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Air Streams

Introduction
Public summary: 

Can you make something float on thin air? Find out how to levitate ping pong balls and why planes can fly in this entertaining experiment.

Balancing balls on upward streams of air, and looking at aerodynamics.
Useful information
Kit List: 

Small scale stuff:
- Bendy straws (consumable), must have reasonably wide holes.
- Ping-pong balls
- Small wooden rocket puzzle
- Anti-bacteria wipes (consumable)
Large scale stuff:
- Big Blower (too big for box, needs 2 people to lift)
- Stream stabiliser for the big blower (a hollow box open on one end and with a grid on the opposite end)
- 'Small' blower (in box)
- 2 x variable power switches for blowers.
- beach balls
- Screwdriver with haztape streamer (for showing streamlines)
- Paper
Wind tunnel stuff:
- Aeroplane wing

Packing Away: 

Everything except the big blower packs into one big blue box. This can be quite hard to do, the trick is to put the small blower and wooden stand in first, then fit everything else around it.
WARNING: Box is heavy to lift, the big blower is even heavier, don't try to lift it on your own.

Frequency of use: 
4
Explanation
Explanation: 

In a nutshell

Show how flowing air acts on object in or next to it.

How to set up the experiment

Note: The big blower is not necessary but makes a nice show. It may cause the circuit breakers to go off, so check first so that you don't cause a power outage a the beginning of the session.

Place the blowers on the ground. Make sure they are not in anyone's way, and also check above them to make sure nothing will be disturbed by the fan (such as lightweight ceiling tiles). Put the stream stabiliser on top of the big blower. Plug the cables to the power switches and to the grid.

Note: The switch for the big blower gives full power for a few seconds after switched on. Afterwards you will be able to regulate it.

Blow up the beach balls and have the straws pingpong balls and aeroplane wing handy.

How to proceed

This is a suggestion on how to proceed with the demonstration. The demonstrator should encourage the kids to try and figure parts of explanation out by themselves by asking the right questions, so that they stay focused. There's a lot in here: you may not explain all of it!

Straws and pingpong balls

Take a straw and bend it in right angle. You can start by asking the kids if they can think of a way to balance the ball on top of the short end (not holding it by hand). Two ways are quite good: sucking and blowing. (Simple balancing it is possible but very hard.)

If you suck the air from under the ball, the air pressure from the outside will hold it in place. You can tell them about the air pressure and that we are constantly pressed everywhere and that suction occurs when we reduce the pressure in an area (eg the inside of the straw). (The concept might be hard to grasp so consider the age of the children.) They might already have some knowledge of it if they have seen the Vacuums experiment.

If you blow into the long end and hold the short end upwards, you are able to balance a pingpong ball on the stream. (The straw end needs to be vertical and you have to blow quite hard.) It requires a bit of practice. Let the kids have a go at it. Give each a new straw and dispose of them afterwards to avoid spread of infection. Demonstrating what goes on is easier with the blowers. It's the same on a larger scale.

A bit of theory - what are forces

Hold a ball in mid air and ask them what is going to happen if you let go. Show them that they are right. Now ask them why the ball falls down. You should get to gravitational force. Explain what a force is. That it is what makes things move, for example pushing, pulling, gravity, magnetic force, friction. Explain that if something isn't moving, the forces must be balanced (they are pulled down by gravity but pushed up by the ground).
Tell them about Newton's third law - action and reaction. If you push or pull something, you get pushed/pulled yourself. You can demonstrate by letting them push/pull your hand.

Wooden Rocket 'Puzzle'

Put the rocket on a flat surface with the tip of the orange 'rocket' pointing upwards. Explain that the aim is to get the rocket out of the container without touching the container. (Hopefully they'll come up with blowing on it after doing the ping-pong straw experiment - though it's really not obvious when you first look at it that it's going to work!). You can then demonstrate that indeed the rocket does jump out of it's container (literally!) if you blow on it. I find that it works best if you blow at about 45 degrees to horizontal - have a practice yourself before hand. (I can generally get it to go the highest when I blow straight across the rocket, but this makes it harder to aim and half the time I just end up blowing the whole thing across the desk). The kids can then have a go themselves. There are some anti-bacterial wipes to use on the rocket afterwards. Explain that the air in the container with the rocket is forced out, bringing the rocket with it - another example of air creating a force. (Air pressure decreases over the top of the rocket when you blow over it creating a pressure gradient, the air needs to be replenished from somewhere and part of it comes from inside the container. The explanation inside the rocket box says 'A short, sharp breath of air directed across the top of the puzzle creates a fierce vacuum, causing the wooden rocket to take off' , however I think vacuum is a bit extreme but if that makes more sense to the group you're explaining it to?)

