Standing waves in water jets

Factual Questions
1

I have some thin background in fluid dynamics, but there’s one question I haven’t seen addressed in my texts. What is the physics behind the standing waves in water jets which makes a flat stream of fluid seem to rotate 90-degrees at specific intervals?

The ubiquitous example is the stream of urine from a man’s penis. The stream emerges from the “slot” as a flat stream, converges, and then flattens out in a plane perpendicular to the original plane.

I’m familar with various wave patterns in jets such as shock diamonds, but these result from reflections due to pressure differences. It doesn’t seem like the pattern described above could be pressure-driven. Is it some sort of standing wave pattern set up by the slot?

I would apologize if this is trivial, but a comment about having too much time on my hands might be misconstrued given my example of the phenomenon.

2

Welcome to SDMB Micco.

I have a question for you. When you say the pattern changes at intervals, does it repeat?

I suspect what you are seeing is not a standing wave but a subtle result of the shape of the orifice. The nozzle that collates the stream exerts a force to the center. With a vertical slit there is more peripheral surface on the sides than top and bottom. The result of that would be a wide flat stram as the elements are pushed to the opposite side. Lots of lateral area/force and little vertical and a vertical stream becomes horizontal.

I can’t say I’ve observed a repeating pattern but I’ll be on the lookout for it. As the stream is weakly bound by surface tension I’d expect it to make the 90º “phase” change again.

Now go wash your hands.

3

Yep, it definitely repeats several cycles until the stream breaks up. Your explanation of the origin of the first cycle makes sense where a radial velocity component is introduced by the nozzle and causes the stream to converge and then diverge, but would surface tension really be strong enough to reverse the divergence and set up a repeating pattern?

Who says you have to get your hands dirty for science? This can be done with a garden hose, faucet, or shower head. Just turn it on and watch the pretty patterns.