Multi-scale flow-induced dynamic response in 710-MW Francis turbine: Numerical investigation

In classical hydraulic turbine fluid–structure interaction (FSI) simulations, gap flows are often ignored, and bearing constraints are typically simplified to reduce computational costs. While this approach conserves resources, it compromises the accuracy of FSI predictions. Addressing these limitations, this study investigates the dynamic response of a 710-MW Francis turbine under multi-scale unsteady flow conditions using a novel FSI framework. Unlike prior studies, the current numerical model integrates millimeter-scale gap flows (in crown gap and band gap) and a complete shaft system with realistic bearing constraints, enabling accurate simulation of complex fluid-structure interactions. A two-way FSI method was employed to accurately characterize the vibration and dynamic stress response of the shaft system, with a focus on pressure fluctuations, runner vibrations, and blade stresses. Study results reveal that pressure fluctuations in flow channels are primarily driven by runner rotation and the first type of rotor–stator interaction (RSI). High-frequency pressure fluctuations (13–24 times the runner rotational frequency) attribute to discontinuous vortices within the band gap. In contrast, runner structural responses are dominated by the second type of RSI, whose effects propagate through the flow system and significantly influence the radial forces exerted on the runner. These forces, intensified by millimeter-scale flows in the crown and band gaps, lead to runner imbalance and large-amplitude horizontal vibrations. This study advances the understanding of multi-scale fluid–structure interaction in Francis turbines, providing a robust simulation method for improving accuracy.