Phase-detection probe measurements in high-velocity free-surface flows including a discussion of key sampling parameters

Air-water high-velocity flows are characterised by strong interactions of air bubbles and water droplets. The void fraction ranges from a few percent in bubbly flows to up to 100% at the free-surface and a reliable measurement instrumentation is the phase-detection intrusive probe. Herein new experiments were conducted on a stepped spillway (θ = 26.6°) in transition and skimming flow sub-regimes yielding new insights into the turbulent air-water flow properties including the turbulence intensities and integral turbulent time and length scales. The integral turbulent scales showed self-similarity independently of the flow regime. A sensitivity analysis was conducted on the phase-detection probe signals to investigate the optimum sampling duration and frequency as well as the data analysis parameters threshold, sub-sampling duration, histogram bin sizes and cut-off effects. The results provide recommendations in terms of optimum sampling and processing parameters for high-velocity air-water flows.