Study of driving force characteristics in the formation of dominant unstable flow events in water jet propulsion pumps

Unstable flow events are pivotal in influencing water jet propulsion pumps' safe and stable operation. However, a notable research gap exists in systematically investigating the underlying driving force characteristics responsible for the formation of predominant unstable flow events in the impeller. To address this gap, we conducted experimental and numerical simulation studies, yielding the following results. As the flow rate decreases, a significant rise in the peak-to-peak value of pressure fluctuations and rigid vorticity occurs on both the blade outlet hub and inlet shroud sides. Two distinct unstable flow events emerge at these locations: the leading-edge separation vortex on the blade inlet shroud side and the horn-like vortex on the blade outlet hub side. To elucidate these phenomena, we conducted an analysis of the driving force characteristics employing the momentum equation of relative motion. Our analysis revealed that, as the flow rate decreases, there is a notable increase in the potential rothalpy gradient (PRG, or the reduced static pressure gradient) at these positions, surpassing the influence of the Coriolis force. Based on these findings, we have constructed the fundamental topological structures of these unstable flow events.