Water Entrainment and Mixing Due to Spillway Discharges

Many spillway designs form surface jets that discharge in relatively large reservoirs. It has been often observed that strong liquid entrainment is caused by these surface jets. Velocity measurements at both model and prototype scale for Wanapum Dam, Washington, and at model scale for Brownlee Dam, Idaho, indicate that the jet configuration and strength affect the entrainment. This water entrainment can have important environmental and ecological impacts. In the abovementioned power dams, water coming from the powerhouse is attracted to the spillway, causing beneficial phenomena. The Total Dissolved Gas (TDG) produced in the spillway is diluted with low TDG water coming from the powerhouse. In addition, low Dissolved Oxygen (DO) waters from the powerhouse are rapidly oxygenated by mixing with the aerated spillway discharge. The mechanisms causing this entrainment are poorly understood. Most of the studies found in the literature explain the entrainment with turbulent mixing. Aeration and free-surface instabilities contribute to modify the turbulence structure and have an effect on the entrainment. Standard RANS numerical models fail to predict the degree of entrainment observed experimentally. In this work a study of the entrainment in free surface jets, with the objective of reveal the reasons for the poor performance of RANS solvers in predicting water entrainment, was carried out. A numerical model was developed to resolve the free surface using the commercial code FLUENT. A submerged round jet parallel to the free surface was modeled and the effect of turbulence models was analyzed. Following a brief description of the models, the paper discusses the mechanisms involved in entrainment in spillways by comparing models with available experimental results.