A measurement based study of the acoustics of pipe systems with flow

Abstract

The focus of this thesis is the measurement of specific aeroacoustic properties in ducts at frequencies below the cut-on frequency of the first higher order mode. A body of measurement results are presented which highlight the effect of flow on some of the aeroacoustic characteristics in ducts as well as describe the aeroacoustic sources of an in-duct orifice and a simple expansion chamber. The results have been compared with published theory where appropriate. Important developments from measurements of the acoustic characteristics of a simple duct with flow include a new experimental method to determine the viscothermal attenuation coefficient. In addition, pressure reflection coefficient measurements of an unflanged duct with flow with two different edge conditions are used in conjunction with a numerical model developed by Gabard [1] to determine the extent of vorticity shed from the duct termination. A novel method is presented for the measurement of aeroacoustic source strengths in ducts with flow. The source is defined in terms of acoustic power and is determined by measuring the acoustic power flux both upstream and downstream of the source region in a duct. The method adopts a plane wave approximation and was assessed experimentally by creating a source in a duct at a number of known frequencies and modifying its magnitude by a known amount. The source measurement technique is applied to an in-duct orifice. The results are used to determine the spectral characteristic and velocity dependence of the source. The results indicate that the duct-to-orifice area ratio has a important effect on the spectral characteristics and velocity dependence of the source. New measurements of the aeroacoustic source strength of a simple flow excited expansion chamber are presented. The results indicate that lock-on flow tones occur when hydrodynamic modes which form in the chamber match the tailpipe resonant frequencies. The results are compared with predictions of a model based on describing function theory

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ORCID for Keith R. Holland: orcid.org/0000-0001-7439-2375

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