High-velocity free-surface flows are complex two-phase flows and limited information is available about the interactions between air and water for void fractions of 50%. Herein a detailed experimental study was conducted in the intermediate flow region on a stepped spillway and the microscopic air-water flow characteristics were investigated. The results showed differences in water and droplet chord times with comparatively larger number of air chord times (0-2 ms), and larger number of water chord times (2-6 ms). A monotonic decrease of chord modes with increasing bubble count rates was observed. Several characteristic time scales were identified based upon inter-particle arrival time analyses of characteristic chord time classes as well as spectral analyses of the instantaneous void fraction signal. Chord times of 3-5 ms appeared to be characteristic time scales of the intermediate flow region having similar time scales compared to turbulent time scales within this region of the flow. A further characteristic time scale of 100 ms was identified in a frequency analysis of the instantaneous void fraction. This time scale was of the same order of magnitude as free-surface autocorrelation time scales suggesting that the air-water flow structure was affected by free-surface instabilities.
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