High intensity sound propagation in flow ducts
A theoretical and experimental study of the propagation of finite amplitude pressure waves, or high intensity sound, in flow ducts was undertaken. A quantitative appreciation is presented describing the propagation of such waves including reflections. Observations showed that time domain descriptions are appropriate for the analysis of such waves, for which the well established method of characteristics proved ideal, providing that both the particle velocity and pressure time histories were established simultaneously. Existing techniques for predicting wave propagation in ducts based on the method of characteristics were reviewed and found to be inefficient for noise emission predictions when the velocity time history was not known. The approach described here gave reliable estimates of the velocity-time history at a reference plane in the duct using a pair of pressure-time histories obtained from closely spaced transducers flush mounted in the duct wall. The essential precautions and conditions required for the measurement procedures necessary to maintain confidence in the estimates of the velocity have been identified. Estimates of velocity and pressure time histories at two positions downstream and one upstream of the reference plane were obtained and found to compare well with observations. The appropriate boundary conditions for describing wave reflections at an open duct termination have been established and evaluated for a systematic set of practical excitations.