Unique characterization of flow region with 50% void fraction in high-velocity free-surface flows.

For 50% void fraction, largest bubble count rate, correlation time scales and turbulence levels.

Analyses of instantaneous void fraction identified characteristic times scales.

Chord times of about 3–5 ms characteristic time scale of flow region with equal air and water entities.

Air–water interfaces in flow region with 50% void fraction affected by free-surface motions.