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Title: Acoustic sources in motion
Author: Dowling, Ann Patricia
ISNI:       0000 0001 2411 633X
Awarding Body: University of Cambridge
Current Institution: University of Cambridge
Date of Award: 1978
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The motion of an acoustic source changes its sound field. Three fundamental topics concerned with radiation from moving sources are investigated in this thesis. In the first we consider the effect of motion on the sound field of two of the simplest realistic sources, a pulsating and a vibrating body. These two problems are often misrepresented as a moving monopole and dipole respectively. But it is found that for real sources that motion introduces additional coupled multipoles whose combined effect generates previously unexpected features. In general the change in the sound field due to source motion is greater than the effect estimated from previous mathematical models. Moreover, and quite unexpectedly, we have found that the radiation is altered in a direction perpendicular to the flight path. Our second problem concerns the production of sound by a jet stream. We derive a generalization of Lighthill's acoustic analogy to account for the interaction of the acoustic field with the mean jet flow. We prove that the jet noise problem can be modelled exactly by convected quadrupoles near a vortex sheet. Each moving fluid particle supports a quadrupole whose strength is given by Lighthill's stress tensor and the sound radiates as if it were adjacent to an instability-free vortex sheet. Although we show that the sound field may be expressed in terms of the turbulence stress tensor, sound is also generated by the flow's instability waves as they grow into turbulence, and this sound appears as an exponentially growing precursor to the main field. Some well known features of the mean flow acoustic interaction issue are an immediate consequence of the theory. We examine the case of a round jet in some detail and concentrate on a new aspect. When the jet density is much lower than that of its environment, the mean flow acoustic interaction results in a considerable amplification of the quadrupole field and the intensity of its sound can scale on an unusually low power of the jet speed. We show that a fourth power law is possible and even a second power law when the density difference is large enough. This may be part of the "excess noise" problem in which the sound of engine produced hot jets is often insensitive to changes in jet speed at low exhaust power. A modern rotary printing press, is a noisy machine. One of the noise sources is the vibration of the paper web as it moves under variable tension between the rollers. Our third problem models this process. We consider a semi-infinite elastic sheet initially at rest with a prescribed displacement. The sound produced by suddenly tugging one end is investigated. It is found that a tension wave travels supersonically through the sheet. There is no motion ahead of this wave but behind it the tensioned sheet supports a vibration. A membrane excited in this way is silent except at the tugged end and at the tension front. The sound field has all the characteristics of a moving line source. The parameters that control the noise output are identified and the dependence of the sound field on these variables is determined.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
Keywords: Motion ; Sound