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The new prototype air demand data are moderate to high compared with existing prototype measurements (Fig. 6, [2]). The large scatter in the model data is due to the varying ζ and L. Actually the model data for a given combination of ζ and L is reasonably described by a power law relation β = aFc b, where a, b are dependent on ζ and L. While the model data shows a good agreement with existing prototype data and equations for free-surface flow, the model data tend to underestimate β for spray flow conditions. Indeed the gas Weber number and the Ohnesorge number indicate that the secondary disintegration of droplets is subject to scale effects i.e. the spray is less pronounced in the model. Additionally the model sluice gate has no gate slots which are a main cause for spray formation [8]. 5. CONCLUSIONS A systematic physical scale model study on the air demand of bottom outlets was performed. The results show that the air demand decreases with increasing air vent resistance and decreasing tunnel length. The flow pattern has a decisive influence on the air demand with the maximum air demand occurring for spray flow. The highest air discharge was observed for free-surface flow. New prototype measurements conducted in two bottom outlets in Switzerland support the main findings of the scale model test. Differences were observed for spray flow conditions, where the local gate geometry plays a crucial role. ACKNOWLEDGEMENTS The first author is funded by the Swiss National Science Foundation (SNF, Grant No. 163415). The prototype measurements are financed by the Lombardi Engineering Foundation and kindly supported by the dam operator Ofible. The project is embedded in the Swiss Competence Centre for Energy Research – Supply of Electricity (SCCER-SoE). REFERENCES [1] SPEERLI J., HAGER W. H. Air-water flow in bottom outlets. Canadian Journal of Civil Engineering, 27, 454–462, 2000. [2] HOHERMUTH B. Air demand of high-head bottom outlets. Proc. 37th IAHR World Congress, Kuala Lumpur, Malaysia, 2956-2965, 2017. 840