What force keeps the ball levitated?

[Change in pressure]

What is reason that ball do not falls down,curved water jet low pressure because of centrifugal force or?
How water jet transfer force to the ball,through pressure?
As I know fluid can only moves object through pressure...

 

[BvU]

Hi $\Delta p$,

Did you have the sound switched on ? What part of the explanation causes you trouble ?

 

[Change in pressure]

Hi $\Delta p$,

Did you have the sound switched on ? What part of the explanation causes you trouble ?

On 1:30 he explain that ball stay in jet because air jet have velocity so from bernulli there is low pressure.
AIR JET PRESSURE HAVE SAME PRESSURE AS AMBIENT PRESSURE,so that is nothing with bernulli..

I don't understand why so many videos explain bernoulli on wrong way,even some professors at high school.This is unbeliveble!
Our children constantly learn wrong theory..

See video bellow,on 1:40 he blow air between two curved plate,nobody rembers to put two flat plates and than start blow air.If you do that, plate will not move...
Becuuse velocity as itslef do not produce low pressure,low pressure is produce when air goes in curved path,changing direction/speed.

 

[BvU]

On 1:30 he explain that ball stay in jet because air jet have velocity so from bernulli there is low pressure

quite correct !

AIR JET PRESSURE HAVE SAME PRESSURE AS AMBIENT PRESSURE

No need to shout. And incorrect. You can even see the turbulence with your own eyes.

 

[boneh3ad]

quite correct !
No need to shout. And incorrect. You can even see the turbulence with your own eyes.

He's not incorrect.

An incompressible jet will have constant pressure across it's profile, and that pressure is equal to ambient. This is a classical result.

Also, turbulence has absolutely nothing to do with Bernoulli's equation.

 

[BvU]

An incompressible jet

Air ?

No but it shows density differences meaning the pressure can't be constant

Ball on water jet is simple momentum transport from water to ball I'd say.

OP creates confusion about which case he/she is talking about: water or air. I concede so does the video

 

[Change in pressure]

He's not incorrect.

An incompressible jet will have constant pressure across it's profile, and that pressure is equal to ambient. This is a classical result.

Also, turbulence has absolutely nothing to do with Bernoulli's equation.

1)What is static pressure of water jet?

2)Does water which flow over upper curved surface of ball,has lower static pressure because of centrifugal force between water molecules so this is why ball will not fall down?

 

[Janus]

On 1:30 he explain that ball stay in jet because air jet have velocity so from bernulli there is low pressure.
AIR JET PRESSURE HAVE SAME PRESSURE AS AMBIENT PRESSURE,so that is nothing with bernulli..

I don't understand why so many videos explain bernoulli on wrong way,even some professors at high school.This is unbeliveble!
Our children constantly learn wrong theory..

See video bellow,on 1:40 he blow air between two curved plate,nobody rembers to put two flat plates and than start blow air.If you do that, plate will not move...
Becuuse velocity as itslef do not produce low pressure,low pressure is produce when air goes in curved path,changing direction/speed.

Here's is an experiment you can do with two flat surfaces. You need a playing card, and thread spool and a pin. Push the pin through the middle of the card and then place the card on one end of the thread spool with the pin going into the hole in the spool.
Put the other end of the spool to your mouth and blow through the hole. Not only will the card not fly off the other end, but as long as you are pushing air through the hole, the card won't fall off even if you tilt the spool so that the card is towards the floor.( if you stop the airflow the card will fall off.)
No curved surfaces, just the flat card surface and the flat end of the spool.

 

[Change in pressure]

Here's is an experiment you can do with two flat surfaces. You need a playing card, and thread spool and a pin. Push the pin through the middle of the card and then place the card on one end of the thread spool with the pin going into the hole in the spool.
Put the other end of the spool to your mouth and blow through the hole. Not only will the card not fly off the other end, but as long as you are pushing air through the hole, the card won't fall off even if you tilt the spool so that the card is towards the floor.( if you stop the airflow the card will fall off.)
No curved surfaces, just the flat card surface and the flat end of the spool.

This is situation where air change direction/speed so normally the pressure will change also.Because fluid can change direction or speed only with pressure difference.Pressure is only force how fluid can change direction or speed or move other obsticles.

read below link and watch video before you make final conclusion,(pay attention on flat plate over tube):

http://www.cabrillo.edu/~jwelch/bernoulli/bernoulli-coanda.html

 

[boneh3ad]

Air ?

No but it shows density differences meaning the pressure can't be constant

Ball on water jet is simple momentum transport from water to ball I'd say.

OP creates confusion about which case he/she is talking about: water or air. I concede so does the video

Air, for most flows in question (specifically, as long as the Mach number is less than 0.3), is incompressible. Water is also incompressible. (Note: I realize that air and even water do, in fact, have compressibility, but in the context of fluid flows, air is often and water is almost always incompressible.)

