Hybrid Investigation of Air Transport Processes in Moderately Slopped Stepped Spillway Flows

In stepped spillway flows, a self-aerated flow region is often found where large quantities of air may be entrained into the water body. This air is then mixed with the water phase leading to an air-water mixture flow with different characteristics than the clear water flow. Thus, air entrainment is an important flow feature which needs to be considered for safe design of these hydraulic structures. Advances in the development of new air-water measurement techniques and numerical modeling capabilities allow addressing these complex problems. Computational Fluid Dynamics (CFD) modeling can be a powerful supplement for physical model tests.
In the presented study, the self-aeration process and the subsequent air transport in the aerated flow region of a stepped spillway model is investigated by means of both, new experimental and numerical methods. The slope of the spillway model is 1:2 and different flow rates are considered involving a skimming flow regime. For the physical model tests, a double-tip conductivity probe and ultrasonic sensors are used to evaluate air-water flow properties and flow depths, respectively. Additionally, high-speed camera recordings help to qualitatively analyze the air-water transport.
In the numerical model, the same stepped spillway is simulated with identical flow conditions. A RANS approach coupled with RNG k-ε turbulence modeling and a VOF technique for free surface tracking is used. The determination of the inception point and the entrained air quantities are estimated employing a subscale model. To verify the numerical model, results are compared to the laboratory air-water measurements. The comparison of results from both techniques helps to identify their capabilities and limitations.