JET-WAVE AMPLIFICATION IN ORGAN PIPES

An envelope-based method to estimate both the jet-wave amplification f actor and the mouth-field strength in organ pipes is developed by usin g flow visualization of a smoked jet with a high-speed digital video c amera. A theoretical envelope of wave growth, which is approximated us ing a negative displacement model of the jet drive, is compared with a n experimental envelope derived from superposing many instantaneous sh apes of jet deflection in the steady-state oscillation. The estimation results are presented in dimensional terms with respect to two partic ular models, where the flue-to-edge distances are, respectively, 15.8 and 10.2 mm, with a common flue thickness of 2.2 mm. In our experiment the jet velocity ranges from 7 to 33 m/s, the Reynolds number from 10 00 to 5000, and the sounding frequency from 130 to 580 Hz. The amplifi cation factor of organ pipe jets, estimated to lie in the 0.18-0.26-mm (-1) range, tends to decrease and saturate with increasing blowing vel ocity in each oscillation mode; the mouth-field strength defined as th e acoustic displacement amplitude, roughly estimated to be 0.5-1.5 mm, tends to increase and saturate with increasing blowing velocity. A ho t-wire anemometer is then used to measure the mouth-field strength, wh ose value shows a good agreement with the estimated one. This result c onfirms the validity of our envelope-based method. A dimensionless rep resentation of the experimental data is used to compare wave character istics between an organ pipe jet and an acoustically perturbed free je t. The applicability of the spatial and temporal theories of jet insta bility is discussed to analyze them. If we can assume a Poiseuille flo w at the flue exit and a subsequent Bickley jet, the spatial theory se ems to be relevant to our organ pipe jets. However, for lack of a reli able experimental measurement of the jet half-thickness we cannot draw a definite conclusion about the wave characteristics of organ pipe je ts. (C) 1998 Acoustical Society of America.