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The bubble dynamics and pressure field generated by a seismic airgun


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de Graff, KL 2013 , 'The bubble dynamics and pressure field generated by a seismic airgun', PhD thesis, University of Tasmania.

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The bubble dynamics of a seismic airgun are investigated in relation to their use for
shock testing naval vessels. A model-scale airgun is used to investigate the dynamics of
a bubble generated at different standoffs from a steel plate and a free surface in an open
top tank. The airgun is fired at 50 and 100 bar initial pressures. Field pressure, wall
pressure and acceleration are measured, and the bubble is imaged using low- and highspeed
photography. The behaviour of the bubble is described and the bubble growth
is determined from shadowgraph photography and compared with the simultaneous
pressure field signal. Four distinct bubbles are evident at the first bubble maximum
as a result of the initial four jets of air. The Rayleigh–Taylor Instability is identified
as playing a role in the bubble break up. The bubble and reverberant frequencies are
identified using Wavelet and Fast Fourier transforms. The interaction of the bubble
with the free surface is similar compared with other bubbles; however, the bubble is
unaffected by the nearby wall for the standoffs tested. Only the initial shock generated
by the airgun significantly impacts the wall, which moves in phase with the bubble
pulsations. As a result of the movement, little pressure is felt at the wall from the
pressure pulses generated by the bubble collapses. Despite the four port arrangement,
the pressure signature of this airgun is not directional.
An analytical model of the bubble dynamics is developed based on the Gilmore equation.
Additional terms for modelling the presence of the airgun body, mass throttling, effective
viscosity and heat diffusion are included and Gilmore’s model for the radiated pressure
wave is used to predict the pressure field generated by the bubble. The results of this
model compare favourably with the model-scale airgun data, with the exception of the
initial shock and first maximum bubble velocity, which are over-predicted. Full-scale
airgun pressure field measurements are predicted well by the model.
Smoothed Particle Hydrodynamics is investigated as a numerical method for modelling
a pulsing seismic airgun bubble. Limitations are found in the available computational
time and power, and in the ability to accurately model the gas-water interface. Some
improvements are identified to enable modelling the bubble pulsation due to the pressure
differential across the bubble surface.

Item Type: Thesis - PhD
Authors/Creators:de Graff, KL
Keywords: seismic airgun, bubble dynamics, pressure filed, Rayleigh-Plesset equation, Gilmore equation, smoothed particle hydrodynamics, underwater explosion
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