The increasing integration of renewable energy into new-type power systems accentuates the critical role of hydroelectric power in grid stabilization and baseload support. Nevertheless, inherent flow instabilities in hydraulic turbines operating under off-design conditions induce power oscillations. These oscillations constrain the flexible regulation capabilities of hydroelectric power systems. This study investigates a large Francis turbine, employing a multiphase flow model to numerically simulate flow characteristics across a comprehensive range of operating conditions. Pressure distribution on runner blades is quantitatively analyzed utilizing a streamline coordinate system. A third-generation vortex method identifies the complex vortex structures within the flow channel. Building upon these analyses, this study delineates conditions predisposing the turbine to power oscillations, elucidates the mechanism of these vortex-induced oscillations, and proposes an optimal air admission strategy for their mitigation. Results show that the turbine is susceptible to power oscillations in below-rated head and overload conditions, with a characteristic frequency of 4.93 Hz, corresponding to flow separation vortices. These vortices generate negative torque on the blades, with the resulting torque fluctuations directly causing power oscillations. An optimal air admission strategy—admission volume of 1 m3/s—significantly reduces the amplitude of power oscillations by up to 81.3%. The conclusions presented herein offer valuable recommendations for optimizing the stable operating range of hydraulic turbines, enabling timely mitigation of output power fluctuations, and enhancing the flexible control capability of hydraulic systems.

Dual-rotor tidal turbines have demonstrated performance advantages over single-rotor systems; however, their optimization remains challenging due to the increased number of design variables and interactions. Existing studies often present inconsistent findings, largely constrained to specific turbine configurations, which limits their general applicability. This study addresses the need for a more unified framework by investigating the influence of axial spacing between the two rotors based on axial momentum theory. To evaluate the applicability of the simplified theoretical approach in practical settings, a numerical analysis was conducted using ANSYS-based simulations of a counter-rotating marine current turbine model. A key assumption in previous theoretical models that the pressure at the inlet of the downstream rotor equals ambient pressure is examined to see if it is necessary and how it affects the turbine's performance. The simulation results are in full agreement with the theoretical predictions. The results show that the optimal power coefficient does not necessarily occur at large axial distances between the rotors. Instead, the results show that the power coefficient improves as the projected cross-sectional diameter of the rear rotor's flow tube at front rotor location (D1int) and axial induction ratio (e1/e2) approach 0.707D and 1/3, respectively, with peak performance observed near these values. These findings can support more efficient optimization of dual-rotor systems.