Ducted propellers have been, for a long time, a viable alternative of propulsion in the ocean engineering community; due to their higher efficiency at high thrust coefficients, less sensitivity to the ambient flow, and more robust mechanical layout, than open propellers. Applications of ducted propellers or thrusters can be found in many types of ships, particularly in floating production, storage and offloading (FPSO) and liquid natural gas (LNG) hulls for maintaining a vessel's position.
A hybrid method that couples a Vortex-Lattice Method (VLM) solver with a Reynolds-Averaged Navier-Stokes (RANS) solver has been developed by the Ocean Engineering Group at UT Austin, to simulate the flow around ducted propellers. The method takes advantage of both VLM and RANS simulations, and has been found to predict very well the thrust and torque on ducted propellers over a wide range of operating conditions, while requiring significantly less computational time than a full-blown RANS simulation. Nonetheless, the prediction of the ducted propeller forces is only one of the objectives in developing this hybrid method. The authors are also interested in more detailed results, such as the pressure distribution on the propeller blades and the duct. Accurate pressure distribution not only serves as an important validation of the method, but is also essential for cavitating flow simulations.
In this paper, full-blown RANS simulations for different types of ducted propellers are performed with sufficient spatial resolution. The hybrid method is also used to simulate the same cases. The predicted forces and pressure distributions on the blades and duct, from the two methods, are compared with each other, as well as with existing experiments.