Complexity in Modelling Electric Marine Propulsive Devices
Advances in electromagnetic technology have enabled the design of novel devices for marine propulsion using structurally integrated permanent magnet brushless motors to drive the propeller by its rim. This has many technological advantages over more conventional shaft driven propulsion by being very compact, it also has no gearbox or shaft either upstream or downstream of the propeller allowing for more efficient propulsion as well as being well suited to bi-directional operation. However, improvement to existing designs of electromagnetically rim driven thrusters is hindered by a limited understanding of the impact of design parameters on the flow features.
Computational modelling of the fluid dynamics of propellers has been performed for many decades with a variety of techniques and assumptions used to make the problem tractable. These methods range from blade element methods based on panel codes to fully three dimensional RANS (Reynolds Averaged Navier Stokes equations) solutions. Despite the efforts to model propellers as best as possible, there are a number of features of the flow that are inherently difficult to emulate. Cavitation, where fast moving fluid vapourises due to the low pressure and then forms bubbles that collapse and erode away the propeller, is very difficult to predict accurately and requires multiphase simulation methods. There is also radial pumping of the flow along the propeller giving rise to three-dimensional boundary layer effects that are too complex to be modelled by eddy-viscosity turbulence models that employ the Boussinesq approximation. The transient dynamics of propellers are also something that has not been fully explored and is something of great interest for an electrical thruster from a control perspective. In addition to these complex flow features of propellers, rim driven thrusters under the correct conditions can produce emergent phenomena such as Taylor-Couette vortices in the gap between the housing and the rim. Also, because of the proximity of electric and magnetic fields to the flow of what is usually a conducting medium, there are possible electromagnetohydrodynamic interactions that are unexplored.
The novel design of rim driven thrusters provides some unprecedented challenges and interesting research questions, the key question being `Is it possible to model all the rich physical interactions occuring within a rim driven thruster to gain insight into the flow features that may inform design decisions to produce a more efficient device?' To answer this research question, an open source CFD package, OpenFOAM, shall be used.
To begin with, a valid hydrodynamic model needs to be established and validated, including a suitable turbulence model to capture all the complex flow features. The hydrodynamic model will then be used, in conjunction with surrogate modelling design optimisation methods, to improve the propulsive efficiency of the device.
Simulation software: OpenFOAM