Title:
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An investigation into the use of aeroacoustic jet noise theory in the interpretation of volcanic infrasound signals
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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.
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