To investigate the flow field characteristics of a low specific speed axial flow pump under different flow conditions ( Q/ Q _des=0.9, 1.0, 1.1), the particle image velocimetry (PIV) technique is employed to measure the axial cross-sections of the pump at four locations: upstream of the impeller, inside the impeller, inside the guide vane, and downstream of the guide vane. And the pressure transducer is used to obtain the pressure distribution of the wall in the flow field of the pump. The results indicate that the velocity distribution upstream of the impeller shows consistent trends with axial acceleration along the axial direction under all three working conditions. A large velocity gradient is observed inside the impeller at different phases, displaying obvious acceleration near the hub along with concentrated high-speed regions. The velocity distribution inside the guide vane is similar, with more uniform high-speed regions and more stabilized flow under the design flow rate. The flow velocity downstream of the guide vane gradually increases in the mainstream direction, slowly accelerating at low flow rates and rapidly approaching the velocity in the mainstream region at high flow rates. As the flow rate increases, the pressure values at the monitoring points in the pump flow channel decrease. The main frequency of the pressure fluctuation is the blade passing frequency. And the pressure transition between the impeller and the guide vane gap is relatively smooth under the design flow rate with reduced flow losses. The experimental results can provide references for the design and performance optimization of the axial flow pump.

The decreased performance of the waterjet pump and the thruster is mainly caused by the inlet flow distortion of the waterjet pump due to the flow separation at the back of the inlet duct of the flush-type waterjet. A typical flush-type intake duct is selected for a waterjet based on the theory of suppressing the possible flow separation by using the vortex generator/vortex generator jet (VG/VGJ). The flow in the flush-type intake duct of the waterjet is then simulated by the blowing model test in the low-speed wind tunnel. The distributions of the static pressure on the wall of the inlet duct and the total pressure on the surface of the waterjet pump inlet were measured to explain the mechanism of promoting the propulsion performance by the VG/VGJ, obtaining the influence of the structural dimension and installation locations of the VG/VGJ on the flow control. At a low advance ratio ( IVR=0.5), the properly installed VG/VGJ can improve the total pressure recovery coefficient of the inlet duct and the uniformity of the axial velocity of the waterjet pump inlet surface, resulting in an increase of thrust about 5%.