Coupling Mechanisms Between Vortex Rope Evolution and Cavitation Within the Draft Tube of Pump-turbines

A technique integrating numerical simulations with experimental observations was hereby employed to elucidate the flow instability mechanisms during transient processes in pump-turbines, and a reversible pump-turbine was investigated for in-depth insights. Concurrently, the coupling phenomena between vortex rope and cavitation under transient conditions were systematically analysed. The results indicate that rotational and pressure gradients predominantly drive the vortex rope evolution, thereby exerting a substantial influence on cavitation development. The formation and collapse of low-pressure cavities disrupt the vortex structures while intensifying the flow instability. High tangential momentum as well as shear layers contribute to forming alternating vortex structures. Cavitation bubbles are either entrained by the vortex rope or diffuse outward, resulting in pronounced fluctuations in cavity volume. As the vortex structure evolves, it gradually transitions downstream into a more stable, axisymmetric configuration. Pressure fluctuation analysis reveals that low-frequency oscillations within the straight conical section are predominantly induced by vortex rope rotation and cavitation collapse. In the elbow region, localized pressure amplification arises from vortex-wall interactions, while the axial non-uniformity of vortex ropes and cavity distribution governs the spatial variability of pressure pulsations. Collectively, this study establishes a theoretical basis for understanding the coupling between vortex ropes and cavitation. It further offers practical guidance for controlling vortex dynamics and mitigating cavitation in engineering applications.