Scale effects in microscopic air-water flow properties in high-velocity free-surface flows

Experiments of high-velocity air-water flows were conducted on two scaled stepped spillways with step heights of h = 0.05 and 0.1 m to investigate scale effects in terms of air-water flow properties for a wide range of discharges in transition and skimming flows. The investigation comprised the complete range of macroscopic and microscopic two-phase flow properties including basic air-water flow parameters, interfacial turbulence properties, as well as cluster properties based upon the near-wake criterion and interparticle arrival time. For both undistorted Froude and Reynolds similitudes, the comparative analysis highlighted scale effects in terms of several gas-liquid flow properties, demonstrating that an extrapolation to full-scale prototype conditions may not be possible. These properties comprised the interfacial area, the turbulence properties and the particle sizes and grouping, affecting any scaling of air-water mass transfer processes. Other key air-water parameters were scaled accurately including the void fraction, interfacial velocity and flow bulking. The present investigation was the most comprehensive to date providing clear guidance on air-water flow properties which may be affected by scale effects. The present results may be also applicable to other types of air-water flows. However detailed testing of air-water flow properties at the prototype scale is needed for final confirmation