Numerical investigation of energy dissipation in stepped spillway using various multiphase models

The safety of dams relies significantly on managing spillway flows, especially during large hydraulic events. Stepped spillways serve the purpose of safely conveying flood water downstream while also dissipating large amounts of kinetic energy. In this study, CFD modeling of a stepped spillway was performed using ANSYS Fluent, and the results were validated against the results of physical model experiments conducted in the Utah Water Research Laboratory at Utah State University. The physical model consisted of a stepped spillway having a slope of 18.4o with a uniform step height of 0.2 m. Homogenous and non-homogenous multiphase models were implemented, considering Volume of Fluid (VOF)-Sharp, Mixture-Dispersed, and Eulerian-Dispersed interface tracking techniques model available in the ANSYS Fluent solver. The physical model results of air concentration, flow velocities, and water surface profile for two different flow rates were evaluated using the numerical model for validation. Additional flow rates were also investigated along with step heights of 0.2 m and 0.1 m. The results revealed that the VOF-Sharp interface tracking model struggled to entrain air into the flow whereas the Mixture and Eularian models displayed aerated flow. Regardless, the VOF-sharp interface tracking model was found to be more suitable for stratified flow with a sharp interface between air and water. The Mixture model was utilized for further analysis with different step heights and flow rates. Air concentration (C) and velocity (V) profiles at selected step edges were compared against the experimental results. Velocity profiles were in close agreement with the experimental results whereas the CFD model consistently overpredicted air concentration. The dispersed interface tracking method demonstrated suitability for highly disturbed skimming flow, underscoring the importance of selecting appropriate models for investigating and analyzing flow over stepped spillways. The length of the inception point increased whereas rate of energy dissipation decreased with increasing discharge over
the chute. This implies the necessity of detailed design and safety evaluation of flood discharges for appropriate chute design.