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Investigation of the potential for sonar detection in bubbly environments

Investigation of the potential for sonar detection in bubbly environments
Investigation of the potential for sonar detection in bubbly environments

In order to investigate the potential for sonar enhancement, a theory for the time-dependent cross-section of a bubble using a non-linear bubble model has been developed.  The model has then been extended to allow the estimation of the acoustic cross-sections of a 1 m3 cloud, neglecting propagation effects.  A further cloud model, which takes into account propagation effects, has been developed to calculate the attenuation from a finite bubble cloud.  The model has also been adapted to allow the input of experimentally measured waveforms.

A laboratory experiment to measure the attenuation is described, and the results (using both pulses and chirped signals) are compared with theory.  The experiments were conducted in an 8 m x 8 m x 5 m deep fresh water tank.  The bubble cloud was generated by electrolysis.  The attenuation from a series of pulse lengths and chirps, with a fixed bandwidth but variable duration, was measured.  Good agreement between theory and experiment is shown but no sonar enhancement was achieved using the current experimental arrangement.  Use of the model allowed confirmation that the type of bubble populations that are usually generated in test tanks would not be expected to show such enhancements, but that enhancements might be seen in population distributions found in the ocean environment.  This is proposed and tested.  It is also an explanation for the conflicting observations in the previous studies mentioned above.

A further series of trials were conducted at a beach on the south coast of the United Kingdom.  An air filled buoy was mounted in the surf-zone and a Furgo-UDI Ltd. Transmit/receive array was used to test a variety of waveforms.  Short pulses were shown to improve target detection at some frequencies.  However a combination of short pulses and optimisation of the transmit frequency gave the best results.

Although for decades the emphasis has been on the effects of resonant bubbles, this thesis uses these experimental results to underline the importance of the contribution made by bubbles close to (but not at) resonance when considering short pulse lengths.  The response of these bubbles can easily mask the ring-up of resonant bubbles.  Furthermore this critical near-resonant bubble response is not included in the simple linear models used in previous studies mentioned above, and is the cause of the discrepancy between the experimental and theoretical results of these previous studies.

University of Southampton
Clarke, James W.L
450725fd-bf4f-4304-a20a-05fe06b88062
Clarke, James W.L
450725fd-bf4f-4304-a20a-05fe06b88062

Clarke, James W.L (2004) Investigation of the potential for sonar detection in bubbly environments. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

In order to investigate the potential for sonar enhancement, a theory for the time-dependent cross-section of a bubble using a non-linear bubble model has been developed.  The model has then been extended to allow the estimation of the acoustic cross-sections of a 1 m3 cloud, neglecting propagation effects.  A further cloud model, which takes into account propagation effects, has been developed to calculate the attenuation from a finite bubble cloud.  The model has also been adapted to allow the input of experimentally measured waveforms.

A laboratory experiment to measure the attenuation is described, and the results (using both pulses and chirped signals) are compared with theory.  The experiments were conducted in an 8 m x 8 m x 5 m deep fresh water tank.  The bubble cloud was generated by electrolysis.  The attenuation from a series of pulse lengths and chirps, with a fixed bandwidth but variable duration, was measured.  Good agreement between theory and experiment is shown but no sonar enhancement was achieved using the current experimental arrangement.  Use of the model allowed confirmation that the type of bubble populations that are usually generated in test tanks would not be expected to show such enhancements, but that enhancements might be seen in population distributions found in the ocean environment.  This is proposed and tested.  It is also an explanation for the conflicting observations in the previous studies mentioned above.

A further series of trials were conducted at a beach on the south coast of the United Kingdom.  An air filled buoy was mounted in the surf-zone and a Furgo-UDI Ltd. Transmit/receive array was used to test a variety of waveforms.  Short pulses were shown to improve target detection at some frequencies.  However a combination of short pulses and optimisation of the transmit frequency gave the best results.

Although for decades the emphasis has been on the effects of resonant bubbles, this thesis uses these experimental results to underline the importance of the contribution made by bubbles close to (but not at) resonance when considering short pulse lengths.  The response of these bubbles can easily mask the ring-up of resonant bubbles.  Furthermore this critical near-resonant bubble response is not included in the simple linear models used in previous studies mentioned above, and is the cause of the discrepancy between the experimental and theoretical results of these previous studies.

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Published date: 2004

Identifiers

Local EPrints ID: 465712
URI: http://eprints.soton.ac.uk/id/eprint/465712
PURE UUID: 11890e52-4970-411c-959b-70ab84d83d58

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Date deposited: 05 Jul 2022 02:41
Last modified: 16 Mar 2024 20:20

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Author: James W.L Clarke

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