Of course this is a matter of momentum transport from the water to the ball, as that is essentially the definition of force. There is always a question of mechanism, though, and in fluids, it's rarely simple. In an example like this one, it seems dominated by the rotation of the ball. The jet gets the ball spinning, which redirects the water. The jet would tend to push the ball up and away from the jet, but when the ball is spinning and redirects water in the direction the ball would ordinarily move, the force exerted to bend the water (and redirect its momentum) has an equal and opposite pair that will tend to push the ball back toward the jet (in addition to holding it up against gravity.

1)What is static pressure of water jet?

1 atm (give or take, depending on where it was filmed).

2)Does water which flow over upper curved surface of ball,has lower static pressure because of centrifugal force between water molecules so this is why ball will not fall down?

It's hard to say exactly since the ball is also rotating. If you assume the water all starts with the same velocity and pressure before it hits the ball, then I am sure the water's interaction with the ball will cause various velocity gradients. That said, they are probably not all generated conservatively, which means that I still wouldn't trust Bernoulli's equation. I am sure there is some degree of pressure gradient radially away from the ball, but it's a complicated flow field.

When the flow first hits the ball, it's a highly viscous problem that causes it to start spinning. Once it reaches a quasi-steady state, I imagine the ball spins with the same tangential velocity as the flow and viscosity is probably no longer dominant. At that point, the dominant forces are probably a pressure gradient tending to hold the water against the ball and the centrifugal force tending to try to rip the water away from the ball. Surface adhesion and surface tension are probably also important.

So... it's complicated.

 

[Baluncore]

https://en.wikipedia.org/wiki/Coandă_effect

 

[Change in pressure]

Air, for most flows in question (specifically, as long as the Mach number is less than 0.3), is incompressible. Water is also incompressible. (Note: I realize that air and even water do, in fact, have compressibility, but in the context of fluid flows, air is often and water is almost always incompressible.)

Of course this is a matter of momentum transport from the water to the ball, as that is essentially the definition of force. There is always a question of mechanism, though, and in fluids, it's rarely simple. In an example like this one, it seems dominated by the rotation of the ball. The jet gets the ball spinning, which redirects the water. The jet would tend to push the ball up and away from the jet, but when the ball is spinning and redirects water in the direction the ball would ordinarily move, the force exerted to bend the water (and redirect its momentum) has an equal and opposite pair that will tend to push the ball back toward the jet (in addition to holding it up against gravity.
1 atm (give or take, depending on where it was filmed).
It's hard to say exactly since the ball is also rotating. If you assume the water all starts with the same velocity and pressure before it hits the ball, then I am sure the water's interaction with the ball will cause various velocity gradients. That said, they are probably not all generated conservatively, which means that I still wouldn't trust Bernoulli's equation. I am sure there is some degree of pressure gradient radially away from the ball, but it's a complicated flow field.

When the flow first hits the ball, it's a highly viscous problem that causes it to start spinning. Once it reaches a quasi-steady state, I imagine the ball spins with the same tangential velocity as the flow and viscosity is probably no longer dominant. At that point, the dominant forces are probably a pressure gradient tending to hold the water against the ball and the centrifugal force tending to try to rip the water away from the ball. Surface adhesion and surface tension are probably also important.

So... it's complicated.

Air and water also have mass,so it is very strange/not intuitive to see how fluids goes in curved surface with high speed,becuse in that curve is huge centrifugal force which wants move fluid outward.(just like when drive car in curve,every stuff inside car wants move outward)That mean there must be some force which will cancle centrifugal force,to keep fluid going in curved path.

What do you think which "force" allows such fluid behavior(pressure gradient or visocosity) ?

 

[russ_watters]

Air and water also have mass,so it is very strange/not intuitive to see how fluids goes in curved surface with high speed,becuse in that curve is huge centrifugal force which wants move fluid outward.(just like when drive car in curve,every stuff inside car wants move outward)That mean there must be some force which will cancle centrifugal force,to keep fluid going in curved path.

What do you think which "force" allows such fluid behavior(pressure gradient or visocosity) ?

There's also friction, surface tension and adhesion. It would be interesting to try this experiment with balls painted with different types of paint to change those properties.

 

[boneh3ad]

Air and water also have mass,so it is very strange/not intuitive to see how fluids goes in curved surface with high speed,becuse in that curve is huge centrifugal force which wants move fluid outward.(just like when drive car in curve,every stuff inside car wants move outward)That mean there must be some force which will cancle centrifugal force,to keep fluid going in curved path.

What do you think which "force" allows such fluid behavior(pressure gradient or visocosity) ?

Did you read my post?

