# Videos of “Lift, Boundary layer separation, and curve balls”

Here are the videos from the “Lift, Boundary layer separation, and curve balls” demonstration. The GIF titles link to the full videos.

Launch

Flight

This one is a little hard to follow. The balls are orange and appear near the top left corner. The ball only rises a little bit and the camera angle makes it hard to see. If you watch carefully you see that the flight appears a lot flatter than a pure projectile motion (because of the perspective the ball appears to float in mid-air at one point)would lead to, indicating that there is a vertical lift force. This is better watched on the full video.

An index of all the demonstrations posted on this blog can be found here. Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.

# Lift, Boundary layer separation, and curve balls

Getting a ball (e.g. a cricket ball, soccer ball, tennis ball, or baseball) to move laterally is somewhat difficult (though I find it all too easy with a golf ball). The fluid mechanics of this phenomenon is quite interesting and provides insight in to how boundary layer separation influences both drag and lift. Here is a simple demonstration for showing how to get a ball to move upward after release by putting backspin on it.

Equipment

1. A ping-pong ball
2. Some sort of track. I used a 30 inch wall mounted shelving frame piece that you can get at a hardware store (I used this one http://www.lowes.com/ProductDisplay?productId=3006188)

Figure 1. Equipment.

Demonstration

1. Hold the bottom of the track and rest the ball on top of your hand (see figure 2 below).
2. Rotate your wrist rapidly so that the ball is forced up the track and forwards (see figure 3 below)

With a bit of practice you can get the ball to fly off the end of the track roughly horizontally and then rise up due to the lift force

Figure 2. Initial setup.

Figure 3. Launching the ball by rapidly rotating the track.

Explanation

The key to the explanation is that the rotation of the ball leads to asymmetric boundary layer separation. As you flick your wrist and rotate the track the ball is accelerated and forced up the track. Friction with the track imparts a backspin to the ball such that, as the ball leaves the end of the track, it is rotating in a clockwise direction as shown in figure 3. The backspin on the ball means that the lower side of the ball is moving faster than the upper part of the ball (see figure 4 below). As a result, provided you are in the right Reynolds number regime such that the separation point is  Re dependent, the boundary layer on the top of the ball will stay attached over a greater distance than the boundary layer on the lower side (see figure 4 below). As a result the airflow will be deflected downward by the ball.

Figure 4. The rotation of the ball causes the underside to move faster and the underside boundary layer to separate further upstream than on the top.

If you draw a control volume around the ball with the control volume moving with the ball then the C. V.  inflow is in the direction of flight and the outflow is deflected downward. Therefore, the ball must be applying a downward force on the airflow to create the downward component of the outflow momentum. As a result, the airflow must be applying a force vertically upward in reaction. This lift force drives the ball upward.

Figure 5. Control volume moving with the ball showing the inflow, the deflected outflow and the force that the ball applies to the flow to generate the downward momentum.

The demonstration requires very little equipment but is a little fiddly. You need to use a ping-pong ball as it is light enough that the lift force can overcome the balls weight and move it upward. An alternate way to do this is to spin it sideways and get it to move laterally. The implications of this physics are discussed with actual numbers in “The physics of baseball” by R. K. Adir. The origin of this particular demonstration is unknown though I have seen basic descriptions of it in a few different books.

An index of all the demonstrations posted on this blog can be found here. Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.

# Videos of “Radial flow, Bernoulli, and levitating an index card”

Here are the videos of the “Radial flow, Bernoulli, and levitating an index card” demonstration. The higher resolution videos are linked from the titles.

Foam plate version

Index card version

An index of all the demonstrations posted on this blog can be found here. Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.

# Radial flow, Bernoulli, and levitating an index card

This experiment was suggested by Dr. John Foss from Michigan State University. He presents a more detailed write up, along with a discussion of Reynolds number effects in Experiments in Fluid Mechanics, R.A. Granger, Ed. Holt, Rienhart and Winston, (1988). The basic idea is that a radial outflow between parallel plates has a velocity that decreases with increasing radial distance. Therefore, if a high Reynolds number situation exists such that the shearing effects are small and a nominal balance exists between the pressure gradient and the acceleration, the pressure will increase with radial distance. If the outflow boundary condition is atmospheric pressure then the pressure between the parallel plates must be a vacuum pressure. This is surprisingly easy to do with very simple equipment.

Equipment:

There are a couple of methods for doing this. Two are described below. The index card version is the easiest to do in large numbers.

• Foam Plate Version
• 1 Foam plate
• 1 Straight pin
• A can of compressed air
• Tape
• Index Card Version
• 1 straw
• 1 3”x5” index card
• 1 straight pin
• Tape
• Adhesive putty (optional, may help hold the straw in the spool)

Procedure:

• Foam Plate Version
1. Place the straight pin through approximately the center of the foam plate. Tape the pin to the bottom of plate to stabilize it and to keep it perpendicular to the plate.
2. Have someone hold the plate so that the pin is horizontal
3. Hold the spool of thread to where it covers most of the pin but is not touching the plate
4. Take the can of compressed air and aim it through the spool of thread. The nozzle does not need to be placed directly in the spool. Just aim it so that the air will flow through the spool.
5. When the air is released from the can the plate should move towards the spool of thread and should stay there without support as long as the compressed air can is blowing

*CAUTION:  Do not aim the compressed air can downward. The air will become very cold and could possibly burn someone

• Index Card Version
1. Place the straight pin through approximately the center of the index card. Tape the pin to the bottom of the card to stabilize it and keep it perpendicular to the card
2. Place the index card on the table with the pin pointing straight up
3. Hold the spool of thread directly above the pin but not touching the index card
4. Insert the straw into the spool. The adhesive putty can be used to attach the straw to the spool so that you only need one hand.
5. Blow through the straw and, if the spool is close enough to the card, you should be able to lift the card off the table. The card should stay as long as there is a steady flow of air

** Trial and error may need to be used to in both experiments to determine the gap distance needed between the spool and card/plate in order to pick the object up.

Foam Plate Version                             Index Card Version

Analysis

Consider an incompressible fluid flowing horizontally and radially out from a point source between two parallel plates separated by a distance T. At any arbitrary radial distance r from the source the area of the flow is

A= 2π r T

(see diagram below). For a constant volume flow rate Q the velocity is given by

U(r)=Q/2 πr T

Writing Bernoulli’s equation from r to the outlet at a radial distance R and taking the outlet pressure to be atmospheric leads to

Pr = (Q/2 π T)2 (R-2-r-2)

Therefore, given that r<R, Pr<0 and the plate/card will be pushed toward the spool. However, for this to work the gap width needs to be small. If T is too large the pressure vacuum pressure over the card will no be enough to overcome the weight of the card.

In reality life is a little more complex and a more detailed analysis of this problem is given by Prof. Foss in Experiments in Fluid Mechanics, R.A. Granger, Ed. Holt, Rienhart and Winston, (1988).

Thanks to John Foss for suggesting the demonstration and helping with the write up. Thanks to Meredith and Alex for testing the procedure, putting together pictures, and making the videos. Videos to follow soon.

An index of all the demonstrations posted on this blog can be found here. Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.

# Videos of “Discharge coefficient calculation and data presentation” demonstration

Here are the videos from the “Discharge coefficient calculation and data presentation” demonstration. The titles link to the full videos. If you really want to watch the whole thing (12 minutes) it is here.

Initial release

Depth measurement

An index of all the demonstrations posted on this blog can be found here. Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me (nbkaye@clemson.edu). I also welcome comments (through the comments section or via email) on improving the demonstrations.