Blowers and beach balls

Preferably use the big blower for this. If you place the ball in the air stream it will balance on it and not fall off. You can demosnstrate that it always balances at the same height. If you hold the ball at the edge of the stream you can feel how it is pulled in. You can let the kids to feel it too. You can use the screwdriver with haztape to demonstrate the direction of the stream.

Explaining why the ball doesn't fall directly down is easy and the kids might tell you that on their own. The tricky part is why is it pulled to the middle of the airstream. There are two ways to go about the explanation. The first is using Newton's third law. When you look at the airstream around the ball when it is at the edge of the stream, you will see that it is bend around the ball. After passing the ball, the stream is not strictly vertical (it is going a bit sideways). That means that the ball effectively pulls the passing air towards itself. The equal and opposite reaction on the ball then is that it is pulled inside. The other way of explaining it is using the Bernoulli's principle - the fact that the pressure is lower where the air has greater speed. (You can demonstrate it by blowing between two parallel sheets of paper hanging vertically next to each other.) So there is atmospheric pressure on one side of the ball and lower on the side of the airstream. The difference in pressure results in a force towards the stream.

This can also be illustrated by tilting the smaller blower, and having it keep the beach ball still, even though it's not blowing from underneath.


A bit on stability

By the time you're reading this we should have a few beach balls of various sizes in the box, if not shout at Zephyr...

Using these we can do a brief discussion on stability of systems and how they will tend to rearrange themselves in the most stable form.

If we put two balls of different sizes into the air stream at once, the most stable configuration is the smallest on the bottom and the largest on top. This is because each ball effectively blocks some of the air hitting it from getting to the ball above (though by no means all, as seen with the bigger at the bottom) and hence there is less upward force on the higher ball.

Thus with the smaller on the bottom, enough air still reaches the second ball to hold it up, whereas the other way up little air makes it to the smaller, higher ball and it is likely to 'fall' off the side of the stream.

Why the quotation marks? Well, if you're lucky (/careful with the settings) the second ball rarely actually escapes the stream, instead the larger ball barges it's way past and then the smaller slots back into place under it. I.e. the system seeks out the most stable configuration.

That said, the smaller ball on top is still an equilibrium state, just less stable, and for more advanced audiences you can talk about the differences between equilibrium and stability.

Note, there is another effect with very small balls, like the ping pongs balls, where they are so light that the force they experience hugely outweighs the difference in amount of air hitting them, so make sure that the balls are of roughly comparable mass, or the lightest will just fly off!

Aeroplane wing

When you tilt the small blower so that the stream of air is horizontal, you can demonstrate the forces acting on the aeroplane wing. Children should be able to feel the air pushing the plane upwards when it is held horizontally. The wings are designed to maximise the upward thrust (which means they push down air significantly) while the backward thrust is minimised (we don’t want the wings to slow down the plane too much). Tilting the wing in the airstream results in greater upward force but also greater backwards force. Let the kids feel it but be careful so that the wing doesn’t fly off and hit them in the face.

Two possible explanations you can use:

One way to think about this is again with Newton's Third Law. The curvature of the wing means that the wing pushes the air blowing at it downwards, and so the air pushes the wing upwards. The size of the force depends on how fast the air is moving, and therefore how much air it is pushing down - you can show this by adjusting how hard the fan is blowing. This means that a plane has to travel VERY fast to get enough lift to keep it in the air (commercial airliners travel around 500mph).

Or using air pressure to explain: When the plane moves forward it pushes air out of the way, over and under the wing. As the top of the wing is curved more than the bottom of the wing, the air has to go further to get over the top and so it has to move faster. Bernoulli's principle says that the pressure is lowered when the air speeds up, so the air pressure above the wing is lower than the pressure below it, and the wing (and plane) is pulled upward (ie the same way things get sucked into a vacuum), opposing the gravity pulling it down.
[note: this doesn't explain why the air has to move faster over the wing - the air stream moving over the top of the wing has to move through a narrowing space due to the curvature, but with a constant volume per time (like water through a tube that then gets narrower).]

Bits from older explanation

*** OTHER THINGS TO TALK ABOUT ***

Have you ever noticed your ears going pop as your train goes through a tunnel?

For the train to move forward the air in front of it has to get out of the way, normally there is lots of space for it to do this, but in a tunnel there is just a little gap around the side, which the air has to squeeze through really fast.

This fast moving air sucks some of the air out of the train, which then sucks on your eardrums, making them go pop!

Why do you think they go pop again when you leave the tunnel?

The air isn't going as fast outside so the pressure goes up and the pop back inwards again.

You may notice the ball is spinning sometimes - think about where the air must be flowing to stop the ball falling off.

This happens when the straw is at an angle so the ball drops down a little and the air stream is faster over the top which is what is holding it up.

