Mechanism of rotational separated flow system formation and synergistic flow rectification optimization in a deflection-type inlet pumping-station forebay

Rectification optimization of the forebay in deflection-type inlet pumping stations mostly relies on a trial-and-error approach, which inevitably leads to resource wastage. To precisely improve the adverse flow patterns in the forebay, this study employs numerical simulations to reveal their formation mechanisms, establishes the intrinsic relationship between flow patterns and hydraulic characteristics, and proposes a targeted rectification measure—a “double-bottom sill + baffle plate” combination. The results indicate that five typical flow structures exist in the computational domain of the pumping station. Among them, the deflection mainstream, which has the most significant impact on the forebay, can be visually categorized into four zones: a concentrated-deflection zone, a gradient deflection zone, a low-speed zone, and a deflection–reflection zone. Helical flows and entrainment vortices predominantly occur in the concentrated-deflection zone, and deflection–reflection zone, localized at the forebay inlet, the degree change section, and the outlet, with intensity proportional to inflow condition. Helical flow intensity correlates with flow uniformity, while vortex strength links to local entropy generation. Energy loss at the forebay outlet shows a positive correlation with inflow condition. Post-rectification redistributes the flow velocity at the bottom sills and enhances the streamlined jet and destruct the vertical flow gradient, results show 36.36% higher flow uniformity, 8.14% less head loss, 1.79% lower turbulent kinetic energy, and 6.6% reduced dissipation rate. This study enhances the understanding of the distribution pattern of undesirable flow regimes in the forebay and provides practical guidance for analogous forebay design.