# Diver's air torus rising from 10m underwater (edited)

[Edit: earlier version was even more messy] I just watched a video of a jellyfish caught in a diver's 'air ring' - a torus blown for the sake of watching it rise. The jellyfish gets drawn into the torus and spun like a top as the bubble rises. I want to know how fast, in rpm, the jellyfish is spinning at a given distance from the surface. I think I need to know a formula for the circumference of the torus section at any given point as it rises , and a formula for the speed of rise.

My assumptions: - air torus is released at 10m, 2 atmospheres. - the exhaled volume at 10m is 500mls, so that'll double to 1000mls at sea-level - the jellyfish is isobuoyant, so is the same mass as the seawater it displaces - so can be treated as water. -the radial rotation of the torus is arbitrary (ie due to the mechanics of how the diver blew the bubble and is at a normal to the axial rotation), so can be ignored.

https://youtu.be/JXkWSgU-CL0

[Apologies if this is completely the wrong forum for these questions - in which case signposting to other places appreciated]

• "torus" is a donut. "taurus" is a bull. This is a nice question, and I don't have any idea how to tackle it. Maybe there's a fluid dynamics whiz on SE who can help. Commented Jul 7, 2019 at 3:15
• Thanks - have edited, for ease of reading, not because I'm vein ;-) Commented Jul 7, 2019 at 4:11
• @S.McGrew What I don't understand is how the jellyfish is drawn in to the torus. How can there be a flow towards it with no outflow? Perhaps the fluid dynamics whiz could comment on that, or it could be another Question. Commented Jul 7, 2019 at 4:12
• Keith, I think it's a 'simple' pressure effect, the moving water creates a lower pressure, Bernoulli's principle, and so the jellyfish is drawn in - if my assumption about mass of jelly being close to seawater is correct, the jelly is showing us what is happening to all the water around the torus. Commented Jul 7, 2019 at 4:23
• " if my assumption about mass of jelly being close to seawater is correct, the jelly is showing us what is happening to all the water around the torus." You mean mass density, but your assumption and conclusion are both correct. @KeithMcClary 's question is interesting though: it appears that there is inflow to the torus without outflow. But I suspect that only an extended object like the jellyfish gets pulled to the long circular axis of the torus. Commented Jul 7, 2019 at 5:14