Try moving your hands across the blower, just above the grill. This will cause the ball to 'dance' as you are changing the air stream so the beach ball adjusts its position to remain in equilibrium.

PLUS Explanation

Aims
- Newton's Laws and Stable Equilibria
- Drag Forces
- Bernoulli's Equation - Lift and Magnus E ect

1 Newton's Laws, Weight and Velocity Pro file

What are Newton's Laws? How might they be relevant here?

1 - still or constant velocity if no net force

2 - acceleration proportional to net force

3 - equal and opposite reaction of same type

Here the ball hangs in equilibrium - weight and drag must balance.

How does weight vary with height?

Weight essentially constant over the range of the room compared to the radius of the Earth (if they are familiar with binomial expansions could show this mathematically by expanding F = - GMm/(r+h)^2 ).

How would we expect the velocity to vary with distance? Can we draw a graph of this (i.e. velocity on y as function of height on x)?

Well collimated at exit - velocity gradient 0 - can show this using streamers. Falls to 0 at large distance - wouldn't feel effect of fan far away. Doesn't keep going negative - no bulk downward motion above the fan caused (there may be turbulence or circulation at the sides). Smoothly interpolate. Full solution very complicated.

2 Drag Forces

What factors will affect the drag?

Relative velocity of wind/object (you feel a force from the air if you run as well as on a windy day, easy to run with wind than into it), size of object, shape (streamlined), also properties of fluid - density, viscosity

In fact the functional form is as follows: At high velocities, inertia effects dominate with constant drag coeffcient:

F = Cpd^2v^2

At low velocities, viscosity dominates. Get Stokes' formula (most likely to have seen this before with Millikan Oil Drop:

F = 6*pi*eta*rv

(Result of drag coeffcient being inverse in velocity i.e.

C = eta/(pdv) )

Note in each case - increasing function of velocity.

Therefore, what would you do if you were building something?

Engineers have to design things to minimise drag by size of area, roughness, stability, smoothness of flow to stop vortices (streamlined tail). Very complicated - get things like vortex shedding which leads to vibration (sounds of wind). Also need to consider laminar and turbulent flow - rough surfaces can actually be better eg golf balls!

Try the "plane wing" (in reality a Pringles tube wrapped in card that you can stick your hand into) in the stream - which way round is it easiest to hold (ignoring the fact it wants to turn a lot) and why? What height is easiest? How does this change when fan turned down?

Should be easier to hold pointing into fan as lower area, more streamlined.

Return to our graph from earlier. Let's assume Stoke's law (where F proportional to v) which means we can relabel y as force. Given weight constant, what should line for weight look like? Where should it be?

Needs to be somewhere between maximum and minimum force such that it crosses the force from the air so we can have equilibrium.

3 Stability

Have a closer look at the graph. When the ball moves above the equilibrium, which is greater and thus what happens? What about the other way?

Up - weight greater - falls. Down - drag greater - rises.

This is an example of being stable, when we move away we return. Also described as negative feedback. What other examples of stability are in this problem? Hint: look for other directions that things can move (ball as a whole has 3 directions it can move, with each point having 3 relative to ball):

- The ball doesn't explode or collapse - e.g. the pressure inside goes up if we try to squeeze it. (Accounts for one degree of freedom - distance from centre of mass.)

- The ball often settles a particular way up with nearish the valve at the bottom. Rotational stability familiar from measuring the centre of mass -
if we move away we get a moment that moves us back. (A second degree of freedom - azimuthal angle from centre of mass.)

- The ball doesn't fall out sideways - due to Bernoulli effect which we will come onto. (Accounts for two degrees of freedom - x and y position of whole ball.)

Other examples across the sciences of feedback/stability:

- Populations - predator increases lead to prey decreases which starves predators.

- Homeostasis - e.g. if glucose levels too high, then insulin reduces them and vice versa for glucagon.

- Chemical equilibrium - in reversible reaction, if we increase products, then backward rate increases and we return - relates to Le Chatelier.

- Bond lengths in atoms - balance of electron energies from attraction to two protons to repulsion of atoms. Can be modelled with Lennard-Jones Potential.

- Balls can sit in valleys but not on top of hills!

- Mass on a spring SHM experiment.

With a good group you could go through stable equilibria being potential minima more - but might need building up idea of force as gradient of potential etc. If they know about Taylor series, you can also discuss all minima being quadratic like SHM.

4 Bernoulli's Equation - Lift and Magnus Effect

In steady flow (only) the arrows show both the direction of the fluid at any one point and also where a particle will go over time.
Where is the flow around the ball? Try sketching this on paper and con firming hypothesis with streamers.

Important to visualise streamlines using streamers at variety of places - note speed and direction at various heights; distances from centre; bottom, side and top of ball with varying proximity.

