Bed sill effectiveness in reducing flow separation at open channel confluences: an openfoam-VOF 3D CFD study

The Flow Separation Zone (FSZ) is a key hydrodynamic feature of open channel confluences, and its reduction contributes to improved channel efficiency. This study employs the open-source computational fluid dynamics (CFD) software OpenFOAM to examine the influence of a bed sill on reducing the FSZ at a right-angled open channel confluence. The flow is simulated by solving the three-dimensional (3D) Reynolds-averaged Navier–Stokes (RANS) equations with the SST k–ω turbulence model, while the interFoam multiphase solver is used to capture the water–air interface via the Volume of Fluid (VOF) method. The numerical model is validated against experimental data from the literature by comparing velocity fields and water surface elevations. Different sill sizes, locations, and configurations are tested to evaluate their effectiveness in reducing the FSZ. The results indicate that strategically placed sills can shorten the FSZ length by up to 55%, while modifying velocity profiles and reducing flow recirculation. Using multiple sills, particularly a three-sill configuration, further enhances this effect, reducing the FSZ length by as much as 74%. Analysis of secondary currents shows that sills intensify these currents, thereby enhancing momentum exchange and reducing the flow separation. Furthermore, energy loss analyses conducted for various sill configurations demonstrate that strategically positioned sills can reduce the energy loss significantly.