VOF-Based Simulation of Turbulent Air-Water Flow over Gravel Beds in Open Channels

Turbulent flow over gravel beds in open channels is a fundamental yet complex problem in hydraulic engineering, as flow behavior is highly sensitive to channel geometry and bed roughness. In this study, the Volume of Fluid (VOF) method coupled with the standard k-ε turbulence model is employed to simulate air-water interactions over gravel beds, with open boundary conditions capturing realistic channel-atmosphere interactions. Numerical simulations are performed to examine how channel design influences the relationship between the friction factor (f) and the Reynolds number (RN). Velocity and VOF contours indicate peak flow near the inlet, with a maximum velocity of 0.64 m/s. The simulations show strong agreement with theoretical predictions, yielding a correlation coefficient of 0.99 for RN, while f and Chezy’s coefficient (C) reach 0.75 and 0.71, respectively. Comparison with experimental measurements shows deviations of approximately 17% for RN, 25% for f, and 12% for C. Moreover, further analysis confirms an inverse linear relationship between f and RN, in accordance with classical models such as Bazin’s curves, the Colebrook equation, and Moody’s approximation. Overall, the results demonstrate that the proposed numerical framework reliably captures flow dynamics over gravel beds, offering a robust tool for hydraulic design and performance assessment of open channels.