@_thegeoff who specifically might find it interesting
Consider a pot with a convex bottom (i.e. it will stand on its midpoint) with sufficiently large radius of curvature to make it stable when filled to any level. When you put such a pot on an electric hotplate and get it boiling, it starts rocking: https://www.youtube.com/watch?v=vB_szsLa3zk
I have a hypothesis on what's going on (the pot rocks away from the area where the boiling is more intense due to density difference, contact with hotplate increases rate of heating, so there's more heating on the side that's currently lower and thus any rocking gets amplified). Sadly, I don't have the pot anymore and didn't thing at that time of any experiments that could falsify this hypothesis.
@robryk (1/2) Lovely! My hypothesis here is something most people don't realise about boiling water with a hot surface:
Turn on an electric kettle. It starts to make noise as the water gets hotter, and gets louder the hotter it gets, but then just before it boils it gets quieter again.
Seriously, I made a graph a few years ago.
@robryk The critical thing is what makes the sound: it's not bubbles going pop (well, it is at the very beginning as the water expels dissolved gas), it's bubbles of gaseous water...not "steam" as in stuff in your bathroom, or mist, but actual gaseous H2O.
This forms close to the hot surface, a pocket of it starts to rise. But it quickly hits cooler water and condenses, and that happens *fast*!
The collapse is violent, a little hammer blow. It's what makes the noise, and maybe moves the pan?
@robryk The reason the kettle gets quiet again is that close to a full boil those pockets of gaseous water can get all the way to the surface, because the water is hot enough to allow that, and they go "sploosh" in the air, not collapsing *BANG* inside the fluid.
Question is, does the pan rock less at a full boil than near it? Cos it could just be a roiling fluid kicking the centre of mass around.
But violent bubble collapses are too cool to not consider 😉
But why would the liquid start roiling in the first place? Liquid surface tilt and pot tilt are obviously coupled resonators, so practically you will get both oscillating if you get either.
But they would impart only angular momentum to the pot, and not momentum (because they leave the surface level roughly constant and unchanged if averaged over horizontal circles a cm or two in radius). So, this explanation could be tested by letting the pot slide ~frictionlessly sideways and seeing whether its center of mass moves sideways, right?
Yeah, you can see how many assumptions one must make to have students model convection: https://ipho.olimpicos.net/pdf/IPhO_2008_Q3.pdf (for example, the convection cell size is clearly mostly a property of the fluid, and yet I've never found any intuitive explanation for what the size should be or even any Reynolds-number-like symmetries).
Hmm... somewhat relatedly: can one get a conservation law from the symmetry that gives the Reynolds number (via Noether theorem on presumably some weird space that includes fluid properties)?
@robryk If I could apply Noether to turbulent flow I'd be in a very different job 🤣
@robryk convection currents love to be conplex. Heat a couple of inches of oil in a small pan and you'll see cool geometric diffraction patterns forming and shifting. Now speed up the idea and add turbulence from boiling!
Boiling water is far from a simple problem, despite being one of humanity's earliest technologies!