# Videos of “Spin up, boundary layers, and tracking tea leaves”

Here are the videos of the “Spin up, boundary layers, and tracking tea leaves” demonstration. The video titles link to the full videos.

spinning up

stopping a fully spun up cup

stopping a partially spun up 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.

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# Videos of Vorticity and walking in circles

Here are the GIFs of the demonstration “Vorticity and walking in circles“. The headings link to the full videos.

(1)    Rotational flow around a circle

(2)    Irrotational flow around a circle

(3)    Rotational flow along a straight line

(4)    Irrotational flow along a straight line

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.

# Vorticity and walking in circles

This is a really simple demonstration that requires no equipment at all, though it is slightly enhanced if you have a stool that you can place at the front of the room. The demonstration shows the difference between rotational and irrotational flow for both straight and circular streamlines.

Equipment

Ideally you would have a stool or chair to walk around for the first two parts of the demonstration. If you do not, then you just need a space on the floor to walk around. The demonstration is exactly the same with or without the stool. You just don’t have the visual reference to walk around if you don’t have a stool.

Demonstration

The demonstration has four parts covering rotational and irrotational flows for both circular and straight streamlines.

(1)    Rotational flow around a circle

Place the stool in the front of the room with space all around it. Start off facing the class with the stool in front of you. Simply walk around the stool while facing the stool the whole time. Stop after one trip around the stool and point to the back of the room. Point out to the class that your body (the model fluid particle) is facing the back of the room. Start walking around the stool again and stop half way around, you should have your back to the class. Point to the front of the room and point out to the class that you are now facing in the opposite direction and have, therefore, rotated. Hence, this is a rotational flow with circular streamlines.

(2)    Irrotational flow around a circle

Repeat the first demonstration, only this time always face the back of the room as you walk in a circle around the stool. Again, stop after one trip around the stool and point to the back of the room. Continue for another half circle and stop. The stool should be behind you and you should be facing the back of the class room. Point to the back wall and point out that, while you were walking in a circle you were not rotating (you always faced the same direction). Hence, this is an irrotational flow with circular streamlines.

(3)    Rotational flow along a straight line

Start at one side of the room with a clear path across the front of the room. Roll yourself across the front wall of the room. That is, walk across the front of the room while rotating as if you were a wheel rolling along the wall. Stop a third of the way along while you are facing the back of the room and point to the back wall telling the class which way you are pointing. Continue rolling and stop a bit further on when you are facing the front wall. Point out to the class that you are now facing the front wall and must therefore have rotated. This is a rotational flow with straight streamlines.

(4)    Irrotational flow along a straight line

This is the easiest part. Simply walk across the front of the room in a straight line while always facing in the same direction. The degree of difficulty can be raised (though only slightly) by facing the students while walking sideways. This is an irrotational flow with straight streamlines.

Discussion

For each of the components of the demonstration you can give an example of such a flow. Examples might include (1) solid body rotation, (2) the bath tub vortex, (3) laminar flow in a pipe, and (4) Wind above the atmospheric boundary layer where there is negligible shear. You can also use examples from outside of fluid mechanics. For example, (1) is analogous to how the moon rotates around the earth such that we only ever see one side of the moon. Example (3) is analogous to a car tire as it drives along a flat road.

# Videos of “Visualizing streak lines, path lines, and streamlines (with lots of Ping-Pong balls)”

Here are some videos of the “Visualizing streak lines, path lines, and streamlines (with lots of Ping-Pong balls)” demonstration. The videos do not show the follow up drawings. The headings are links to higher resolution videos. Thanks to Scott Black of the Glenn Department of Civil Engineering for help with the videos and all the technicians in the department for help in building this and other demonstrations. Also, don’t forget to follow @nbkaye on twitter to get updates on this blog.

Path line (this GIF is a little unclear. I recommend the full video)

Streak line

Streamline

# Visualizing streak lines, path lines, and streamlines (with lots of Ping-Pong balls)

The difference between path lines, streak lines and streamlines is often hard to visualize. There is a great video illustrating the differences between path lines and streak lines for an unsteady flow but the flow gets a little complex at times. I use a larger scale demonstration with Ping-Pong balls acting as the fluid particles. This one takes a little more equipment and a fair bit of clean up.

Equipment

1. Lots of Ping-Pong balls (50+)
2. A bucket
3. A Ping-Ping ball gun capable of shooting 10 or so balls at a time.
4. An air compressor

The Ping-Pong ball gun can be made from a four foot length of 1½” PVC pipe capped at one end. Attach a quarter turn valve to the cap and a compressed air connector to the valve (see picture below). You will need either a compressed air line or a portable air compressor in the class room to fire the gun.

Demonstration

Path lines (the trajectory of an individual particle)

Write the definition of a path line on the board. Load a single ball into the gun. Ask the class to take a mental image of where the ball is at each of the times that you call out during flight. Fire the ball across the front of the class room in front of the white board (use orange balls if you have a white board or white balls if you have a chalk board). While the ball is in flight, quickly count out loud from 1 up to say 5 or 6 (the actual number is not important). After the flight draw a series of circles on the board (one for each number you counted) roughly following the arc of the ball’s flight. Label them as (t=1), (t=2), … with (t=1) being the circle nearest the outlet of the gun. The labels refer to the locations of the ball at the times called out. Draw a line through circles to form the path line.

Streak line (a line connecting all the particles that have passed through a single point)

Write the definition of a streak line on the board. Load about 10 balls into the gun. Ask the class to take a mental image of where all the balls are when you call out ‘NOW’ during flight. Fire the balls across the front of the board and call ‘NOW’ when the balls are in the air. After they have landed draw up a series of circles in an arc across the board. Label them (P1), (P2), … with (P1) being the particle that came out of the gun first. The labels indicate the order in which they left the gun. Draw a line connecting the circles to form the streak line.

Streamline (a line that is instantaneously tangent to the velocity vectors of the flow)

Write the definition of a streamline on the board. Tell the class that you are going to throw a bucket full of balls across the front of the room and ask them to take a mental image of where each ball is and what its velocity is when you call “NOW”. Throw the balls across the front of the room and call out “NOW” when the balls are in the middle of the board. Once they have landed, draw a whole series of circles all over the board with arrows for the velocity vectors. Draw the vectors such that you can draw lines tangent to the arrows passing through multiple balls. I typically draw up about 20-30 circles. I often ask the class if the drawing is accurate. Then draw a series of streamlines that pass through the circles and are tangent to the velocity vectors.

At the end of the class I ask each student to pick up a couple of balls each to help tidy up.

Discussion

The demonstration usually gets a good laugh, particularly when 100+ balls get thrown across the front of the room (I think the students don’t actually believe I am going to throw the whole bucket full). However, it does illustrate that a path line is formed over time (the counting during flight) as a single particle moves through space, whereas the other two lines are snap shots in time (when you shout “NOW”) connecting multiple particles.

Videos to follow shortly.