Us geeks have strange hobbies. My old boss, Paul D., plays the corrugated plastic tube, also known as a “whirly.” I just posted a new episode of my Science Teaching Tips podcast where he plays the whirly (like a true master) and explains the science behind the sound. It’s not quite what you might think! I was surprised. I figured, as do most people, that it’s air moving across the top of the whirly, like blowing across the top of a soda bottle. It’s not.
From Paul’s website:
While playing the whirly:
1. Cover the stationary end with your hand. Notice that the sound stops immediately.
2. Hold the stationary end near the burning candle, notice that the flame bends into the whirly.
3. Hold the stationary end of the whirly near a pile of confetti, or other small paper pieces. For the best effect hold the confetti in a strainer so that air can flow around the confetti. Notice that when the whirly is signing a note the paper pieces flow into the whirly and are sprayed around.
Notice that, when the whirly sings, air flows through the tube.
What’s Going On?
Why does air flow through a whirly?
Picture a whirly full of marbles. If you twirled such a whirly, the marbles would fly out of the spinning end. This is what happens to the air in the whirly. The faster you spin the whirly the faster the marbles and the air will flow through it. The wall of the whirly moves in a circle, to make the air inside the whirly move in a circle a centripetal force is needed. The whirly is open at the end so in the absence of centripetal force the air inside the whirly accelerates along the rotating tube. Air flows through the rotating whirly.
One way to say this is that “You throw the air out of the whirly.”
And see a video on Steve Spangler Science here.