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

setup

Ignition (with low flame height)

ignition

fire whirl (with much larger flame height)

whirl

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.

Fire whirl and stretching a vortex

Background

A fire whirlwind, otherwise known as a “fire devil”, “fire whirl” or “fire tornado,” occurs naturally in wild fires (see photos from National Geographic). They occur when a vortex forms around a fire plume. The hot air from the fire plume stretches the vortex vertically narrowing it and intensifying it. In nature the vortex can form due to the ambient wind being deflected around a fire by local topography. In the lab there are a couple of ways to form a fire whirl. The Phaeno Science Center uses an array of air jets blowing tangentially in a circle around the flame to create the vortex. At a smaller scale one can just rotate the flame.

Equipment:

  • Nonflammable Turntable (Lazy Susan)
  • Wire Mesh Trash Can (the finer the mesh, the better)
  • 1 Glass Pyrex Bowl (or any nonflammable container)
  • Cotton Balls
  • Rubbing Alcohol (or any fuel source)
  • Duct Tape
  • Lighter
  • Damp Towel (or any form of extinguisher)
  • fire

Procedure

  1. Using the duct tape, affix the trash can to the center of the turntable.
  2. Place cotton balls, 10-15, into the glass bowl and pour rubbing alcohol over the cotton balls. Pour enough alcohol to lightly dampen the cotton balls, careful not to pour too much; do not completely saturate the cotton balls.
  3. Using the duct tape again, affix the glass bowl inside the trash can, careful to center it in the bottom.
  4. Use the lighter to ignite the cotton balls. Observe the flame.
  5. Begin to rotate the turntable and observe the effect on the flame. Vary the speed (without being reckless) and note how the flame stretches with higher rotational speeds.
  6. Put out the fire using the damp towel.

CAUTION:

  • If not attached well, the trash can will slide off of the turntable when rotated.
  • Also, if not attached, the glass bowl will slide around the inside of the trash can.
  • Be careful when removing the glass container, it becomes very hot during the experiment.

no-whirlwhirl

Flame height (left) before spinning and (right) when spinning

Discussion

This is a good ‘wow that’s cool’ demonstration for any fluid mechanics class. It is particularly useful when discussing vortex dynamics and vortex stretching. Essentially, once the vortex forms around the fire the buoyancy generated by the fire stretches the vortex vertically. This makes the flame and vortex narrower and, by conservation of angular momentum, increases the intensity of vortex. this process is self reinforcing and the flame length can then dramatically increase. While this basic mechanics is fairly well understood, the actual prediction of when such fire whirls will occur in nature is very much an open question.

Thanks to Alex, Meredith, and Ali 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 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

CR

(2)    Irrotational flow around a circle

CI

(3)    Rotational flow along a straight line

SR

(4)    Irrotational flow along a straight line

SI

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.