Scrutable

We have lots of balloons in the kitchen at the moment (party balloons, not Montgolfier).

When we give a balloon whack, it appears to set off at whacking speed, then after about 1.5 - 2m suddenly decelerates.

Any thoughts on why?

Probably not anything to do with this but it's interesting to know about.

The Reynolds number of a 30cm balloon at 10 m/s is about 2 10⁵, and 4 10⁵ at 20m/s. (According to an online calculator I found)

That puts it across the sharp link in the middle right of the plot, so that the drag coefficient rises rapidly from something like 0.1 to 0.5 as it slows.

Of course, that might not be it.

Although I think it is it.

It's also interesting that a cricket or baseball gets to a Reynolds number of 2 10⁵ at around 40m/s or 90 mph, which is roughly the dividing line between fast bowling/balls and swing bowling/curve balls.

The dividing line between hard shots and floating passes in football is probably also around that kind of Reynolds number, although closer to 15 m/s there.

The Magnus effect that produces the swing on a ball reverses sign around that critical point, which is why swing and reverse swing can happen around these kinds of speeds.

Physically that kink is to do with what happens to the air flow around the back of the sphere by the way.

shpalman wrote: Fri Jan 03, 2020 10:58 pm Physically that kink is to do with what happens to the air flow around the back of the sphere by the way.

Probably a nice big Von Karman Vortex Street. Its a while since I've whacked a balloon, but IIRC they wobble in air which would be explained by shedding vortices. Perhaps Beaker would care to check the wobbling?

Matatouille wrote: Sat Jan 04, 2020 8:51 am

shpalman wrote: Fri Jan 03, 2020 10:58 pm Physically that kink is to do with what happens to the air flow around the back of the sphere by the way.

Probably a nice big Von Karman Vortex Street. Its a while since I've whacked a balloon, but IIRC they wobble in air which would be explained by shedding vortices. Perhaps Beaker would care to check the wobbling?

I would love it if a town planner somewhere would design a road with endless mini roundabouts and call it Von Karman Street.

Matatouille wrote: Sat Jan 04, 2020 8:51 am

shpalman wrote: Fri Jan 03, 2020 10:58 pm Physically that kink is to do with what happens to the air flow around the back of the sphere by the way.

Probably a nice big Von Karman Vortex Street. Its a while since I've whacked a balloon, but IIRC they wobble in air which would be explained by shedding vortices. Perhaps Beaker would care to check the wobbling?

Well I hadn't considered that [1], but what I meant was that slowing down through that kink, the boundary layer separates earlier around the sphere. So you go from the air flow following the sphere all the way around the back and leaving only a narrow cylinder of wake, to the air flow leaving the surface of the sphere half way around and leaving a cylinder of wake which has the same diameter as the sphere. This is hand-wavy, fluid dynamics is always more complicated than that [1].

Roughness causes the separation to happen at a lower speed, which is why dimpled golf balls travel further.

1. Maybe https://doi.org/10.1006/jfls.2000.0362 is relevant: KINEMATICS AND DYNAMICS OF SPHERE WAKE TRANSITION.

Matatouille wrote: Sat Jan 04, 2020 8:51 am

shpalman wrote: Fri Jan 03, 2020 10:58 pm Physically that kink is to do with what happens to the air flow around the back of the sphere by the way.

Probably a nice big Von Karman Vortex Street. Its a while since I've whacked a balloon, but IIRC they wobble in air which would be explained by shedding vortices. Perhaps Beaker would care to check the wobbling?

Ooh, thanks, you have answered a question for me. I always wondered why some tall chimneys had helical strakes.

Thanks !

After the whack, it rises then dips, then a little bounce upwards. Then it dramatically slows and falls. Whilst rotating around a vertical axis.

https://youtu.be/1_UNm1WXl38