Let me acknowledge that my friend found out about this effect first, who later showed it to me.
When you slowly open an aerated tap, bubbles that form near the opening of the tap start to merge to form a big cavity inside the water flow.
This is normal water flow before cavity formation
Water flow after cavity formation
What's more interesting is that the cavity pulsates as it grows bigger. (Hopefully, it is visible in this gif)
Oscillations of the cavity
I couldn't see this effect when the tap was fully open. So the effect, as correctly mentioned by @niels, has something to do with the velocity of water flow.
Furthermore, I couldn't see this effect for taps without aerators.
- (Cavity formation in water).
- (Cavity formation in water flowing at slightly higher speed)
- (Cavity oscillation - angle 1)
- (Cavity oscillation - angle 2)
So what is going on here? How does the cavity form, and why does it pulsate?