Unveiling the transient energy flowpath in the turbine mode of pump shutdown: From inertial buffering to irreversible dissipation

The normal shutdown of Low-Head Tubular Pump Systems (LH-TPS) traverses a critical turbine mode, characterized by a complex conflict between energy recovery potential and structural risks. This study employs high-fidelity overset mesh simulations, validated against prototype field measurements and scaled model experiments, to decode this nonlinear transient evolution. A cross-scale analytical approach is proposed to elucidate the energy conversion mechanism. First, multivariate regression of the proposed Normalized Energy Output Coefficient (EOC) identifies a distinct transition: initial acceleration is dominated by a transient hydraulic surge, while the subsequent deceleration is governed by the release of stored rotational kinetic energy, creating a hydraulic-inertial coupling buffer. Second, time-frequency diagnostics reveal a spectral bifurcation phenomenon. Governed by the vector cancellation effect, fundamental blade-passing frequencies are spatially filtered from the resultant torque, while axial forces bear broadband stochastic shocks. Finally, thermodynamic entropy production diagnostics trace the origins of hydraulic loss. Peak EOC coincides with global streamline alignment and vortex suppression. Conversely, the deceleration phase is dominated by rotation-driven turbulence, where the Coriolis force promotes relative vortex stretching, accelerating the energy cascade into irreversible boundary layer shear entropy. This investigation bridges macroscopic mechanical hysteresis with microscopic turbulent topology, offering critical insights for optimizing transient control strategies and mitigating structural fatigue.