 

[OmCheeto]

https://en.wikipedia.org/wiki/Coandă_effect

I liked figure #1.
It allowed at least a basic free body diagram.

I'm not sure about the effect that the water spinning off the ball has.

All I know is that I don't like Derek's guessed explanation.
@≈3:22

From looking at the high-speed footage, what I really think is going on, is this water getting thrown over and down by the ball, and since the ball is putting a force over and down on the water, that water is putting a force up and in on the ball, which keeps it right in that stream, [etc, etc, etc.]​

From my screen captures, most of the water is flung upwards. Which if it weren't for adhesion forces, I would say tend to force the ball down. But due to adhesion, um, I'm not sure.

So... it's complicated.

I could not agree with you more.

 

[boneh3ad]

I'm not sure about the effect that the water spinning off the ball has.

All I know is that I don't like Derek's guessed explanation.
@≈3:22

From looking at the high-speed footage, what I really think is going on, is this water getting thrown over and down by the ball, and since the ball is putting a force over and down on the water, that water is putting a force up and in on the ball, which keeps it right in that stream, [etc, etc, etc.]​

From my screen captures, most of the water is flung upwards. Which if it weren't for adhesion forces, I would say tend to force the ball down. But due to adhesion, um, I'm not sure.

View attachment 239334

There is nothing wrong with his explanation. Any redirection of the water from its initial trajectory is a change in momentum, which requires a force. Even if most water is flung upward, that is still the same as the original direction of movement. That upward flinging is likely slower than the original stream would have been, so the upward water momentum has been reduced, corresponding to a corresponding upward force on the ball or disk. That is what holds it up against gravity.

For the lateral force (and therefore a stable levitation in the stream), the important question is whether more water is ejected away from the stream (corresponding to a force on the water away from the stream and therefore a force on the ball toward the stream), or if more is ejected toward the stream (corresponding to a force on the ball away from the stream). It's pretty clear that more water is being ejected away from the stream when operating at steady state. If the ball moves too far toward the stream, more water is ejected toward the stream, pushing the ball away. If the ball moves too far away from the stream, less is deflected away from the stream, therefore pulling the ball back toward the stream. This is how it is maintaining a stable equilibrium. I am sure there is an inflection point in either direction where, upon crossing it, the ball is now no longer stable.

So based purely on momentum/force pairs, this makes complete sense. Digging into the underlying physics of how the water sticks to the ball and those sorts of questions is where it all gets sticky.

 

[rcgldr]

boneh3ad's explains it well. The flow is deflected slightly outwards resulting in a Newton 3rd law pair of force, the ball exerts an outwards force on the jet, and the jet exerts an inwards force on the ball.

read below link and watch video

http://www.cabrillo.edu/~jwelch/bernoulli/bernoulli-coanda.html

The website mentions a pressure drop for a flow moving across and over the open end of a tube, but doesn't explain the cause. When the air blows over an open ended tube, vortices form and the pressure at the end of the tube is reduced. Some carburetors use this principle in addition to venturi pathways to draw fuel into the air stream. For an enhanced version of this effect, the open end of the tube is angled and the opening faces away from the direction of air flow.

By attaching a flat plate to the end of the open tube, the equivalent of a static port is created, where the opening "hides" within the boundary layer that forms along the flat plate, and the pressure at the opening will be the same (or very nearly so) as that of the static pressure of the flow across the flat plate.

You need a playing card, and thread spool and a pin.

In this case, the flow is being spread out as it moves radially outwards, the same amount of mass traveling through an ever increasing volume, which is why the pressure is reduced.

 

[Change in pressure]

By attaching a flat plate to the end of the open tube, the equivalent of a static port is created, where the opening "hides" within the boundary layer that forms along the flat plate, and the pressure at the opening will be the same (or very nearly so) as that of the static pressure of the flow across the flat plate.
.

Yes this is prove that air jet have same static pressure as ambient.Hole must stay prependicular to the airflow,holes edges must at 90 degrees with sharp edges so airflow minimize micro turbulenece on hole etc...in video you can see if air jet have low pressure (like 90% peopele and high school proffesor think,becasue of bernoulli) than air jet will also lift flat paper

 

[OmCheeto]

Digging into the underlying physics of how the water sticks to the ball and those sorts of questions is where it all gets sticky.

After doing an experiment a few minutes ago, I've decided I have no idea what is going on.

Bathroom sink, since there's snow on the ground outside
The camera was moved between images, so the apparent shift in water stream can be ignored.

Since the spheres are attached to the rubber band with duct tape, they did not spin.

The angles between the water stream and the rubber band are approximately the maximum angle I could obtain.
From that, it may be possible to at least determine the lateral force upon the balls.

But as I said, I now have only guesses as to what is actually going on.

ps. Fun experiment! I'll have to recreate it at the river this summer, when it's NOT 0°C outside.