Cavity Breathing: How to Quench Low-Frequency Oscillations in a Supersonic Combustor
Hidden in the hot flow of a supersonic engine is a recess in the wall: the cavity. Its job is to keep the flame from blowing out, like a flag flapping in the wind. But if the cavity is made long and shallow, it starts to behave like lungs: inhale, it draws in a gulp of gas; exhale, it pushes it back out. The frequency of this 'breathing' is about 156 hertz, meaning 156 breaths per second.