Optimization design of the duct of a rim-driven thruster using the adjoint approach

In this study, the adjoint method is employed for the duct optimization of a rim-driven thruster (RDT). The work is mainly focused on the efficiency improvement of the thruster by adopting a more effective duct profile. As baseline design, a duct with symmetrically round-shaped ends and a flat middle part with a Ka4-70 propeller inside is considered. For applicability considerations, steady-state numerical simulations are carried out to solve the Reynolds-averaged Navier-Stokes equations (RANS) with the moving reference frame (MRF) method. The adjoint method modifies the geometry of the symmetric duct into a more streamlined shape while keeping the aspect ratio (the ratio of duct length and thickness) constant. The original and final profiles of the duct and their influence on the overall performance of the RDT are presented and analyzed. The results indicate that after optimization, the efficiency of the thruster is increased by an absolute value of 3 to 10%, depending on the advance coefficient. The thrust torque ratios of the propeller and rim have also increased, meaning the RDT can provide a higher thrust for an equivalent absorbed torque. Moreover, a preliminary examination of the cavitation number based on the lowest pressure on the blade surface indicates that the RDT with optimized duct has a better cavitation performance as well.