JET FORMATION AND JET VELOCITY FLUCTUATIONS IN A FLUE

Flow visualization of the initial transient in a small recorderlike fl ue organ pipe is presented and the various stages of the jet formation are related to measurements of the acoustic response of the pipe. An initial acoustic signal, due to the unsteady volume flow of the jet, a ppears before the forming jet reaches the labium. This signal can easi ly be modeled using a low-frequency approximation. The initial traject ory of the jet makes a curve towards the exterior of the pipe. Under c ertain conditions, the jet may even, at first, miss the labium. This e ffect is related to the steepness of the pressure rise in the foot of the pipe. The initial impact of the jet with the labium appears to be a crucial factor in the triggering of the transient. Moving the labium towards the exterior of the pipe, using a steep pressure rise or putt ing ears around the mouth increase the chance that the jet will hit th e labium. This initial impact is followed by an impulsive vortex shedd ing at the labium and subsequently a high-frequency varicoselike oscil lation is observed on the jet. This oscillation is also observed witho ut labium. After about three periods of the fundamental mode of the pi pe, turbulence appears therefore destroying these coherent structures. Whereas the time dependency of the jet velocity dominates the first s tage of the starting transient, the jet velocity fluctuations during s teady-state result in a non-negligible. damping. This loss mechanism i s, for the fundamental mode of our experimental organ pipe, of the sam e order of magnitude as the radiation or visco-thermal damping.