Abstract:
Ab initio transfiguration of the jet disturbance at the sound initiation in organ flue pipes is captured by visualizing a smoked jet with a digital high-speed video camera. The greatest interest centers on the ``two-crest'' mode of the jet wave disturbance appearing prior to the ``one-crest'' mode, which corresponds to the steady-state oscillation. This two-crest mode may be sustained by a ``simultaneous double feedback'': The lowest dominant frequency, which is lower than the first mode of the pipe resonance, is determined by the overall separation--reformation process of the two-crest jet; the second mode of th epipe resonance is simultaneously excited by the rear-crest jet after the two-crest jet is separated and a stagnant vorticity is formed at the mouth. Mode locking is not established between the harmonics until the two-crest mode has been transfigured into the one-crest mode. Moreover, a pictorial analysis of the steaady-state jet wave reveals that these experimental results of the phase speed and the amplification factor show excellent agreement with the theoretical results obtained from a spatial analysis of the jet instability.