Multi-scale turbulence characteristics and energy transfer analysis of lateral inlet/outlets based on POD/DMD

Local head losses in lateral inlet/outlets of pumped storage power stations account for 6.7–8.3% of total system losses, yet the underlying multi-scale turbulent mechanisms remain poorly understood. Prior work focused on time-averaged and single-scale flow features, leaving cross-scale energy transfer pathways unexplored. This study employs Large Eddy Simulation (Re = 2.5 × 107, 42-million-cell mesh, 35 flow-through periods) combined with Proper Orthogonal Decomposition (POD) and Dynamic Mode Decomposition (DMD) to characterize multi-scale turbulence and quantify energy cascade mechanism in lateral inlet/outlets. Results show geometric expansion creates a middle transition zone (x/D = 2.0–5.0) where turbulent kinetic energy (TKE) production peaks at 12 times the entrance baseline, identifying it as the primary site of hydraulic loss. POD identifies hierarchically distributed coherent structures, with the leading five modes capturing over 70% of total TKE: the dominant mode (0.09 Hz) governs large-scale low-frequency separation oscillations, intermediate modes exhibit dual-peak spectra reflecting cross-scale coupling, and higher-order modes represent small-scale dissipation. DMD reveals a clear spectral bifurcation between large-scale low-frequency and small-scale high-frequency modes. Critically, the framework uncovers a dual-pathway energy transfer mechanism departing from the classical Kolmogorov cascade: 43% ± 0.73% of large-scale energy bypasses the sequential cascade via non-local energy transfer into intermediate scales, while 65% ± 1.02% of high-frequency energy originates from coupling with low-frequency structures (phase synchronization Index = 0.78 ± 0.02). These findings demonstrate that suppressing low-frequency global instabilities, rather than solely targeting small-scale turbulence, is key to optimizing hydraulic performance and provides a theoretical basis for flow control in lateral inlet/outlets.