Numerical investigation of air demand by the free surface tunnel flows By WANGRU WEI, JUN DENG and WEILIN XU, J. Hydraulic Res. 59(1), 2021, 158--165, https://doi.org/10.1080/00221686.2020.1744747

The authors developed a novel design equation based on numerical model results by taking into account the effects of water velocity and residual tunnel cross-sectional area above the water surface. The discussers acknowledge the presented approach, but also express their concerns regarding the following points: (i) the validation of the numerical model is superficial as it is based on bulk parameters only and ignores known model limitations; (ii) effects of the air vent characteristics are not considered in the numerical model and consequently neither in the presented design approach; (iii) air entrainment is not considered in the authors' approach but may be important for Froude numbers above 6; and (iv) the general applicability of the design equation due to the small range of considered Froude numbers and the negligence of air vent characteristics which have been shown to significantly impact air demand in outlet tunnels. This discussion aims at clarifying some facts not clearly presented by the authors and identifying limitations of the authors' design equation. Overall, we highlight that the approach describes the unrestricted air discharge and should be limited to the parameter range and assumptions considered for its derivation, i.e. for large air vent parameters (A* > 0.1) and flows with Froude numbers below 10.