Numerical investigation of a pump-jet with ring rotor using an unstructured mesh technique

Abstract

In the present study, the hydrodynamic characteristics of a pump-jet were numerically investigated using an incompressible RANS flow solver based on pseudo-compressibility. The focus of the study is the assessment of the flow characteristics and performance, particularly of a ring rotor with the rotor blades interconnected with a circumferential ring at the tip. For this purpose, a vertex-centered finite- volume method on unstructured meshes was used in conjunction with second-order Roe's FDS to discretize the inviscid fluxes, while the viscous fluxes were computed based on central differencing. For the unsteady time integration, a dual-time stepping method and the Gauss-Seidel iteration were employed. An unstructured overset mesh technique was adopted to treat the relative motion between the rotor and the main body of the pump-jet. For the closure of turbulence, the Spalart-Allmaras one equation model was employed. To validate the flow solver, the calculations were initially made for the High Reynolds number pump (HIREP) configuration at several advancing ratios. Reasonable agreements were obtained between the present results and the experiment in terms of the pressure coefficient on the blade surface, velocity distribution, and integrated blade loadings. To investigate the effect of the ring on the flow characteristics and propulsion performance of the pump-jet, additional calculations were made for the HIREP configuration installed with the circumferential ring at the rotor blade tip, and the results were compared with those of the original HIREP configuration. The addition of the ring at the blade tip helped reduce the tip vortex strength, which might be beneficial in suppressing the formation of water vapor. Furthermore, the cavitation performance on the suction surface of the rotor blades was degraded by the reduced axial velocity because of the passage area reduction after the ring was attached. With the addition of the ring, the blade loadings of the pump-jet, such as the thrust and torque, were reduced even though the efficiency, defined as the ratio of thrust and torque, was maintained.