Ducted propellers have been used as an effective method of propulsion due to its higher efficiency especially when high thrust is required at low speeds. In particular, the ducted propellers are highly useful in station keeping of floating offshore structures. For example, both the floating production, storage and offloading unit (FPSO) and liquid natural gas (LNG) hulls keep their positions with the help of the dynamic positioning by ducted propellers with a rotating pod. Predicting the performance of the ducted propellers in low advance ratios is very important since most of the floating structures are designed to keep their position at a fixed location.
In the 21st Offshore Symposium of Texas SNAME, Ocean Engineering Group (OEG) of UT Austin presented a full wake alignment scheme in which the trailing vortex wake sheets of the blades are aligned with the local flow velocity considering the effect of duct and duct wake. The repaneling process applied to the existing panel method was also investigated to show the improved accuracy of the predicted forces on the blade and duct.
In this paper, improvements in the predicted pressure distribution on the blade by using a panel method are described. A square-tip propeller inside a sharp-trailing edge duct is considered. The two repaneling processes on the duct and duct wake are also applied to improve the predicted forces and pressure distributions on the blade and duct. In order to improve the predicted results at lower advance ratios special care must be given to the arrangement of control points on the duct, as well to avoid penetration of the blade wake into the duct surface.
The results from the panel method with the two repaneling processes are compared with those from a three-dimensional full-blown RANS simulation as well as the experimental data. The predicted pressure distributions on the blades show very good agreements with the results from the full-blown RANS simulation. Also, the forces on the blade and duct are in very good agreement with the experimental data, at design and at lower advance ratios.