Numerical Investigation of the Effect of Inflow Froude Number on Flow Pattern in Expansive Gradual Transitions in Open Channels

Transitions are commonly used structures in both natural and artificial open channels. With increasing the transition dimensions along the flow, the flow decelerates. Under subcritical steady state flow conditions, reducing the flow velocity increases the water pressure and reverses pressure gradient. This phenomenon creates separation zone and turbulent eddy flow, and causes the flow energy losses. Due to the complexity of flow pattern and scale effects, physical models can solely provide a clear understanding of the physical principles governing the flow field, so it is necessary to study flow pattern numerically along with the field and experimental studies. In this study, the flow pattern in a rectangular to rectangular expansive transition, has been simulated under subcritical flow with RSM turbulence model using Fluent software. Water surface and flow velocity profiles obtained by the two methods at different sections of transition were compared with experimental results. The results showed a good agreement between the simulated and experimental data. After validation of the numerical model, the effects of inflow Froude numbers on strength of secondary current, hydraulic efficiency of the transition, turbulent kinetic energy and bed shear stress at different cross sections were simulated. The results showed that with increasing the inflow Froude number, the strength of secondary current along the transition increased, while the hydraulic efficiency decreased. Maximum efficiency (60.83%) occurred in Fr1=0.40. Also, with increasing the Froude number, turbulent kinetic energy and bed shear stress increased, so that, from transition inlet to the outlet, turbulent kinetic energy for total Froude numbers decreased with a 30.71% reduction in turbulent power.