This paper takes the push-type duct azimuth propulsor, considering the maximum speed condition, adopts the SST k-ω model in the RANS method with Schnerr-Sauer cavitation model to investigate the numerical simulation results of cavitation and fluctuating pressure performance. Compared with the results of the model test, the cavitation pattern is generally in accordance with the test results, and the blade back sheet cavity, blade face cavity, tip vortex cavity and gap vortex cavity are captured very well. Because of the close distance between the stud and the duct propeller, the phenomenon of Propeller-Hull Vortex (PHV) cavity appeared. The relationship between the strut and the duct propeller and the strength of PHV is analysed, and the distance between the stud and the front of duct to inhibit the generation of the PHV was proposed. The evolutionary mechanism of PHV cavitation is investigated. And the fluctuating pressure of 15 monitor points on the connection block is researched. The distance of 0.285D has much larger fluctuating pressure amplitude. The distance which is greater than 0.35D basically eliminates the PHV, and it provides an important support for the optimization and design of the duct azimuth propulsor based on the performance of the cavitation.
Duct azimuth propulsor is a special ship propeller. The shaft of azimuth propulsor is vertical shaft, and the propeller can rotate around the axis for 360 °. The azimuth propulsor can get the maximum thrust in any direction and make the ship in situ slewing, transverse movement, rapid backward and in the range of micro-speed rudder for steering and other special driving operations. Because of its high degree of integration and good maneuvering performance, the duct azimuth propulsor is widely used in ships which have dynamic positioning and high maneuvering requirements.
The duct azimuth propulsor generally consists of pod, strut, duct, propeller, duct-pod connecting block and other parts. The pod of push-type duct azimuth propulsor is in front of the duct and propeller, and the flow field of the duct propeller is affected by the forward components. For conventional propellers, Feng et al. (2013) used the RANS method to investigate the numerical computation of the propeller's cavitation under multiple conditions in a uniform flow field, and established a reliable numerical computation method. Yang et al. (2011) investigated the effect of the wake field on the hydrodynamic and cavitation performance of the propeller, and concluded that the wake field can effectively delay the cavitation incipient of the blade face sheet of the propeller, but at the same time, it will cause the cavitation incipient of the blade back sheet to be advanced and the range of the cavitation to be increased significantly. Lu et al. (2012) compared the numerical simulation results of the propeller cavitation patterns under the wake flow field using the RANS method and the LES method, and the LES is able to capture the details of the flow and cavitation patterns in a better way. The duct propeller is able to provide more thrust compared to the conventional propeller, so it is widely used in ships that require dynamic positioning and bollards with large thrust. Kong et al. (2019) conducted numerical prediction of hydrodynamic performance and optimal design of low-speed high-thrust duct propellers, and optimized the distribution of thrust loads between conduit and propeller to achieve the purpose of low-speed operating conditions with high thrust. Park et al. (2004) used structured mesh to investigate the hydrodynamic performance of duct propellers containing a post stator, and the simulations were performed using a single-fluid channel method, with the detailed analysis of the flow details of the pressure and velocity fields. Hu et al. (2017) investigate the hydrodynamic characteristics of a ducted propeller with different duct lengths and tip clearances by numerical simulations using a structured grid, and the effects of the inclined flow on the thrust, torque, and efficiency of the ducted propeller are explored. The thrust of the duct propeller generated by the duct is greatly enhanced at low inlet velocities, and the presence of the duct can significantly reduce the pulsations of the thrust and torque of the propeller in the inclined flow. The hydrodynamic and cavitation performances of azimuth propulsors are generally studied by modeling tests and numerical simulations. Li et al. (2018) used the RANS method to calculate the thrust and torque generated by the propeller under different slewing angles, and obtained the variation rule of its hydrodynamic performance during the full-slewing process. The simulation results show that the thrust coefficient shows an M-shaped curve during the full slewing process, and the thrust reaches the maximum value at 120° and 240° slewing angles respectively. Xu et al. (2019) conducted a numerical prediction study of the hydrodynamic performance of two tandem propellers under uniform incoming wake field with different deflection angles. Under the influence of the upstream propeller wake, the thrust loss of the downstream propeller is about 70% when the upstream propeller has no deflection angle, and the thrust loss of the downstream propeller is about 15% when the upstream propeller is deflected by 10°. Tian (2016) The cavitation generation state of a fully rotating pod thruster (CRP) was investigated by the RANS method using the Zwart et al cavitation model.
