Investigation into the coupling mechanism of tailwater vortex dynamics and cavitation during pump-as-turbine operations

In order to investigate the coupling mechanism between tailwater vortex and cavitation bubbles (CB) during the operation of a low-head pump-as-turbine (PAT), the method of combining experiment and numerical simulation is used to quantify the vortex dynamics characteristics in combination with the vorticity transport equation under high flow conditions for PAT mode. The results show that the decrease in Thoma number significantly regulates the symbiotic evolution of tailwater vortex and CB: prolonging the residence time of CB in the draft tube (DT) and changing its evolution mode, resulting in the extension of vortex rope (VR) generation period, length contraction, and the increase in breaking vortex in DT. The peak volume of CB is 7 times that in the rotor region, squeezing the channel vortex and the wake vortex, weakening its contribution to VR. Vortex dynamics shows that the relative vortex stretching term is the core driving force of vorticity, which causes velocity gradient distortion and VR high-frequency oscillation synchronously with vertical vorticity. The baroclinic torque term (BT) only generates pulse contribution in the early stage of CB collapse. Under critical cavitation, BT converts the cavity collapse energy into vortex energy through density-pressure gradient coupling, which expands the vortex core radius to 0.03 m, increases the circulation peak to 2.3 m2/s, and shifts outward by 27.3%, resulting in vortex energy diffusion and high-frequency oscillation of the flow field. This study provides a theoretical basis for cavitation suppression and operational optimization of low-head PAT.