists of three foil-shaped brackets: two that extend from the hull to support the propeller and shaft and a third that extends downwards from the hub. All brackets are located slightly forward of the propeller disc and therefore affect the propeller inflow. By manipulating the radial pitch distribution of these brackets, pre-swirl can be generated to counteract propeller induction and potentially reduce the energy lost to flow rotation. Rather than manually rotating the brackets using a computer-aided-design tool, mesh deformation is employed to twist the brackets using a single baseline geometry and mesh. The pitch distributions of the brackets are parameterized to facilitate a simplified design space exploration to determine if performance benefits can be realized in terms of power delivered and cavitation generation. Coupled viscous and potential flow calculations are used to consider the influence of the rotating propeller on the aft-ship region. Furthermore, a simplified one-way alternative is shown to provide similar results with a reduced computational requirement when compared to the typical iterative approach. In the preliminary design space exploration, it was found that applying twist to the outboard bracket permits a slight power reduction over the baseline case but comes with an associated penalty in cavitation behaviour.