Mechanisms of the generation of external acoustic radiation from pipes due to internal flow disturbances

Extensive spectral measurements of the internal wall pressure fluctuations, pipe wall acceleration, and external acoustic radiation due to the disturbance of a fully developed turbulent air flow in a pipe by 90° radiused bends, 45° and 90° mitred bends, and butterfly and gate valves are presented. The measurements were made at sufficient distances downstream of the disturbance for an undisturbed hydrodynamic regime to be re-established, and for the only remaining disturbance to be a superimposed internal acoustic field radiated from the disturbance. This internal acoustic field, which can be responsible for extending the influence of the disturbance throughout a piping system, comprises, in general, both plane wave and higher order acoustic modes, although its detailed character depends on the particular characteristics of the individual fittings. It gives rise to increases in pipe wall vibration and external radiation, by large amounts in the cases of the more severe disturbances, over those due to undisturbed turbulent pipe flow. The background theory of pipe wall response and radiation which provides the framework for interpretation of the experimental results and identification of sound generation mechanisms is also presented. At frequencies at which only plane acoustic waves can propagate in the pipe, both forced peristaltic motion and resonant modes of the pipe wall contribute to the acceleration response of the wall. For a circular pipe with uniform wall thickness, theory predicts no coupling between plane waves and resonant wall modes of circumferential order m>0; but departures from the ideal state result in excitation of apparently all pipe modes with resonance frequencies in this range. High subsonic and supersonic structural modes and the peristaltic motion (which, because of augmentation of acoustic velocity by the flow, is supersonic with respect to the external fluid) all have high radiation ratios and make the dominant contributions to the external sound radiation. Increases in pipe wall response and external radiation are largest at frequencies at which higher order acoustic modes can propagate, in which case coincidence effects lead to strong excitation of supersonic pipe modes. A previous conclusion that coincidence occurs at a frequency close to the cut-off frequency of the acoustic mode involved is confirmed. The results show the effect of the combination of the set of discrete resonance frequencies of the pipe wall and the influence of the flow on the wavenumber-frequency relation for the acoustic modes in producing four different principal wavenumber coincidences for each acoustic mode, in agreement with theoretical predictions.