Runner hydraulic thrust decomposition and pressure pulsations influenced by multiscale flow during the startup process of the pumped storage unit

Pumped storage units experience strong hydraulic transients during startup, which may cause unstable runner hydraulic thrust and pronounced pressure pulsations. To investigate the evolution of runner hydraulic thrust and the associated internal-flow mechanisms during startup, a multiscale three-dimensional unsteady numerical model of the full flow system, including the surge tank, is established. The model is validated using field-measured rotational speed and power data. Axial hydraulic thrust and radial force are decomposed to identify the critical instability periods and their dominant contributors. Pressure-pulsation analysis and rigid-vortex diagnostics are then used to clarify the underlying flow mechanisms. The results show that axial hydraulic thrust instability during startup is mainly associated with the crown clearance region, whereas radial force instability is mainly related to the main flow passage. Time-frequency analysis in the vaneless space and runner passage shows that circumferentially non-uniform pressure pulsations in the main passage are closely related to radial force instability. In contrast, pressure pulsations in the clearance passages play a dominant role in the axial hydraulic thrust response through the coupling between pressure differences and effective force-bearing areas. Rigid vortex analysis at selected instants further reveals a strong spatial correspondence between enhanced enstrophy transport and force-sensitive regions, suggesting that rigid enstrophy evolution is closely related to force fluctuations and low-frequency broadband pulsations. These findings provide useful guidance for startup control optimization and load-risk assessment in pumped storage units.