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Title: An investigation into the use of aeroacoustic jet noise theory in the interpretation of volcanic infrasound signals
Author: Swanson, Elizabeth
ISNI:       0000 0004 5923 8822
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 2015
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Researchers in volcanic infrasound have proposed links between the acoustic signals of industrial jets and those recorded during sustained plume-generating volcanic eruptions (Matoza et aI., 2009; Fee et aI., 201Ob). If valid, this link offers a scaling law between the peak frequency of the volcanic signal and the exit velocity, a key parameter in determining the behaviour of eruption columns and predicting the dispersal of volcanic products. This PhD explores the validity and practicality of applying the engineering aeroacoustic results to volcanology through laboratory experiments with a range of nozzle geometries and a field case study. PIV analysis of a series of laboratory jet experiments demonstrated significant differences between the near to intermediate fields (NIP) of jets exiting from convergent nozzles, such as those commonly reported in the engineering literature, and those exiting from straight and divergent nozzles. As the NIP is the main region for sound generation, changes in its flow have implications for jet noise generation. Spectral analysis and adaptive beamforming of the acoustic signals of these jet flows showed them to be dominated by internal noise sources. Though the identification of differing noise sources was achieved, retrieval of the jet operating conditions via the standard empirical spectra was not possible. These results have important implications for the interpretation of infrasonic signals from sustained volcanic eruptions. The complex non-convergent geometry and large diameter of volcanic vents mean that high levels of internal volcanic noise are expected in infrasound from sustained explosive volcanic eruptions; As a substantial component of infrasound generated by a sustained eruption might not be from the plume itself, it is important to separate generation regions of different sources before inferring source parameters from infrasound data. Such separation requires a good understanding of the propagation path. Results from a multi-array field study at Sakurajima volcano, Japan, demonstrated the influence of local topography on the recorded infrasound signals. These influences require robust modelling before the vertical location of volcanic infrasound signals can be identified. This thesis serves as a reminder of the infancy of the infrasonic monitoring in terms of sustained events and identifies key goals for future laboratory experiments.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available