Computational fluid dynamics simulation of a rim driven thruster

Abstract

An electric rim driven thruster is a relatively new marine propulsion device that uses a motor in its casing to drive a propeller by its rim and the fluid dynamics associated with their operation have not been fully investigated. There are many interacting flow features that make up the flow field of a rim driven thruster that pose a number of challenges when it comes to simulating the device using computational fluid dynamics. The purpose of this work is to develop a computational fluid dynamics solution process that accurately simulates features including vortex generation and behaviour, radial pumping and rotor-stator interaction while attempting to minimise computational costs. This will enable the method to be used to calculate an objective function, typically the thrust or propulsive efficiency of the device, in a design optimisation study. Implementation within a design optimisation study also requires the numerical methods to be easily repeatable and robust in both mesh generation and solution.Mesh generation was performed using snappyHexMesh, a meshing program that is part of OpenFOAM, and a thorough mesh verification procedure has been conducted. Validation of the computational fluid dynamics solution of a standard series propeller, as a baseline case with good experimental data from MARIN, using the open source Reynolds-Averaged Navier-Stokes solver MRFSimpleFoam (part of the OpenFOAM software) has been performed. Results show a great sensitivity to computational domain size that suggest that similar previous works may have used an insuffcient domain size. In particular, it is shown that a number of boundary conditions may be used if the domain is large enough. Also, comparisons are made between the Re-Normalisation Group (RNG) k-e and k-w Shear Stress Transport (SST) turbulence models (the most widely reported models in the literature), and the k-w SST model is found to be robust due to its better handling of the separation that occurs at low propeller advance ratios. Validation against experimental data for the standard series propeller shows good agreement to within 5%.The validated solution method is then applied to a rim driven thruster and key design areas are highlighted by the results. The rim is found to be an important region of the flow, the drag on which comprises almost half of the torque losses in the device. Interaction between the rotors and the stators is also a key area, with both thrust and torque changing as the position of the blades is varied