Thermodynamical mode selection rule observed in thermoacoustic oscillations

A gas column in tube with temperature gradient spontaneously begins to oscillate when it is driven away from equilibrium. We observed transition from the standing- to the traveling-acoustic-wave modes through quasiperiodic state using looped tube attached to resonator. Through the measurements of entropy flow, we experimentally propose thermodynamical mode selection rule, which acts as guiding principle for sequence of unstable modes appearing in nonequilibrium systems.