Numerical analysis on hydrodynamic performance of a rim-driven thruster near a free surface

To investigate the hydrodynamic characteristics of a rim-driven thruster (RDT) operating under near-free-surface conditions, numerical simulations of the RDT flow field were conducted using a Computational Fluid Dynamics (CFD) approach. The Reynolds-averaged Navier–Stokes equations (RANS) were solved in conjunction with the shear stress transport SST k - ω turbulence model and the Volume of Fluid (VOF) multiphase model. The simulations encompass a range of submergence ratios, namely I / R = 1.0, 1.2, 1.4, and 1.6, and advance coefficients spanning J = 0.1 to 0.7. The results demonstrate that the presence of the free surface exerts a pronounced influence on the hydrodynamic performance of the RDT. As the submergence ratio decreases, the ventilation effect becomes progressively more intense, leading to reductions in both thrust and torque relative to open-water conditions. The most severe degradation in performance occurs at the combination of a low advance coefficient ( J = 0.1) and a shallow submergence ratio ( I / R = 1.0). Flow field analysis reveals that, owing to the influence of the free surface, the pressure distribution over the blade surfaces exhibits notable asymmetry. This asymmetric pressure field induces periodic fluctuations in the vertical and lateral forces acting on the RDT and is accompanied by the formation of free-surface vortices and air-ventilation phenomena. Furthermore, with specific attention to the characteristic rim-gap configuration of the RDT, the study finds that the entrained air forms an air–water mixture flow within the gap. Consequently, the rim frictional torque decreases as the volumetric air fraction increases. This investigation elucidates the evolutionary mechanisms governing the hydrodynamic performance of an RDT in the vicinity of a free surface and provides a valuable reference for the optimal design and operational deployment of RDTs in such environments.