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Research on the Optimal Design and Fluid Flow Control Mechanism of a Rim-driven Thruster

  In conventional shaft-driven propulsion systems, the shaft has to be connected to the main engine by another intermediate transmission device, in which way too much space is taken up. Moreover, the complex structure causes great mechanical loss, resulting in low efficiency and creating high vibrations and noise. On the contrary, the rapidly developing rim-driven thruster (RDT) is gaining more and more popularity in recent years. Comparing to conventional propulsors, the RDT has many outstanding advantages and is surely a major topic of development in the future. The Rim-driven thruster, also known as integrated motor propeller, has gained much attention all over the world since the concept was brought up for the first time. The RDT integrates the driver motor with propeller blades to form a more compact design, which is totally different from conventional propulsors. The propeller blades are connected to the rotor of the motor, while the stator is embedded in the duct, and when the thmster works, the rotor drives the propeller blades to rotate and produce thi-ust which is then delivered by bearings. In this way, the high integrated structure cuts the need for a large design space, improves working efficiency significantly and more importantly reduces vibration and noise greatly, making vessels quieter and more comfortable. The RDT was applied in the military field in the beginning, but nowadays the application of RDT's has been extended to yachts, civil boats and AUVs. Despite the many benefits the RDT possesses, there are still several technical problems in the optimal design of a RDT. Therefore, the study of the optimal design and fluid flow control mechanism of a RDT is definitely a worthy topic. The flow field generated by a RDT in open water is quite complicated and varies randomly, making it a tough challenge to capture the very details of the flow characteristics with experimental methods. With the development of computer technology, fortunately, the CFD methods promote a convenient way of addressing these kind of problems. Costs can be reduced with CFD simulation since the CFD method occupies relatively less resources and provides acceptable results. The nature of the fluid flow in a RDT is turbulent and currently there are 3 ways of numerical simulation of turbulence, which are Direct Numerical Simulation (DNS), Reynolds Averaged Navier-Stokes (RANS) and Large Eddy Simulation (LES). The advantage of the DNS method is that precise information of the fluid flow can be acquired and the disadvantage is that the demand for computer power cannot be met when employing the DNS method to calculate flows with high Reynolds number. RANS methods can solve high- Reynolds problems, at the cost that small details of flow field are missing because the results are obtained by computing mean motions of the fluid. LES can be used to calculate the motion of large-scale vortices based on the transport mechanism of kinetic energy, and the influences of the small scales on the large scales can be simulated by establishing models. As such, more details of the flow field can be revealed at less cost, and that's why the LES method is applied more widely now. RANS methods are widely used due to its common requirements for computer resources and extensive application scope. At present, many studies on the performance analysis of RDT's are carried out with CFD methods employing RANS to achieve the steady flow field. However, RANS has its limits in the sense that detailed information about flow characteristics is lost with this method, therefore flow mechanisms cannot be revealed. Moreover, the performance derived from a propeller, and the inevitable losses, arise from the interaction of a large number of flow features, each adding to the complexity of the flow and rendering the task of its simulation more difficult. A solution to this problem can be found in the unsteady RANS method, were the large scale transient flow features are resolved. This method seems feasible for simulation of a RDT, but this method is currently not applied frequently. Therefore, this project will focus on the optimal design and fluid flow control mechanism of a RDT with URANS and LES methods. By establishing numerical models with different parameters and different methods, the steady and unsteady results and details will be analyzed to optimize the structure design of RDT, and unsteady characteristics, like cavitation, will be studied, and the flow field will be assessed as well as the flow mechanisms.

Date:29 Oct 2018 →  Today
Keywords:Rim-driven thruster, CFD, OpenFOAM, URANS, LES
Disciplines:Other engineering and technology
Project type:PhD project