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

This paper presents a computational study to better understand the underlying fluid dynamics associated with various duct shapes and the resultant impact on both total vehicle drag and propulsor efficiency. A post-swirl propulsor configuration (downstream stator blade row) was selected with rotor and stator blade number kept constant. A generic undersea vehicle hull shape was chosen and the maximum shroud radius was required to lie within this body radius. A cylindrical rim-driven electric motor capable of generating a specific horsepower to achieve the design operational velocity required a set volume that established a design constraint limiting the shape of the duct. Individual duct shapes were designed to produce constant flow acceleration from upstream of the rotor blade row to downstream of the stator blade row. Ducts producing accelerating and decelerating flow were systematically examined. The axisymmetric Reynolds Averaged Navier–Stokes (RANS) version of fluent® was used to study 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 maximum rotor radius was held constant with similar circulation distributions intended to generate equivalent amounts of thrust. Computations predicted that minimum vehicle drag was produced with a duct that produced zero mean flow acceleration. Ducted designs generating accelerating or decelerating flow increased drag. However, propulsive efficiency based exclusively on blade thrust and torque was significantly increased for accelerating flow through the duct and reduced for decelerating flow cases. Full 3D RANS flow simulations were then conducted for select test cases to quantify the specific blade, hull, and shroud forces and highlight the increased component drag produced by an operational propulsor, which reduced overall propulsive efficiency. From these results, a final optimized design was proposed.