Nonlinear gas oscillations in pipes. Part 2. Experiment

Forced nonlinear acoustic oscillations near the resonant frequency of closed and open tubes are studied experimentally. In particular, the motion in tubes terminated with different orifice plates is studied, and comparison is made with secondand third-order theories of the motion which contain an adjustable endwall reflexion coefficient. It is found that oscillations at resonance in an open tube exhibit remarkably large amplitudes despite the fact that in some cases shock waves are emitted from the open end. For oscillations at resonance in a closed tube, the effect of substituting an orifice plate for the solid end wall is to reduce the amplitude and thicken the compressive portion of the shock waves which occur under these conditions. In both the open-tube and closed-tube experiments the reflexion coefficients which are evaluated by fitting theory to experiment are found to increase with increasing amplitude, in agreement with the observations of previous investigators (Ingard & Ising 1967). In fact, for the open end the same linear dependence upon amplitude is observed, but the constant of proportionality is different. Qualitative differences are observed between the reflexion coefficients of a given orifice at the open-end and the closed-end resonant frequencies; at the open-end frequency the reflexion from the given orifice is less ideal than at the closed-end frequency. The implications of reflexion coefficients dependent on the wave forms are discussed.