The free surface begins to deform slightly, but no air is yet trapped.
Air entrainment is not a singular event but a multi-stage process driven by the instability of the air-water interface: Air Bubble Entrainment in Free-Surface Turbulen...
The free surfaces enclose, releasing a single or group of air bubbles into the turbulent flow. Key Drivers and Models The free surface begins to deform slightly, but
A surface cavity develops, reaching deeper into the water while the sides remain smooth. often following a power-law scaling.
The cavity collapses due to local instabilities, often following a power-law scaling.