# Explanation for ball hovering in a vertical stream

• rcgldr
In summary, the conversation discusses the self-stability of a spinning ping pong ball in a diagonal stream of air. One explanation suggests that the pressure difference between the faster and slower portions of the stream causes the ball to remain centered. However, it is believed that the Coanda and Magnus effects are responsible for the ball's stability. The Coanda effect causes the flow to follow the surface of the ball, while the Magnus effect stops the ball from falling down. The conversation also notes that the expanding stream plays a role in the ball's stability, as the outer portions of the stream are angled outwards while the inner portions are angled less outwards.
rcgldr
Homework Helper
Example video I made a few years go:

Note that most of the air interacting with the ball flows around the ball, as demonstrated near the end of the video where a tube is used to prevent flow around the ball, causing the ball to shoot upwards.

The main point of this post is the explanation for the self stability of the ball to remain centered in the stream, even when the stream is tilted significantly.

One explanation I often read states that the pressure in the higher speed middle of the stream is less than the pressure on the slower speed outer portions of the stream, so the ball tends to hover in the lower pressure zone. The issue with this idea is that the impact effects of the flow with the ball result in the change in momentum being greater at the center of the ball where a small portion of the flow stops in a stagnation zone, than the change in momentum of the air impacting the ball away from the center of the ball.

It's my belief that the ball's stability is due to Coanda / Magnus effect. Say the flow around the the outer surface of the ball tends to follow the surface of a the ball for a nearly fixed angle of deflection, detaching away from the ball somewhere along the upper surface of the ball. The flow at the sides of the stream diverges outwards, while the flow in the center of the stream remains vertical (until it collides with the ball). If the ball shifts to one side of the stream, then the flow on the oustide of the stream has a greater angle from vertical than the flow on the inside of the stream. Since the flow remains attached for the same angle of displacement, the flow on the outside stream detaches sooner with less horizontal deflection than the flow on the inside of the stream. The net result is that the wake of the flow is diverted outwards. There is a Newton third law pair of forces involved with the Coanda / Magnus effect, the off center ball exterting an outwards force on the air, diverting the wake of air outwards coexistant with the air exerting an inwards force on the ball. I believe this inwards force of the air on an off centered ball is why the ball returns back to the center of the stream and why the ball's position within the stream is stable.

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rcgldr said:
It's my belief that the ball's stability is due to Coanda / Magnus effect.

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

A spinning ping pong ball is held in a diagonal stream of air by the Coandă Effect. The ball "sticks" to the lower side of the air stream, which (in combination with the Magnus effect) stops the ball from falling down. The jet as a whole keeps the ball some distance from the jet exhaust, and gravity prevents it from being blown away.

A spinning ping pong ball
As seen in the video the ping pong ball doesn't need to spin to remain stable, even in a diagonal stream.

After reading the side article on the Wiki Coanda effect, showing the ping pong ball being stable in a diagonal stream, the explantion is Coanda effect (the point I've been making) in combination with Magnus effect also occurring when the ball ends up spinning in the "right" direction. My point is that the Coanda effect alone is sufficient for stability, and Magnus effect is a minor contributor, since I can reverse the angle of the diagonal stream, and in spite of the ball initially spinning the "wrong way", it remains stable.

What's missing from the side article is noting that the stream is expanding (larger cross sectional area) as it slows down (mass flow is somewhat constant, there is interaction due to viscosity with the surrounding air). The expanding stream means the outer portions of the stream are angled outwards relative to the stream, while the inner portions are angled less outwards. It's this difference in angle relative to the stream combined with Coanda effect that results in an offset ball producing an outwards wake and experiencing an inwards corrective force.

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## 1. How does a ball hover in a vertical stream?

The ball hovers in a vertical stream due to the phenomenon of Bernoulli's principle. As the ball moves through the stream, the air above it has a higher velocity and lower pressure than the air below it. This creates an upward force that counteracts the force of gravity, causing the ball to hover.

## 2. What is the role of air pressure in the hovering of a ball in a vertical stream?

The difference in air pressure between the top and bottom of the ball is what allows it to hover. As the ball moves through the stream, the air pressure above the ball decreases while the pressure below remains constant, creating an upward force that keeps the ball suspended.

## 3. Can any object hover in a vertical stream or only a ball?

Any object with a suitable shape and size can hover in a vertical stream. The shape of the object determines how the air flows around it, and if the air pressure above and below the object can be balanced to create an upward force.

## 4. Why does the ball stop hovering when it is removed from the vertical stream?

The ball stops hovering when it is removed from the vertical stream because it is no longer experiencing the upward force created by the difference in air pressure. The force of gravity then becomes dominant, causing the ball to fall back to the ground.

## 5. Can the hovering of a ball in a vertical stream be explained by any other scientific principles?

The hovering of a ball in a vertical stream can also be explained by the Coandă effect, which states that a fluid (in this case, air) will follow a curved surface rather than a straight path. This helps to keep the ball suspended in the stream as the air flows around it.

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