Experimental and numerical investigation of water flow behaviour in sharply curved 60° open-channel bends

Understanding secondary flows in sharply curved open-channel bends is key for analysing the flow patterns in rivers and and designing effective hydraulic structures. This study employs both experimental and computational methods to investigate the flow characteristics in a sharply curved 60° open-channel bend. The primary objective is to enhance understanding of flow behaviours in such configurations. For numerical simulations, we utilize the Reynolds-averaged Navier–Stokes equations, applying the volume-of-fluid free surface model to simulate air-water interactions alongside the standard k-ε and renormalized group (RNG) k-ε turbulence models. Our findings reveal the emergence of helical currents driven by centrifugal forces at the bend's onset, which guide the fluid particles from the channel bottom to the convex (inner) bank and then to the concave (outer) bank at the surface. We observe a progressive increase in secondary flow intensity and energy dissipation along the bend, peaking at the terminal section. Notably, maximum flow velocity occurs near the convex wall accompanied by nonlinear water surface behaviours. Additionally, flow separation tendencies near the convex wall are noted after two-thirds of the curvature. Quantitatively, the flow velocity at the convex bank was observed to be 1.70 times higher than at the concave bank within the bend. The mean absolute errors between experimental data and the standard k-ε and RNG k-ε models are 3.20 and 3.12, respectively, indicating the accuracy of the RNG k-ε model.