A pictorial analysis of jet and vortex behaviours during attack transients in organ pipe models

摘要

Attack transients in organ pipes are investigated by analyzing slow-motion pictures from the smoked-jet visualization with a high-speed digital video camera. In our experiment the attack is very slow and the 90% rise time of the blowing pressure is over 50 fundamental periods. Also, lower final blowing pressures (typically 150Pa) and longer cutup lengths (10.2 and 15.8 mm) are used. Although the jet issuing from the flue almost always deviates exteriorly, the jet impinging straight on the edge is incidentally observed. This head-on impinging jet is followed by two symmetrical forward-spinning vortices. The exteriorly-deviating jet is characterized by the type of the vortex formed during initial jet-edge interaction: (1) no vortex; (2) a forwardspinning vortex just beneath the edge; (3) an almost stationary vortex before the edge; (4) a backspinning vortex just beneath the edge (this example is borrowed from M. P. Verge ef al. [1], and is observed when a very fast attack is applied to a short pipe with a short cutup). A forwardspinning vortex seems to promote a smoother buildup of the jet wave; a backspinning vortex yields complicated interactions among the main jet flow, its side flow, the entrained flow, and the edge; a stationary vortex largely retards the jet-wave buildup by inducing the two-crest (hydrodynamically second) mode; two symmetrical vortices need reaction time to adapt themselves to asymmetrical sinuous disturbance along the jet. Also, a kind of subharmonic jet oscillation occurs during this transformation. The two-crest mode is followed by complicated transfigurations such as the three-crest mode before the ordinal one-crest mode is established. The presteady state just prior to the steady state is characterized by the acoustic vortex shedding from the edge surface, which however disappears at the steady state. [References: 15]