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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.

enter image description here

This is normal water flow before cavity formation


enter image description here

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)

enter image description here

Oscillations of the cavity


Updates

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.

Video samples


So what is going on here? How does the cavity form, and why does it pulsate?

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  • $\begingroup$ Do the pipes in your house/apartment not have air shocks on them? I ask because those surface waves on the Water flow after cavity formation pic look like vibration-induced waves (at least that's my best guess). $\endgroup$ Sep 16 at 13:22
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    $\begingroup$ If you meant water hammer arrestors, Yes, we have those. $\endgroup$
    – AlphaLife
    Sep 16 at 14:56
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    $\begingroup$ I've always called them air shocks but they are the little, dead-end lines that off a T-joint near the actual outlet. They let water ram into the end, dissipating the momentum of the moving water, without shaking your pipes off your walls. My house is old enough not to have these, so I wake up in cold sweats some nights worrying a pipe sweat burst because of this (only half joking about this last part). $\endgroup$ Sep 16 at 14:59
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    $\begingroup$ Yup, those are real headaches! $\endgroup$
    – AlphaLife
    Sep 16 at 15:02
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an aerator nozzle contains air passages that draw outside air in through slits in the sides of the nozzle into a mixing chamber inside, where the air is mixed with the water and then squirted out through a series of small holes in the outlet of the nozzle. Those holes break up the air bubbles into small ones, giving the water a fizzy appearance.

The air inlets into the mixing cavity are so designed that by Bernoulli's law, a suction is created in the mixing chamber which actively draws the air in when the water is flowing fast through the assembly.

But when the water is not flowing fast enough, a suction is not created and the air entering the mixing chamber does not get pulled in and swirled around with the water. Instead it populates the center of the mixing chamber while the water dribbles past it around the periphery of the mixing chamber (where the little water nozzles are distributed).

So in this case you get a slow-moving water stream with a hollow core in it. I'll have to think some more about the oscillation phenomenon.

BTW most aerator nozzles are made to be disassembled for cleaning, and so you can take yours apart and inspect all the tiny features that are molded into the parts inside which let the nozzle do its job.

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  • $\begingroup$ Very good answer. If you look at the video closely, you will see some bubbles appearing near the tap opening. For some reason, they appear exactly when the cavity is pulsating. Does the appearance of these bubbles have any connection with the oscillation? $\endgroup$
    – AlphaLife
    Sep 10 at 8:22
  • $\begingroup$ @AlphaLife, beats me! -NN $\endgroup$ Sep 10 at 17:37

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