軸対称噴流-直管系の自励振動現象

抄録

An experimental investigation is presented for acoustic oscillation sustained by axi-symmetric jet impinging a straight pipe. The experiment has been carried out in axi-symmetric air jet facilities with nozzle diameter d=28 mm. Data presented for this nozzle are for U_0 ≈ 10 m/s corresponding to the jet Reynolds number Re(=U_0d/υ) of 1. 7*10^4. The pipe of finite length is positioned in line with the axis of a jet. As impinging length l is increased from zero, an acoustic tone abruptly appears and dominant frequency of the tone decreases with increasing l. With further increase in l, at a certain length a frequency jump to a higher frequency occurs, making the beginning of a new stage. The average frequency remains the same at successive stage and is the instability frequency of the shear layer stabilized by feed back from the impinging edge. Flow visualization shows that the vertex shedding from the nozzle appear to be locked in this frequency. Acoustic tone corresponding to resonant modes of the pipe are produced when the condition are such as to ensure close coincidence between the instability frequency of the shear layer and a natural frequency of the pipe. These pure tone, however, are not stimulated when a screen mesh is introduced in between the nozzle and the pipe, since the discrete vortex shedding is a almost completely damped out.