CS12A - Concurrent Session 12A: The State of Using CFD to Calculate Air Entrainment on Spillways

Abstract Description: Predicting air concentration in spillways is important both for assessing cavitation damage potential and flow bulking that could lead to overtopping the spillway training walls. Free-surface air entrainment occurs when the turbulent boundary layer reaches the water surface where small bubbles are then drawn down in the water column. Observations and theory by others predict where the boundary layer reaches the free surface and air entrainment begins. Air concentration measurements have been used by others to develop empirical equations to predict equilibrium air concentration. However, most of the air concentration measurements which the equations are based on have been made on small flumes; only a limited number occurred at prototype scale.

In 2024, Gessler et al. presented a paper at ASDSO describing how to calculate the near spillway surface air concentration at Oroville. The paper used empirical approaches for calculating where the boundary layer reaches the free surface and then the distance until equilibrium air concentration is reached. Using field observations from Oroville and Cottonwood Chute, which has a similar slope to Oroville, the methods were tested and the best equation for Oroville was identified.

This paper builds on the 2024 paper by presenting the state of CFD for predicting air concentration in spillway flows. FLOW-3D HYDRO is widely used for spillway modeling and has an air entrainment module. There is interest in the dam safety community in further understanding how the model works and the applicability of the model to various types of spillways. Using the geometry of Cottonwood Chute, the FLOW-3D HYDRO model is compared to the air concentrations predicted by the empirical equations. Model results are also compared to a very small set of field data from Cottonwood Chute.

Discussion is provided if CFD, in its current state, is justified for calculating air concentration or if the historic empirical equations remain the best option. Empirical equations have a long history, however, a future in which validated CFD models can accurately predict air entrainment for any spillway geometry is the goal. There is also a discussion of the data needed to increase confidence in the CFD models.