Studying Liquid Vortices

A vortex, or whirlpool, is a liquid or gas flowing about an axis. Vortices appear in nature on large and small scales. The small swirl of water in a draining sink is a vortex. The devastating winds of tornadoes and hurricanes are also vortices. You can easily build an apparatus to study liquid vortices with scientific precision.

Materials

• 2-L soda bottle with extra caps
• scissors
• one-hole paper punch
• metric ruler
• drill and bits (requires adult help)
• ring stand and 2 clamps (One ring clamp should be larger in diameter than the bottle, the other smaller.)
• three 15-cm pieces of uninsulated wire
• food coloring
• notebook and pencil
• small cork
• 30-cm string
• timer
• tubing (Latex and Tygon tubing work well. The tube should be large enough to fit over the faucet. A hardware store can sell you a special adapter that will screw into the faucet. You may find you get better results if your tube runs from the faucet to a second tube of narrower diameter.)

Procedure

To Assemble the Apparatus

1. Cut off the bottom of a soda bottle with scissors. Punch three holes about 4 cm apart around the bottom edge of the bottle, using a paper punch.
2. Ask an adult to drill a hole in the center of the cap. You should try different size holes to see which is best for making vortices. If you have extra caps, use a range of bits from I to 2.5 cm to make a different size hole in each cap. If you have only one cap, start with the smallest bit and make the hole larger with each trial.
3. Screw the cap on the bottle, then mount the bottle upside down in the sink. You can do this by placing the mouth of the bottle through a small ring clamp that supports it, and encircling the bottle at the middle with a larger clamp.
4. Connect one end of a tube to the faucet. Mount the other end inside the bottom edge of the bottle. Several centimeters of the end of the tube should be horizontal, following the inside curve of the bottle. Pass a piece of wire through each punched hole and twist the ends to fasten the tube in place.
5. Turn on the water, starting with a gentle flow and increasing slowly.

To Explore the Vortex Effect

1. At what water elevation in the bottle do vortices begin to form?

Vortices can be a problem. Vortices in the wake of ships and aircraft waste fuel because they use up energy. Vortices become a serious menace during forest fires because they are capable of picking up large flaming timbers and dropping them elsewhere to start new fires.

2. What are the dimensions of a vortex?
3. Drop food coloring slowly into the water and sketch the path of the water. What can you determine about how vortices form?
4. What diameter hole works best for producing vortices?
5. Hang a small cork from a string in the vortex to measure the number of revolutions per minute of the water. Determine this value (a) at the mouth of the vortex and (b) at different depths in the vortex.

Vortices occur in air and water because these substances exhibit the properties of friction and adhesion—they rub together and stick to themselves to some degree. Release a drop of detergent into a vortex and watch it disappear as the surface tension of the water is chemically destroyed. Adhesion and surface tension are explained in the next chapter.)

References

Hurricanes and Tornadoes (When Disaster Strikes series) by Keith Greenberg (Chicago: Twenty-First Century Books, 1995).

Hurricanes and Tornadoes by Neil Morris (Hauppauge, N.Y.: Barron's, 1999).

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