110. Rijke Tubes

Several large metal tubes ROAR impressively when held vertically over Meker burners. The heat maintains the open-end acoustic standing wave mode (about 100 Hz).

A "Magic" Rijke tube has a wire gauze inside about one quarter the way from the bottom. Keeping the gauze concealed from the class, heat it red hot by holding it over a Meker burner on the floor. The tube will ROAR for some time after being removed from the flame from the stored heat. You can "tip the sound out" by tilting the tube horizontal, or block it by placing the lower end on the floor, and the ROARing will restart when the tube is vertical away from the floor if the gauze is still hot. Also, if you "tip the sound out", and then run across the classroom with the tube horizontal, bottom first, the ROARing will restart.

Heat up the magic Rijke tube, hold it away from the flame so it ROARS with the retained heat, and hold the normal tube over the flame. The tubes beat in their ROARing since, although they have the same lengths, they have slightly different diameters.


Rijke Tubes and Variable Stars
Similar to the standing sound waves exhibited in the case of the Rijke tubes, the pulsation mechanism for variable stars such as the Cepheids can be understood as a resonance of pressure waves of a particular period in the interior of those stars.

In the case of the Rijke tubes air can move in and out of both ends. A heated metal mesh placed a quarter of the way up from the bottom heats the air flowing past it. This flow of air is a combination of the convection current caused by the transfer of heat from the metal mesh and the sound wave that is set up for the condition of two open ends. For half of the oscillation cycle of the sound wave air moves in from both ends as it flows towards the center generating a pressure antinode (displacement node) there. Even though some of the air moving past the hot metal mesh has already been heated during the cycle prior to this, some additional cool air flows in, passing through it and acquiring thermal energy and further increasing the pressure, thus reinforcing the oscillation. For the remaining half cycle air passing by the metal mesh while flowing outward from the center of the tube is already heated and therefore energy transfer is minimal.

In the case of a Cepheid variable star, the partial ionization occurring in a He envelope allows for an increased opacity with increasing pressure, and is responsible for setting up the pulsation mode. The stage when the opacity is increasing corresponds to the part of the vibration cycle in the case of the Rijke tube when energy is transferred and the increase in pressure is amplified.

In both cases analogy can be drawn to an internal combustion engine.