# trying to understand how a ball is kept in a constant stream of air

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#1
I'm trying to understand why if you place a ball in a stream of air (i.e a ping pong ball in the stream produced by a hair drier), why the ball stays within the stream when the stream is tilted.
Is this caused by the Coanda effect?
How can we work out how far we can tilt the stream until the ball falls out (and why it does?)

Thanks!
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6 years ago
#2
(Original post by mediterraneo)
I'm trying to understand why if you place a ball in a stream of air (i.e a ping pong ball in the stream produced by a hair drier), why the ball stays within the stream when the stream is tilted.
Is this caused by the Coanda effect?
How can we work out how far we can tilt the stream until the ball falls out (and why it does?)

Thanks!
I suppose it's something to do with the ball having a low density/high surface area.

Posted from TSR Mobile
0
6 years ago
#3
Yeah coander effect but I think that's going to be a tough calculation.

When electrical retailers still had high street stores they'd sometimes fly a beachball in the window display using the inclined jet of air from a vacuum cleaner as an eyecatche, but iirc it didn't hold a steady position but cycled... More like a chaotic motion.
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#4
(Original post by Joinedup)
Yeah coander effect but I think that's going to be a tough calculation.

When electrical retailers still had high street stores they'd sometimes fly a beachball in the window display using the inclined jet of air from a vacuum cleaner as an eyecatche, but iirc it didn't hold a steady position but cycled... More like a chaotic motion.

Yeah that's exactly the example. I initially assumed that the ball was held in place just by Bernoulli's Principal, however on further reading I found other people saying it was due to Coanda effect. I've never heard about Coanda effect until now so I'm not exactly sure how it fits into this problem. Yeah I've realised it's a difficult problem to work out haha!
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#5
(Original post by Andy98)
I suppose it's something to do with the ball having a low density/high surface area.

Posted from TSR Mobile
Thanks for the reply, but it's not really about low density or high surface area. Take a piece of paper for example (low density and large surface area) and you would not get the same effect. It's to do with the shape of the object, in this case the air acting as a fluid around a sphere.
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6 years ago
#6
(Original post by mediterraneo)

Yeah that's exactly the example. I initially assumed that the ball was held in place just by Bernoulli's Principal, however on further reading I found other people saying it was due to Coanda effect. I've never heard about Coanda effect until now so I'm not exactly sure how it fits into this problem. Yeah I've realised it's a difficult problem to work out haha!
I think this is a legit explanation

but some of the Coanda stuff on youtube is dubious
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6 years ago
#7
(Original post by Joinedup)
I think this is a legit explanation

but some of the Coanda stuff on youtube is dubious
Yes, that's exactly it.

When the airstream is vertical, the leeward air stream will create turbulence as the flow converges. These random 'eddy' type flows, will induce pressure changes which cause the ball to bounce up and down under harmonic type motion.

Tilting the air stream creates a pressure differential between the upper and lower surfaces of the ball which then behaves like a conventional wing surface. This creates a new force vector perpendicular to gravity and the ball shifts in that direction.

Tilt it too much and the ball will fall out of the airstream altogether.
0
6 years ago
#8
(Original post by mediterraneo)
Thanks for the reply, but it's not really about low density or high surface area. Take a piece of paper for example (low density and large surface area) and you would not get the same effect. It's to do with the shape of the object, in this case the air acting as a fluid around a sphere.
Yeah, twas just a guess. However I know that someone answering with the wrong answer can spark a chain effect until someone gets it right.
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