Rotating Stall Characteristics in the Hump Region of a Pump Turbine: Application of Modal Decomposition

A reversible pump turbine is a large-scale commercial energy storage device that serves as the core component of a pumped storage power station. Under pump operating conditions, the reversible pump turbine often exhibits hump characteristics on the head–discharge performance curve, leading to operational instability and limiting both regulation performance and safety. However, the mechanism underlying flow instability in the hump region is not adequately understood. Existing analysis methods are limited in scope, and they struggle to identify key flow structures accurately in application. In this study, large eddy simulation (LES) is used to investigate unsteady flow characteristics under typical hump conditions. Proper orthogonal decomposition (POD) and dynamic mode decomposition (DMD) methods are applied to a transient flow field to extract dominant modal structures and analyze their dynamic behaviors. The results show that rotating stalls in the guide vane and runner regions are the primary factors creating hump characteristics. Both the POD and DMD methods effectively capture major vortex structures and their changes, demonstrating their suitability for analyzing complex flow dynamics. These findings provide theoretical insight into the flow instability mechanism in the hump region and offer guidance for improving the operational performance of pump turbines.