# Video of “Flow attachment, wakes, and blowing a candle out around a cup”

Here is animated GIF of the “Flow attachment, wakes, and blowing a candle out around a cup” demonstration. The full video is here.

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.

# Flow attachment, wakes, and blowing a candle out around a cup

Most undergraduate engineering textbooks have a schematic diagram of the flow around a cylinder at different Reynolds numbers. The one below is styled on the diagram from Munson et al.  However, the implications of the diagram can seem a little abstract. Here is a simple demonstration that illustrates how, at the right Reynolds number and positioning, you can blow out a candle with a glass or cup between your mouth and the candle.

Schematic diagram of flow regimes for flow around a cylinder for increasing Reynolds numbers. From left to right and down Laminar fully attached wake, steady separation bubble, von Karmen vortex street, wide turbulent wake, flow reattachment with narrow turbulent wake (drag crisis). Adapted from Munson et al.

Equipment

1. A candle
2. Matches
3. A cylinder such as a drinking glass

Demonstration

1. Light the candle
2. Place the glass a few diameters away from the candle
3. With your mouth a similar distance from the glass as the glass is to the candle, blow out the candle.

Explanation

The main point of the demonstration is that the glass does not create and endless wind shadow but rather there is a wake behind the glass that gradually decays as the flow moves downstream. Therefore, provided the glass is not too close to the candle, the velocity in the wake regains sufficient velocity to extinguish the candle. The only exception to this would be if the candle were a low flow portion of the steady separation bubble that exists for low to moderate Reynolds numbers. However, it is difficult to achieve this Reynolds number while still blowing hard enough to blow out the candle. I found that my peak air speed is around 25 mph (11 m/s). The glass diameter was approximately 3” (7.5 cm) which gives a Reynolds number of 55,000. This places the flow squarely in the wide turbulent wake (pre drag crisis) regime (or it would if it were a uniform flow as opposed to a round air jet coming from a small opening). The demonstration takes a little practice.

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 “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.

# Spin up, boundary layers, and tracking tea leaves

Background:

Boundary layers play an important role in many fluid mechanics applications including drag, lift, and flow in conduits. This demonstration illustrates the role of boundary layers as part of the classic spin-up problem. The demonstration is a cheap and easy version of one written up by Nicholas Rott in the book ‘Experiments in fluid mechanics’ (out of print but worth getting a second hand copy of). The demonstration uses tea leaves to visualize the secondary vortex that forms during spin-up. See here for more on tea cup fluid dynamics.

Equipment:

• Turntable (Lazy Susan)
• Bag of tea
• Scissors
• Half-filled glass of water
• Tape (to secure the glass to the turntable)

Procedure:

1. Place the tea bag in a glass of hot water to wet the tea leaves.
2. Fasten the half-filled glass of water to the turntable with tape.
3. Using the scissors, cut open the used tea bag and dump roughly half of the tea leaves into the glass of water fastened to the turntable.
4. Spin the turntable quickly, so that the tea leaves move to the outer edge of the glass. Keep spinning until the water is fully spun up (at least thirty seconds for the glass we used you will need to test this out prior to using the demonstration).
5. After the elapsed thirty seconds, stop the turntable abruptly.
6. The tea leaves should move from the outer edge and settle in a heap in the center of the bottom of the glass.
7. Alternatively, if you do not allow the water in the cup to fully spin up, when you stop it the tea leaves will form a circle at the edge of the secondary vortex (see analysis below).

CAUTION: If not attached well, the glass of water can slide off of the turntable when rotated.

The images below show the tea leaves location when, from left to right, the cup is being spun up, the cup is stopped having been fully spun up, and the cup has been stopped after partial spin up.

Analysis (qualitative)

When, starting from rest, the cup is initially spun, a boundary layer forms along the base of the cup. This drives the fluid in a circumferential direction. However, in the absence of any force to balance the resulting normal acceleration, the water in the boundary layer is driven radially outward. This drives the tea leaves to the edge of the cup. The radial outflow is then forced up the side of the cup, though the tea leaves stay in the corner at the base as they are denser than the water.

The vertical flow then turns back in toward the cup center and then down when it reaches the water surface. This creates a cylindrical vortex around the edge of the cup (see figure below). Inside the cylindrical vortex is a non-rotating core with a flat water surface.

Over time, the cylindrical vortex grows toward the center of the cup until there is no longer a non-rotating core and the water surface is curved all the way across (see figure below). At this point the flow is fully spun up and the tea leaves should still be at the corner of the cup.

When the cup is abruptly stopped, the water in contact with the base also stops moving. There is, therefore, no longer anything driving the flow radially outward. Instead, there is a hydrostatic pressure gradient toward the center of the cup due to the curved water surface (the water surface remains curved as all the fluid outside the boundary layer does not know the cup has stopped and is still rotating). Therefore, the flow in the bottom boundary layer reverses and the tea leaves are driven into the center of the cup (see figure below).

In the event that the cup is not fully spun up (step 7 in the procedure section), the hydrostatic pressure gradient only extends from the side of the cup to the edge of the cylindrical vortex (recall that the water surface in the non-rotating core is horizontal). Therefore, the lower boundary layer only flows radially inward to the edge of the cylindrical vortex. The tea leaves thus accumulate at the inner edge of the cylindrical vortex (see figure below).

This is a remarkably robust experiment. It is almost impossible for it not to work (provided that the cup is secured to the center of the turntable). Thanks to Alex, and Meredith for putting together this write up and demonstration. 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 “Fire whirl and stretching a vortex”

Here are the videos from the “Fire whirl and stretching a vortex” demonstration. The full videos are linked from the GIF titles (the entire demo video is here).

Setup

Ignition (with low flame height)

fire whirl (with much larger flame height)

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.