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Ball caught in a stream of air

  1. Jul 24, 2011 #1

    Femme_physics

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    http://img508.imageshack.us/img508/3161/pressure1.jpg [Broken]

    Say I put my hand slightly above the middle of this air-flow producing machine that holds the ball up. Where would I "feel" it affecting more force on my hand, on the middle, or on the sides?

    I'd imagine on the sides, right? Even though the air moves SLOWER on the sides, when I put my hand there, I'll feel more "force" applied on my hand, yes?
     
    Last edited by a moderator: May 5, 2017
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  3. Jul 24, 2011 #2

    I like Serena

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    The text in the drawing is misleading.

    The air immediately next to the ball, left and right, where the lines are closer together, is moving faster and has lower pressure (that is what Bernoulli says and what feels a bit counter intuitive at first).

    However, the air that is entirely outside the stream of air, left and right, is standing still and has normal pressure.
    So only relatively speaking, does the still air completely outside the stream "move slower" and has higher pressure.
    That is, the pressure is only higher than the pressure in the compressed stream of air.

    If you put your hand in the stream, you'll mostly be feeling the force of the upward streaming air.
    The speed of that streaming air is highest immediately left and right of the ball.
    However, this is not the "pressure" that keeps the ball in place.
     
    Last edited: Jul 24, 2011
  4. Jul 24, 2011 #3

    rcgldr

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    I question the pressure oriented explanations for why the ball remains centered in an upstream of air. It think it's more likely that wake deflection related to Coanda effect is the reason the ball remains centered in an air stream. If the ball gets off center, the air flow on the "inside" surface of the ball probably remains attached longer than the air flow on the "outside" of the ball , resulting in an net outwards wake deflection, causing the ball to recenter itself.
     
    Last edited: Jul 24, 2011
  5. Jul 24, 2011 #4

    I like Serena

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    Interesting! :smile:

    I didn't know this effect yet.
    As I understand it, the difference would be that due to the Coandă effect the ball would be pulled diagonally upward, whereas Bernoulli would draw it back horizontally.

    I would assume both effects are applicable.

    Here's a video that we can use to analyze (Googled the title of this thread): :cool:
    http://www.mindbites.com/lesson/4578-physics-in-action-ball-caught-in-a-stream-of-air [Broken]

    As it is, I'm not certain which effect is dominant.
     
    Last edited by a moderator: May 5, 2017
  6. Jul 24, 2011 #5

    I like Serena

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    Btw, if I understand the Coandă effect correctly, it's still a difference in pressure caused by the Bernoulli's principle that causes the ball to be pulled back.
    The difference in pressure is the mechanism by which the change in momentum is carried over.

    The Coandă effect only predicts how the air changes its flow exactly when the ball is no longer centered.



    And here's a picture! :smile:

    explain.gif



    And here's another picture:

    angleOfAttack.jpg

    In this picture you can see the Coandă effect at work in the middle picture, where despite the great angle of attack, the air is still flowing along the wing, letting Bernoulli do its work.

    But if the angle of attack becomes too great (lower picture), Coandă does not do its job any more, so suddenly Bernoulli can not do its work any more either.
    The result is that the plane suddenly loses its lift, resulting in WHOOAAAH!!! :smile:
     
    Last edited: Jul 24, 2011
  7. Jul 24, 2011 #6

    rcgldr

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    In the videos the ball oscillates up and down and well as side to side, probably due to variations in the air flow, so it's stable but moves around a bit. Both effects could cause these movements.

    My issue with the Bernoulli explanation is that the fastest moving and lowest pressure air flow is at the sides of the ball, while the slowest moving and highest pressure air is at the center (directly below) the ball (because the ball slows the air flow).

    Any sideways movment of the ball corresponds to a difference in pressure and wake deflection in the opposite direction of the ball's movement (in order for momentum to be conserved). The question here is if Coanda effect, a difference in how long the flow remains attached on the upper portion of the ball, is the primary reason for the wake deflection and stablity.
     
    Last edited: Jul 24, 2011
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