The characteristics of flow instabilities as well as the cavitation phenomenon in a centrifugal pump operating at low flow rates were studied by experimental and numerical means, respectively. Specially, a three-dimensional (3D) numerical model of cavitation was applied to simulate the internal flow through the pump and suitably long portions of the inlet and outlet ducts. As expected, cavitation proved to occur over a wide range of low flow rates, producing a characteristic creeping shape of the head-drop curve and developing in the form of nonaxisymmetric cavities. As expected, the occurrence of these cavities, attached to the blade suction sides, was found to depend on the pump's flow coefficient and cavitation number. The experiments focused on the flow visualization of the internal flow patterns by means of high-speed digital movies and in the analysis of the inlet pressure pulsations near the impeller eye by means of fast response pressure transducers. The experimental results showed that the unsteady behavior of the internal flow in the centrifugal pump operating at low flow rates has the characteristics of a peculiar low-frequency oscillation. Meanwhile, under certain conditions, the low-frequency pressure fluctuations were closely correlated to the flow instabilities induced by the occurrence of cavitation phenomena at low flow rates. Finally, the hydraulic performances of the centrifugal pump predicted by numerical simulations were in good agreement with the corresponding experimental data.

When a pump-jet propeller rotates at high speeds, a tip vortex is usually generated in the tip clearance region. This vortex interacts with the main channel fluid flow leading to the main energy loss of the rotor system. Moreover, operating at a high rotational speed can cause cavitation near the blades which may jeopardize the propulsion efficiency and induce noise. In order to effectively improve the propulsion efficiency of the pump-jet propeller, it is mandatory to research more about the energy loss mechanism in the tip clearance area. Due to the complex turbulence characteristics of the blade tip vortex, the widely used Reynolds averaged Navier–Stokes (RANS) method may not be able to accurately predict the multi-scale turbulent flow in the tip clearance. In this paper, an unsteady numerical simulation was conducted on the three-dimensional full flow field of a pump-jet propeller based on the DES (detached-eddy-simulation) turbulence model and the Z-G-B (Zwart–Gerber–Belamri) cavitation model. The simulation yielded the vortex shape and dynamic characteristics of the vortex core and the surrounding flow field in the tip clearance area. After cavitation occurred, the influence of cavitation bubbles on tip vortices was also studied. The results revealed two kinds of vortices in the tip clearance area, namely tip leakage vortex (TLV) and tip separation vortex (TSV). Slight cavitation at J = 1.02 led to low-frequency and high-frequency pulsation in the TLV vortex core. This occurrence of cavitation promotes the expansion and contraction of the tip vortex. Further, when the advance ratio changes into J = 0.73, a third type of vortex located between TLV and TSV appeared at the trailing edge which runs through the entire rotational cycle. This study has presented the dynamic characteristics of tip vortex including the relationship between cavitation bubbles and TLV inside the pump-jet propeller, which may provide a reference for the optimal design of future pump-jet propellers.

A pump-jet, which is generally and widely adopted on underwater vehicles for applications from deep sea exploration to mine clearing, consists of a rotor, stator, and duct, with the properties of high critical speed, high propulsion efficiency, great anti-cavitation performance, and low radiated noise. The complex interaction of the flow field between the various components and the high degree of coupling with the appendage result in the requirements of in-depth research on the hydrodynamic performance and flow field for application and design. Due to the initial application on the military field and complicated structure, there is scant literature in the evaluation of pump-jet performance and optimal design. This paper, in a comprehensive and specialized way, summarizes the pump-jet hydrodynamic performance, noise performance, and flow field characteristics involving cavitation erosion and vortices properties of tip-clearance, the interaction between the rotor and the stator and the wake field, as well as the optimal design of the pump-jet. The merits and applications range of numerical and experimental methods are overviewed as well as the design method. It also concludes the main challenges faced in practical applications and proposes a vision for future research. It was found that the compact structure and complex internal and external flow field make the pump-jet significantly different, also leading to higher performance. As the focus of cavitation research, vortices interact with the complex structure of the pump-jet, leading to instabilities of the flow field, such as vibration, radiated noise, and cavitation erosion. The effective approaches are adopted to reduce radiated pump-jet with minimal influence on the hydrodynamic performance, such as eliminating the tip clearance and installing the sawtooth duct. Advanced optimal technology can achieve high performance, cavitation performance, and acoustic performance, possessing good prospects. Further developments in investigation and the application of pump-jets in the multidisciplinary integration of fluid dynamics, acoustics, materials, chemistry, and bionics should be the main focus in future research.

The paper presents the results of an activity carried out on a three-bladed inducer and compares the experimental data from other similar inducers. All the inducers considered in the present study have been designed by means of the simplified analytical model recently developed by some of the authors. The main effects of different geometrical solutions on hydraulic performance and flow instabilities of the pumps under noncavitating and cavitating conditions are presented in the paper.