Integrated Motor/Propulsor Duct Optimization for Increased Vehicle and Propulsor Performance

Integrated electric motor/propulsor technological development offers the potential to increase usable volume for undersea vehicles by locating the electric motor in the duct. This has the added advantage that the electric motor has increased usable torque due to the increased radius. For many torpedo and unmanned undersea vehicle applications, however, the maximum vehicle diameter is limited by design. This places significant constraints on the vehicle and propulsor design in order to maximize hydrodynamic performance. The electric motor requires a significant duct thickness that both increases hydrodynamic drag due to the presence of the duct as well as limiting the maximum propeller radius. Both constraints result in diminished propulsor performance by both increasing overall drag and reducing the propulsive efficiency. In order to meet vehicle design objectives related to maximum vehicle speed and associated power requirements, a computational study was conducted to better understand the underlying fluid dynamics associated with various duct shapes and the resultant impact on both total vehicle drag and propulsor efficiency. As a baseline to this study, a post-swirl propulsor configuration was chosen (downstream stator blade row) with a 9 blade rotor and 11 blade stator. A generic torpedo hull shape was chosen and the maximum duct radius was required to lie within this radius. A cylindrical rim driven electric motor capable of generating a specific horsepower to achieve the design operational velocity required a set volume and established a design constraint limiting the shape of the duct. With this constraint, the duct shape was varied to produce varying constant flow acceleration from upstream of the rotor blade row to downstream of the stator blade row. The mean flow acceleration was derived from a constant mass flow relation. The axisymmetric Reynolds Averaged Navier-Stokes version of Fluent® was used to examine the fluid dynamics associated with a range of accelerated and decelerated duct flow cases as well as provide the base total vehicle drag. For each given duct shape, the Propeller Blade Design Code, PBD 14.3 was used to generate an optimized rotor and stator. To provide fair comparisons, the circulation distribution and maximum rotor radius were held constant to generate equivalent amounts of thrust. Propulsor efficiency could then be estimated based on these calculations. Calculations showed that minimum vehicle drag was produced with a duct that produced zero mean flow acceleration. Ducts generating accelerating and decelerating flow increased drag. However, propulsive efficiency based on blade thrust and torque was significantly increased for accelerating flow through the duct and reduced for decelerating flow cases. Full 3-D RANS flow simulations were then conducted for select test cases to quantify the specific blade, hull and duct forces and highlight the increased component drag produced by an operational propulsor, which reduced overall propulsive efficiency. Based on these results, an optimum rotor balancing vehicle drag and propulsive efficiency is proposed.