Numerical Simulation of Air Entrainment on Stepped Spillways

Air entrainment is a common phenomenon in hydraulic engineering. It has significant effects on flow depth, potential cavitation inception and resulting damages, and aquatic organisms. For a long time, physical scale model tests have been the dominant approach to investigate two-phase flow characteristics at hydraulic structures. However, scale effects became apparent when comparing the experimental results with prototype data. Benefitting from rapidly increasing computing power and deeper understanding of air entrainment and related transport phenomena, numerical simulations have gradually become an alternative and effective tool for air-water two-phase flow investigations. The results from the numerical simulation of self-aerated flow on stepped spillways are given in this paper in consideration of reported experimental data. It was found that the air entrainment model in the commercial software FLOW-3D^® is able to correctly reproduce the turbulence development in the non-aerated region and the inception location of the surface air entrainment. Moreover, the results show that the computed bubble diameter is reasonable and the drag coefficient and Richardson-Zaki coefficient multiplier have an insignificant effect on the air concentration results. Much attention should be paid to the cell size as the turbulence parameters are very sensitive to spatial discretization. However, the model still has to be improved since the calculated velocity and air concentration profiles remain unsatisfactory and therefore further research is necessary.