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Building a science exhibit demonstrating Bernoulli's Principle

  1. Jul 23, 2011 #1
    Hi guys, I'm building a science exhibit demonstrating Bernoulli's Principle for a competition. It's similar to the setup found here () at 2:51 to 2:57.

    So, I tried a ping pong ball and a hair blower. Doesn't work. Firstly, the hair blower is pretty weak, so when the setup is in upright position, it's hard to tell if the ping pong ball is "hovering" right above the nozzle (like in the video) or on top of the air stream (the regular beach ball above a fan). Secondly, the nozzle of the hair blower is way too large, as compared to the setup in the video, where the nozzle is like half the size of the ball.

    I then tried a really powerful industrial blower with a smaller nozzle (just slightly smaller than the ping pong ball, but nowhere like the video). The ping pong ball just shot right out the tube. Perhaps it's too strong? Like, really really powerful. When I switched it on there's a kickback. Like firing a pistol, maybe?

    Any advice?

    Thanks.
     
    Last edited by a moderator: Sep 25, 2014
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  3. Jul 23, 2011 #2

    rcgldr

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    That video is not a good demonstration of Bernoulli effect because other effects are involved as well, Coanda, viscosity, momentum, ...

    A youtube video showing the opposite effect of a hovering ping pong ball in an upstream that is shot upwards when a tube is placed just above the ping pong ball (this is done towards the end of the video, all done in one take so no tricks involved):

    http://www.youtube.com/watch?v=PdT3ChUl-zk&hd=1

    Wiki article for Coanda effect.

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

    Here is a better example of a Bernoulli demonstration:

    http://scifiles.larc.nasa.gov/kids/D_Lab/activities/bernoulli.html [Broken]

    A link to a device that uses a Venturi to produce enough suction to drain water from an aquarium.

    http://andysworld.org.uk/aquablog/?postid=247

    Attached is a photo showing the internals, figure 4 is when the device is in a suction mode. Figure 5 has the end cap shut so water flows into the aquarium (to refill it).
     

    Attached Files:

    Last edited by a moderator: May 5, 2017
  4. Jul 24, 2011 #3
    So Coanda effect dictates that the air will flow AROUND the ping pong ball (because it's attracted to the surface), while Bernoulli states that the lower pressure of the fast-moving air will cause the ball to remain stationary?

    I'm hoping to continue with my exhibit because the WOW factor is one of the judging criteria.

    About the video with the ping pong ball shooting out, why does it happen? My guess is that the tube interferes with the air flow somehow, although I don't know exactly why.
     
    Last edited by a moderator: May 5, 2017
  5. Jul 24, 2011 #4

    rcgldr

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    That's the key to the experiment shown in the video. The hose and nozzle size is 1/3rd or less (maybe 1/4th?) than the diameter of the tube and the ball. Similar to the spool and cardboard link I posted, the higher pressure from the small nozzle generates a force away from the nozzle on the ball, but the much larger surface area of the ball outside the diameter of the nozzle that experiences a somewhat lower pressure, results in a net force towards the nozzle.

    Even without the ball, the narrow nozzle feeding into the tube is probably going to generate vortices near the nozzle, which will have lower pressure.

    Coanda effect is that a gas or fluid tends to follow a convex (curving away) surface. In the process of following that surface, there is centripetal acceleration towards the surface, and this acceleration combined with momentum of the gas or fluid results in a reduction of pressure, creating a low pressure zone in the vicinity of the convex surface.

    I'm not sure of all the details. I assume that in the free stream air can somewhat flow around the ball in a Coanda like effect, and that the wake upstream of the ball is smaller than the diameter of the ball, reducing the drag on the ball. When the tube is held in place just above the ball, (note in the video the tube doesn't have to be lowered all the way down to the ball before the ball is catapulted upwards), it interferes with that Coanda effect, increasing the size of the effective wake and increasing the drag. A simpler way of stating this is that in the free stream, the air can somewhat flow around the ball, but with the tube restriction of that flow, the air can't flow around the ball and has to push the ball upwards in order to flow.

    back to the original post again:

    In the immediate vicinty of any powered fan or propeller, there is a pressure jump (this is how the fan or propeller increases the energy of the air). Depending on the length and shape of the nozzle after the fan, the high speed air may have higher or lower than ambient pressure. You'd need to setup a static port horizontal to the flow from the nozzle to determine the pressure of the high speed air. In order to get the results as shown in the video, you'd need a tube with much larger diameter than the nozzle size of the blower to reproduce the effect. I'm not sure how large the ball needs to be before it's no longer blown out of the tube. My guess is a tube large enough to hold a small beach ball (basketball size) should be enough, but that would be a big tube.
     
    Last edited: Jul 24, 2011
  6. Jul 24, 2011 #5
    Thanks for the guidance. I guess the ratio of the diameter of the ball to the diameter of the nozzle is key here. So I'm going to play about with different sizes to see which one works.

    And about the pressure jump in the immediate vicinity of a powered fan/propeller, you're saying that I need to have like a hose or something so that the "turbulence" generated by the propellers will decrease and the air pressure will be more even?
     
  7. Jul 24, 2011 #6

    rcgldr

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    and the ratio of tube diameter versus nozzle diameter.

    I'm not sure how much turbulence there is. If the hose restricts the flow rate, it may increase the pressure. If the hose is not restrictive (so that flow rate remains about the same as without the hose), it may reduce the pressure due to friction, or if the hose is tapered, due to Venturi (Bernoulli) effect.

    This NASA article should help explain what I mean by pressure jump at the propeller:

    The engine, shown in white, turns the propeller and (the propeller) does work on the airflow. So there is an abrupt change in pressure across the propeller disk.

    http://www.grc.nasa.gov/WWW/K-12/airplane/propanl.html
     
  8. Aug 15, 2011 #7
    Hi,

    I am still unable to replicate the results in the Youtube video, although I have managed to produce a similar outcome which is equally eye-popping.

    Considering that I'm running out of time (I have a deadline to adhere to!), my team has decided not to waste valuable time trying to mimic the Youtube video, but instead modify the entire exhibit to demonstrate the new effect.

    So basically, an industrial blower is pointing downwards (air is blown out of a nozzle about 1/2" in diameter and the nozzle (which is actually a metal tube) is in a clear, plastic tube). The bottom end of the tube is sealed, and a ball placed in the tube. When the air flow is turned on, the ball mysteriously flies upwards towards the nozzle and hovers there. When the seal is broken, the ball drops.

    I have no idea what causes this effect. Anybody has any inkling?
     
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