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

Stefano Gaggero, Diego Villa, Giorgio Tani, Michele Viviani, Daniele Bertetta (University of Genova)

CHALLENGE - 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.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.

SOLUTION - This study proposes an optimization approach for the design of the accelerating
and decelerating nozzles for ducted propeller applications. The design workflow combines a parametric description of nozzle shape, a RANSE solver (OpenFOAM) and a genetic algorithm provided by the modeFRONTIER optimization environment. The design of nozzles through optimization was achieved starting
from two reference geometries available at the cavitation tunnel of the University of Genoa. In both cases, the design consists in the analysis of an initial population of 150 members. 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

BENEFITS - In the specific case of the accelerating duct, the optimization procedure demonstrated the possibility to improve the propulsive efficiency at unvaried cavitation inception speed by maximizing the delivered thrust of the nozzle and, in the end, by redesigning the propeller in the more favorable (unloaded) functioning condition allowed by the increase of the nozzle thrust.