Bernoulli's Equation


For a unit volume of incompressible fluid (suprisingly good even for gas), energy conservation gives us (derivation not needed here, we are looking at mathematical reasoning):

P + 1/2pv2 + pgh = constant

What do the terms in this equation mean?

P is pressure, p is the density, v is the velocity, gh is gravitational potential

What happens if the velocity at a given height changes?

Pressure must change to balance.

Thus, why does the ball stay in? Try the ping-pong ball. Why does it not stay in?

Pressure difference between side of ball if off-centre. Pressure difference too small on ping-pong ball.

Shows diagram illustrating how the pathlines/streamlines go around a aerofoil (wing). Where are the lines more spread? What does this mean for velocity? Therefore, how does the pressure vary between sides?

Lines spread below and bunch up above. Mass conservation means that lower velocity below meaning higher pressure - net upwards force - lift. Bird Wings also use similar principles - also have narrow tips to minimise vortices - see cross section comparison.

Magnus Effect

Demo - a pair of plastic cups taped back to back. Wrap (chain of) elastic bands around them. Hold off of one finger like slingshot and release.
Have a look at this pair of cups. What does it do immediately after firing?

Cups loop up into air.

Why does this happen? Can we draw diagram of flow lines?
Cups drag air as they rotate- adds to motion on one side and detracts on the other meaning there is a velocity and hence pressure difference.

This is known as the Magnus effect. One example is rotor ships. Perhaps more common is the spin on a ball in sports e.g. football, golf, table tennis.

Risk Assessment
Date risk assesment last checked: 
Fri, 24/01/2020
Risk assesment checked by: 
jaredjeya
Date risk assesment double checked: 
Mon, 27/01/2020
Risk assesment double-checked by: 
Polly Hooton
Risk Assessment: 
Hazard Risk Affected Person(s) Likelihood Severity Overall Mitigation Likelihood Severity Overall
Blower (intakes) Trapping fingers/long hair in the intakes/gaps. All 3 4 12 Keep visitors away from the intakes. There are guards designed specifically to keep fingers out of the danger area. 1 4 4
Blower (weight) Injuries from lifting/moving the apparatus from place to place. Also the possibility of falling from a height onto people, causing injuries to the public. Demonstrator 2 3 6 Keep air conditioning generator on floor. Make sure that it is stable in the mount. Take care when moving it (refer to lifting advice on CHaOS website). 1 3 3
Electrical cables/electrical parts Tripping over cables. Electrical hazards as detailed in Electrical Parts RA. All 3 3 9 Tape wires to floor and lay sensibly (not across the middle of the room). If necessary, attach something brightly coloured so that cables are clearly visible. Ask children to put down cups of water before interacting with blower. Follow electrical parts RA. 1 3 3
Ping pong balls Slip hazard from ping-pong balls. Risk of child injuring mouth from falling over with a straw in his/her mouth. Also, children are liable to run across people's paths after escaped ping pong balls, which can lead to accidents. 4 2 8 Keep track of where the ping pong balls are and do not leave them on the floor. Get the kids to sit down if possible when playing with the ping pong balls so that they don't run around with the straws in their mouths and fall over. Encourage them not to chase balls. Do not allow young children to take straws away with them after experiment. 2 2 4
Saliva on straws/rocket Transmission of illness through contact with saliva. Public 2 3 6 Give every child a fresh straw. Wipe the rocket with anti-bacterial wipes, after attempts at making it take off. 1 3 3
Straws/model wing blown out of children’s hands Injuries as the result of collisions with flying objects, particularly with eyes/face. All 3 3 9 Make sure model wing is held securely - do not let small children hold unassisted when blower at full power. Discourage dropping of straws into air streams. 2 3 6
Blowing too hard when making air streams Dizziness, possible collapse Public 3 2 6 Warn before blowing into the straws not to blow too hard and to stop if dizzy. Call a first aider in the event of an accident. 2 2 4
Wooden rocket Rocket may hit child in face when it takes off - It's not very heavy nor does it go that high so will probably only be a problem if it hits their teeth. Splinter from wooden rocket. All 3 3 9 Make sure the child isn't too close to the rocket when blowing on it. Demonstrator to check for splinters before use and not use if any are noticed. Children should not need to touch the rocket anyway. Call a first aider in the event of an accident. 2 3 6
Objects above the fan Blower, or objects lifted by the blower, may disturb overhead objects such as lightweight ceiling tiles, causing them to fall down and potentially injure someone. All 3 2 6 Be careful to check above the blower for anything that might be disturbed when choosing where to place it. If this is unavoidable, then make sure the power is sufficiently low that nothing will be disturbed. If an incident occurs, turn off the fans, clean up and call a first aider if necessary 2 2 4
This experiment contains mains electrical parts, see separate risk assessment.
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