The State of Using CFD to Calculate Air Entrainment on Spillways

 Air concentration in spillway flows is important for assessing cavitation potential and flow bulking, which can lead to overtopping of spillway training walls. Free-surface air entrainment occurs when the turbulent boundary layer intersects the water surface, drawing small bubbles into the flow. Presently, empirical equations are used to predict the location of boundary layer intersection and resulting equilibrium air concentration. 

FLOW-3D HYDRO includes a physics-based air entrainment model and is widely used for spillway simulations. The dam safety community has expressed interest in understanding the model's underlying assumptions, performance, and applicability across different spillway geometries.

The paper also discusses whether current CFD provides sufficient accuracy to replace empirical methods for predicting air concentration. The goal is to use validated CFD models to simulate site specific air entrainment. Recommendations are provided on the types of data needed to improve CFD model validation and build confidence in its predictions.