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.

Take home lab experiment – flow rate from a hose

I typically use this lab as the 3rd or 4th lab of the semester. The lab is simple enough, they have to use two different methods to measure the flow rate out of a hose. They can use kinematics, conservation of volume, or even momentum (though this is a little more tricky). This is one of the labs where I ask my students to use their estimates of measurement uncertainty, through some basic linear error analysis, to estimate their calculated flow rate uncertainty. If their 2 measurements are not the same (within the bounds of uncertainty they calculated) they have to discuss why not. It is a great experiment for discussing errors because, even though the measurements are simple to make, they often have significant percentage errors that propagate into very large percentage error in their calculated flow rates. For example, measuring the diameter of the hose outlet can be tricky and a 10% error in the measurement becomes a 20% error in the hose area. There are also challenges with repeatability of the experiments as it is hard to get the hose to have the same flow rate each time you turn it on. I do not explicitly ask them to discuss repeatability but rather I discuss it when I return the graded reports and ask them to think about repeatability as part of their next take home lab.

As with all the take home labs I will not publish methods for conducting the tests as I still use them in class and want my students to figure it out on their own.

Introduction

In this class we have looked at a range of different flow analysis techniques (conservation of mass, kinematics, Bernoulli, momentum, etc.). In this 3rd lab you need to use 2 different approaches to calculate the flow rate from a garden hose.

1. Run a series of experiments to establish the flow rate our of the flow from a regular garden hose. There is a hose in the fluids lab that you could use. You can use buckets, measuring tapes, and stopwatches. If you wish to use anything other than that you will need to check with me first. You are not to use laboratory flow rate measurement devices such as the venturi meter.
2. Write a brief report (3 page max) that:
1. Includes photos of you running your 2 experiments.
2. Describes how the test was run
3. Includes diagrams showing what you measured
4. Presents the theory and equations you used in your calculations
5. Lists what data was collected and estimates of your measurement error (in a table)
6. Error analysis (see class notes) to estimate your uncertainty in your calculation of flow rate.
7. A table listing the two calculated flow rates and your uncertainty estimation.
8. If the two measurements to not agree (within the error range you calculated) then discuss why not.

Due date in 2 weeks

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.

Take Home Lab – Measuring the mass flow rate from a compressed air can

The mass flow rate of a fluid appears many times in an introductory fluids class (pump power, conservation of mass, momentum, etc.). This take home lab requires students to use at least two different approaches to measure the mass flow rate out of a compressed air can. My experience with this is that some students struggle with finding a second way to do the measurement. This has led to some rather creative, if not physically appropriate, measurement methods.

It is a nice experiment because there are substantial difficulties in taking accurate measurements. For example, it is hard to accurately measure the diameter of the straw connected to the can as it is so small. It is not unusual to have a measurement uncertainty of 50-100% in the diameter leading to an error/uncertainty of up to 400% for the straw area. These errors  can make a substantial difference to the resulting calculated mass flux. There can, therefore, be substantial differences between the two sets of measurements that are still within the bounds of uncertainty. There is also a repeatability problem with this lab that is easily observed and explained if one is paying attention to the data.

As with other take home labs that I use later in the semester, the students are required to do some basic error analysis to explain the differences between their two measurements. This is a very useful complement to their fluids lab class that runs in parallel with the main lecture class. The students need to estimate the uncertainty in each measurement they take and then use that data to estimate the uncertainty in their measured mass flow rate.

As with all the take home labs I will not publish details on specific methods for conducting the tests as I still use them in class and want my students to figure it out on their own.

Instructions to students

Introduction

In this class we have looked at a range of different flow analysis techniques. In this lab you need to use 2 different approaches to estimate the mass flow rate coming out of a compressed air can such as are used for cleaning computer keyboards.

1. Run a series of experiments to establish the mass flow rate out of a compressed air can. You can borrow an air can from me when you are ready to do your testing. You may use an electronic scale and a stopwatch but otherwise only non-lab equipment is to be used without permission. If you would like to use something else you need to check with me.
2. Write a brief report that
1. Is a maximum of 3 pages including photos of you running your experiments.
2. Describes the experiment(s) you used to establish your result including
1. How the test was run
2. What data you collected including estimates of your measurement uncertainty
3. How you performed your calculations including diagrams (with control volumes), equations, and relevant theory
4. A quantitative discussion of the uncertainties in your measurements and calculations including an analysis of the differences between your two sets of measurements.

Due in 2 weeks

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 “Momentum – air jets and paper plates”

Here are some video of the “Momentum – air jets and paper plates” demonstration. The full videos are linked from the GIF titles

Jet impinging on a plate

Jet impinging on a cup

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.