Fluid Mechanics Question: Why Will A Ball Float On A Jet Of Water?

[ralph124c]

If you have a fountain which sprays vertically, you can place a light ball (like a pingpong ball) on the top of the water column, and it will stay there.
Why is such a position stable? One would think that it would be unstable-as a small movement of the ball (in the horizontal plane) would push it off the water column.

[robert_columbia]

Originally Posted by ralph124c

If you have a fountain which sprays vertically, you can place a light ball (like a pingpong ball) on the top of the water column, and it will stay there.
Why is such a position stable? One would think that it would be unstable-as a small movement of the ball (in the horizontal plane) would push it off the water column.

At the National Air and Space Museum (in DC), they have a demonstration of the equivalent phenomenon with a jet of air that is holding up a beach ball.

[Joey P]

I wonder if it's the same for water and air? For air it's because there's a column of low pressure air traveling around the ball all the time. When the ball tries to move to one side or the other the high pressure still air pushes it back in.
If it's sitting at the top of a water stream I'm not 100% sure that principle would hold up. I've never seen it done with water though.

[beowulff]

The column doesn't even have to be vertical - you can "trap" a beach ball in the exhaust of a vacuum cleaner hose which is tilted at a substantial angle.

[Mangetout]

Originally Posted by Joey P

I wonder if it's the same for water and air? For air it's because there's a column of low pressure air traveling around the ball all the time. When the ball tries to move to one side or the other the high pressure still air pushes it back in.
If it's sitting at the top of a water stream I'm not 100% sure that principle would hold up. I've never seen it done with water though.

I agree - with air, I think it's Bernoulli's Principle at work.

With water, I think what happens is that when the ball drifts off-centre, the jet tends to wrap around the side still touching and be deflected off in the direction of drift, resulting in reaction force correcting the drift. I don't think this is as stable a system as a ball balanced on an air jet.

[naita]

This video seems to show the same as with a light ball in a stream of air: http://www.youtube.com/watch?v=Bg4A-R_ANvs

[md2000]

My guess - the side most off-center shows the most cross-sectional area being hit by the stream and the angle of that area is such that the deflection force is toward the center of the ball, so the ball is pushed back to the center. So if the ball is too big compared to the stream, this won't work (that extra area will be outside the stream).

Equilibrium is where all sides are being hit by equal force, and all force being radial, pushes the ball to the middle of the stream.

[Fear Itself]

Originally Posted by ralph124c

If you have a fountain which sprays vertically, you can place a light ball (like a pingpong ball) on the top of the water column, and it will stay there.

I have never seen this.

[Rhythmdvl]

Originally Posted by robert_columbia

At the National Air and Space Museum (in DC), they have a demonstration of the equivalent phenomenon with a jet of air that is holding up a beach ball.

I used to work for the Smithsonian. One of the benefits was wandering around the attics and back halls with researchers and talking about their projects. Very much like a live-action Ted Talk. Anyway, the point being that with a badge you had access all over the museums (I worked for NMNH, but I could go just about anywhere). The basements were fascinating, not just the infrastructure and exhibit preparation shops, but the inner workings of some exhibits. I remember how fascinated I was with the beach-ball exhibit--something few people get to see is that embedded in the basement ceiling/exhibit floor is a treadmill.

[md2000]

On reflection - the edges of the column would need mor force than the middle. Not sure of the dynamics of a fan-blown air column, but I assume the area by the outer fan blades, the air is blown faster than near the axle. Probaly would explain why a forced jet of wate will not ususally do this - the pressure is the same all the way across, if not higher in the center, so it is dynamically unstable. if you create a circular spray with a low-pressure center, it will be dynamically stable.

[Rick]

Originally Posted by Fear Itself

I have never seen this.

Not only have I seen this, I have in very heavy rain storms seen cast iron manhole covers "floating" on a column of water. Very impressive to watch.

[RearEchelon]

I think it's something about the pressure in the moving stream being lower than the pressure in the surrounding (relatively immobile) air, so the atmospheric pressure pushes the ball into the lower-pressure area (center of the flow).

[Mangetout]

Originally Posted by RearEchelon

I think it's something about the pressure in the moving stream being lower than the pressure in the surrounding (relatively immobile) air, so the atmospheric pressure pushes the ball into the lower-pressure area (center of the flow).

That's the Bernoulli Principle, but I'm not sure it can be the effect at play here between two fluids of such different density.

I stand by my earlier assessment - in fact, if you look at the video linked upthread by naita, you can see the water wraps around the golf ball - and when the ball moves out of the stream in any direction, the uneven flow tends to deflect the stream leaving the ball so as to point in the same direction the ball was going - and this jet creates a reaction force pushing it back into the stream.

[Mangetout]

Here's a very rough-and-ready diagram of what I'm trying to describe:

http://sdrv.ms/KXRh8q

In fig1, the ball is in the centre of the stream and the water flows around it - and when the streams rejoin above the ball, they cancel each other out exactly.

In fig2, the ball has moved to the right of the stream - this means that more water is flowing around the left side of the ball, and when this travels around to the top of the ball, it more than cancels out the other streams, resulting in a jet being shed in a leftward direction. For each action (the jet) there is an equal and opposite reaction (the purple arrow) and the ball is pushed back into the stream.

[Mangetout]

Originally Posted by Mangetout

Here's a very rough-and-ready diagram of what I'm trying to describe:

http://sdrv.ms/KXRh8q

In fig1, the ball is in the centre of the stream and the water flows around it - and when the streams rejoin above the ball, they cancel each other out exactly.

In fig2, the ball has moved to the right of the stream - this means that more water is flowing around the left side of the ball, and when this travels around to the top of the ball, it more than cancels out the other streams, resulting in a jet being shed in a leftward direction. For each action (the jet) there is an equal and opposite reaction (the purple arrow) and the ball is pushed back into the stream.

And I got the description wrong. The ball moves right, the jet is shed in a rightward direction, pushing the ball back left.

[dtilque]

Originally Posted by Rhythmdvl

I used to work for the Smithsonian. [...] I remember how fascinated I was with the beach-ball exhibit--something few people get to see is that embedded in the basement ceiling/exhibit floor is a treadmill.

Aha! That's the cause. It has nothing to do with air pressure or the Bernouli principle. Because of the treadmill the ball can't either land or take off, so it just sits there in mid-air.<sup>1


1</sup> This phenomenon is somewhat related to that of a cat with buttered toast strapped to its back.

[dtilque]

Originally Posted by dtilque

Aha! That's the cause. It has nothing to do with air pressure or the Bernouli principle. Because of the treadmill the ball can't either land or take off, so it just sits there in mid-air.

After I posted I realized I should explain this a bit better.

There are actually two treadmills in this display. The one that Rhythmdvl saw is below the exhibit. Obviously there must be a second treadmill in the ceiling above the exhibit. So if the beachball lands on one treadmill, it will be, in effect, taking off from the other. This violates treadmill physics (well-known to the SDMB), so the ball just sits in the middle between the two, undecided.

[crowmanyclouds]

Julius Sumner Miller explains all, Bernoulli Pt. 1, Bernoulli Pt. 2 (including the cool trick of how placing a funnel at the end of the stream will instead of levitating the ball will trap it in the funnel).

CMC fnord!

[MetroGnome]

Anyone remember the old Planters Peanuts commercial where some guy levitates a peanut a couple inches above his mouth just by blowing on it? Couldn't find the actual commercial, but here's a random person doing the same trick (don't know why the camera's turned 90 degrees though).