Scale effects in high-velocity air-water flows on a stepped spillway.

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Stefan Felder, Hubert Chanson

Monday 29 june 2015

14:20 - 14:35h at Africa (level 0)

Themes: (T) Water engineering, (ST) Experimental facilities and instrumentation

Parallel session: 2F. Engineering - Instrumentation

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A comparative analysis of air-water flow properties was performed between two geometrically scaled stepped spillway models with step height of 5 and 10 cm respectively. The experiments were conducted in the same large size facility with chute slope of 26.6° and with the same phase-detection intrusive measurement devices. A full range of macro- and micro-scopic air-water flow properties was tested for Froude and Reynolds similitudes. Significant scale effects were found in transition and skimming flows. The present finding stressed that the notion of scale effects must be defined in terms of explicit set of air-water flow property(ies). Some air-water flow parameter are more affected by scale effects than others, even in large-size facilities. On one hand, the void fraction and flow bulking are not much sensitive to scale effects. On another hand, the data analyses confirmed scale effects in terms of bubble count rate, turbulence properties and air bubble and water droplet chord sizes, while the present investigation highlighted also scale effects in terms of further air-water flow properties including particle clustering, inter-particle arrival times as well as auto- and cross-correlation functions and time scales respectively. For all air-water flow properties, a close agreement was found between transition and skimming flow outcomes. The findings highlighted that a scaling of the air-water flow properties is rarely possible and measurements at a prototype scale are needed to identify the limitations of scaled air-water flow experiments.