Design of ducted propeller nozzles through a RANSE-based optimization approach

Marine propellers design requirements are always more pressing and the application of unusual propulsive configurations, like ducted propellers with decelerating nozzles, may represent a valuable alternative to fulfill stringent design constraints. Accelerating duct configurations were realized mainly to increase the propeller efficiency in the case of highly-loaded functioning. The use of decelerating nozzles sustains the postponing of the cavitating phenomena that, in turn, reflects into a reduction of vibrations and radiated noise. The design of decelerating nozzle, unfortunately, is still challenging. The complex interaction between the propeller and the nozzle, both in terms of global flow feature and local (tip located) phenomena, is not yet fully understood. No extensive systematic series, as in the case of accelerating configurations, are available and the design still relies on few measurements and data. On the other hand, viscous flow solvers appear as reliable and accurate tools for the prediction of complex flow fields and their application for the calculation of ducted propeller performance and nozzle flow was almost successful. Hence, using CFD as a part of a design procedure based on optimization, by combining a parametric description of the geometry, a RANSE solver (OpenFOAM) and a genetic type algorithm (the modeFrontier optimization environment), is the obvious step towards an even more reliable ducted propeller design. An actuator disk model is adopted to include efficiently the influence of the propeller on the flow around the duct; this allows avoiding the weighting of the computational effort that is necessary for the calculations of the thousands of geometries needed for the indirect design by optimization. Design improvements, in model scale, are measured by comparing, by means of dedicated fully resolved RANSE calculations, the performance of the optimized geometries with those of conventional shapes available in literature. For both nozzle typologies, dedicated shapes reducing the risk of cavitation and increasing the delivered thrust are obtained, showing the opportunity of customized nozzle design out of usual systematic series. In addition, by analyzing the results of the optimization histories, appropriate design criteria are derived for both accelerating and decelerating nozzle shapes.