Calibration of an Air Entrainment Model for CFD Spillway Applications

Abstract

Air entrained has become one of the main variables in the study of large spillways performance since it can help avoiding cavitation. Moreover, high rates of air concentration produce significant bulking of the flow as well as a water–solid friction reduction, generating flow acceleration and increasing maximum velocities at the inlet of the energy dissipation structure. Air entrained also affects turbulence inside the flow producing different energy dissipation rates. Aerated spillways physical models are affected by scale effects, with Weber and Reynolds numbers being usually too low to adequately reproduce observed flows. Alternatively, simulation of air–water flows can be carried out by means of Computational Fluid Dynamics techniques in 1:1 scale. However, 3D numerical simulations of spillway flows are time expensive and air–water interfaces need fine resolution meshes which would require extensive computing. Thus, the use of a subgrid scale in air entrainment models can be useful to predict the inception point and the air concentration profile of the flow along the spillway. Computational techniques can handle a more accurate momentum distribution over the spillway sections with affordable costs. In this research, FLOW-3D® routine for turbulent air entrainment is used, coupled with variable density evaluation. VOF and κ-ε RNG turbulence model are also employed. Over 200 spillway flow simulations have been carried out to obtain optimal values of the air-entrainment model parameters, which can be used in future spillway simulations. The calibration of the model is carried out employing prototype data. Interesting conclusions are obtained concerning air entrainment model performance.