Vortical structures in the wake of a rim-driven thruster operating at maximum efficiency

Abstract: The rim-driven thruster (RDT) has drawn much attention worldwide and experienced rapid development. Instead of being shaft-driven as in a traditional propulsion system, RDTs have a much more compact design by integration of the driving motor with the propeller. The structure of an RDT resembles a ducted propeller, yet without a tip clearance as the propeller is directly connected to the rim. Despite being an emerging technology, the flow structures present in the wake of RDTs are still largely unknown. However, the structure and dynamics of the RDT wake are crucial in performance prediction and estimation of noise production. Therefore, this paper studies the vortical structures found in the wake of an RDT operating at maximum efficiency. Delayed detached eddy simulations are performed and the wake flow is analyzed using both visualization of the instantaneous vortical structures and large scale structure extraction by proper orthogonal decomposition (POD). Three main structures have been identified. The first one is the central hub vortex, which is the strongest one and it is aligned with the flow direction and precesses around the central axis of the RDT. Next to the hub vortex, small scale vortices are formed after the duct in the shear layer between the wake flow and the surrounding and these are convected downstream. The last structures are the trailing vortices shed by the propeller blades and these are dissipated quite fast downstream. Keywords: Rim-driven thruster, Delayed detached eddy simulation, Vortical structures, Proper orthogonal decomposition