Tuesday, November 24, 2009

The Rijke Tube or The Gondolo

So apparently the first year professor Gondolo was teaching here at the U he had a demonstration set up in which a wire mesh is heated inside a cardboard tube by a propane burner. The bottom of the tube is obstructed while the mesh is heating up because if an air current were allowed to flow it would blow out the propane. When the mesh is hot enough you turn off the propane and remove the obstruction on the bottom. The mesh causes convection heating of the air and an air current starts to flow through the tube. Standing wave patterns form and the tube becomes a resonator. The demonstration is actually rather an old one and is called the Rijke tube. If you don't know what I am talking about or have never seen it here is a youtube video of a small one being built and put into action. The video is pretty long you can just skip to around 3:30 to see the tube work.

As I was saying our young professor Gondolo (well... younger anyway) had a Rijke tube demonstration all set up but the tube was not like the one in the video it was a tube of more than a foot diameter and 10 feet or so tall. The tube was held by ropes hanging from the ceiling of the extremely large physics classroom. Professor Gondolo made the mistake of starting the propane burner and then teaching the physics of it. He taught the physics of the device for too long though and the wire mesh got rather hotter than it should have. When he was done explaining the physics and unblocked the bottom of the tube the mesh was so hot that the new airflow did a lot more than just allow resonance it allowed the cardboard tube to catch on fire. After that particular disaster when the demonstration was rebuilt it was built out of metal ducting and the name "THE GONDOLO" was painted on its side in red along with some nice decorative flames. Since professor Gondolo is going to hopefully become my research advisor I don't think I shall mention it until after he decides to accept me as his student, but I can't help but wonder how he feels about the giant metal demonstration tube that now bears his name.

Either fortunately or unfortunately I never saw the original demonstration with the cardboard tube. The cardboard tube was supposed to have been even larger and louder than the current metal one but the newer one is still damn impressive. The natural frequency of the tube is lower than you can hear (or at least hear well) so mostly what you hear when the tube goes off is the second and third harmonics. But the vibration is so loud and so low that you can definitely feel it.

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