Numerical investigation of air-entrainment in skimming flow over stepped spillways

• Simulation is performed of the air-entrainment in skimming flow using the volume of fluid (VOF), mixture and Eulerian.
• The VOF+large eddy simulation (LES) method is able to capture the splashing water droplets.
• The mixture+LES method predicts the inception of air entrainment most accurately.
• The Eulerian+Reynolds-averaged Navier–Stokes (RANS) method fails to capture the free-surface aeration.

As a widely used flood energy dissipator, the stepped spillway can significantly dissipate the kinetic or hydraulic energy due to the air-entrainment in skimming flow over the steps. The free-surface aeration involves the sharp deformation of the free surface and the complex turbulent shear flows. In this study, the volume of fluid (VOF), mixture, and Eulerian methods are utilized to simulate the air-entrainment by coupling with the Reynolds-averaged Navier–Stokes/large eddy simulation (RANS/LES) turbulence models. The free surface deformation, air volume fraction, pressure, and velocity are compared for the three different numerical methods. Only the Eulerian+RANS method fails to capture the free-surface aeration. The air volume fraction predicted by the VOF+LES method best matches the experimental measurement, while the mixture+LES method predicts the inception point of the air entrainment more accurately.