Free-Surface–Duct–Rotor Interactions in Shallow Confined Flows: Experimental and CFD Analysis of a Flanged Ducted Hydrokinetic Turbine

This study examines the performance and flow characteristics of a flanged ducted horizontal-axis hydrokinetic turbine functioning in shallow open-channel flow under free-surface circumstances. A hybrid experimental and numerical methodology is utilised, integrating towing-tank data with transient computational fluid dynamics simulations via the Volume of Fluid (VOF) technique. The numerical model utilises transient Reynolds-Averaged Navier–Stokes equations, using the SST k–ω turbulence model and a sliding mesh methodology. Experiments are performed at a towing velocity of 0.85 m/s, indicative of a subcritical flow regime (Fr = 0.27) and a Reynolds number of roughly 5 × 10⁴. Results indicate that the ducted arrangement improves turbine performance by 20–30% in CFD models and 40–60% in experiments, attributed to diffuser-induced flow acceleration and augmented blockage effects. Nevertheless, the augmented energy extraction generates a more pronounced velocity deficit, leading to a delayed wake recovery. Bypass flow acceleration, generated by blockage, somewhat mitigates this effect by enhancing shear-layer formation and turbulent mixing. In shallow submergence, the free surface functions as a deformable boundary that modifies the flow structure, diminishing performance enhancement by approximately 10% and further impeding wake recovery by restricting vertical flow expansion. Notwithstanding these impacts, the ducted turbine sustains a reliable performance advantage. This paper presents a comprehensive analysis of turbine performance considering the interactions of blockage, duct, and free-surface effects, thereby advancing prior research on duct-augmented turbines and contributing valuable insights for the design and implementation of hydrokinetic systems in constrained settings.