Numerical design of a RIM-driven thruster using a RANS-based optimization approach

A simulation-based design optimization (SBDO) tool is proposed for the design of rim driven thrusters. The optimization framework consists of a parametric description of the rim blade geometry and a multi-objective optimization algorithm which makes use of the results from high-fidelity RANS calculations to drive the choice towards optimal blade shapes. Maximization of the propulsive efficiency and minimization of cavitation, monitored through the simplest cavitation inception criterion based on the analysis of the non-cavitating pressure distribution over the blade, are the contrasting objectives selected for the design. A constraint on the delivered thrust fixes the functioning condition of the devised propellers. Four distinct design runs, changing the number of blades, from three to six, are considered. The reference performance are those of a ducted propeller operating in a decelerating nozzle which is exactly used in the case of rim configuration. Results of the optimizations prove the flexibility and the reliability of the SBDO framework in dealing with unconventional configurations. A generalized reduction of the risk of cavitation is observed regardless the number of blades; five- and six-bladed propulsors ensure remarkable (about 40%) higher margins with respect to any (leading edge and midchord) cavitating phenomena at the cost only of a slight reduction of efficiency. Also, the structure of the tip vortexes results significantly modified.