Acoustic vortex appearing at the pre-steady state and disappearing at the steady state in organ pipes

Abstract

In visualizing the motion of the smoked jet during the attack transient, an acoustic (acoustically induced) vortex was observed alternately above and beneath the pipe edge at the pre-steady state just prior to the steady state. Interestingly enough, this acoustic vortex disappeared at the steady state. This finding is contrary to Fabre’s, in which the acoustic vortex appears at the steady state [Acust. Acta Acust. 82, 863–877 (1996)]. The rotation of the acoustic vortex is the reverse of that of the hydrodynamic vortex produced at the initial phase of the attack transient. According to Howe’s theory [J. Sound Vib. 70, 407–411 (1980)], our acoustic vortex seems to absorb the final excess in acoustic energy generation occurring at the pre-steady state and to lead the finally saturated amplification of the jet stability wave. The acoustic vortex may then be convected by the jet flow into regions where the vorticity can no longer continue to interact with the acoustic field. On the other hand, Fabre’s acoustic vortex was interpreted as some sound source corresponding to higher harmonics. Slow-motion pictures captured with a high-speed digital video camera will be demonstrated.