Experimental and numerical modelling of aerated flows over stepped spillways

Stepped spillways are a popular design choice for reservoir overflows due to the high rates of energy dissipation and air entrainment compared to smooth spillways. Air entrainment is important in spillway flows as it affects the pressures acting on the spillway surface, which in adverse conditions can damage the spillway. Air entrainment also causes flow bulking which increases the depth of flow. This study presents free surface and pressure data for aerated flows over an experimental stepped spillway, with pressures measured at different positions across the width of the channel. Within the step cavities, recirculating vortices are observed in both the stream-wise and cross-stream directions, with the direction of circulation alternating at each subsequent step. These 3D effects cause the pressures acting on the step edges to vary across the width of the channel. The Volume of Fluid (VOF) and Eulerian multiphase numerical models are used to predict flows over the spillway. The Eulerian multiphase model shows high levels of air entrainment and is able to predict the position of the free surface to reasonable accuracy. The VOF model, conversely, does not show any air entrainment and therefore under predicts the position of the free surface. The accuracy to which each numerical model predicts pressures on the step faces varies depending on the measurement location. Both of the numerical models accurately simulate the direction of circulation of the 3D vortices within the step cavities. Simulations with varying channel widths, conducted using the VOF model, show that the pattern of 3D vortices repeats as the channel width is increased.