  The pressure wave emitted from the air gun contains many frequencies, among
which the low-frequency waves are desirable for exploration and imaging, while
the high-frequency waves need to be suppressed as they are harmful to marine
species. The high-frequency waves originate from the fast oscillations of the
flow during the release of the air, such as the impingement of the gas jet into
the liquid, the expansion of the air gun bubble, and the interaction between
the air gun body and the bubble. However, those dynamic and the emitted waves
are adjustable by the special design of the air guns. To analyze the underlying
relations, we present a numerical study with a compressible air gun bubble
model using the volume of fluid (VOF) approach combined with the finite volume
method (FVM) implemented in STAR-CCM+. The venting process of an air gun is
investigated to reveal the influence of the air gun body. The results show that
air gun pressure for the far field is mainly proportional to the expansion
acceleration of the whole gas. Our results also indicate that the opening and
chamber shape of the air gun affects the gas expansion acceleration, which
influences the first peak of the pressure wave significantly. The larger the
opening is, the faster the gas is released, the greater the amplitude of the
first peak is. The larger the chamber length/diameter ratio, the slower the gas
is released and the lower the amplitude of the first peak.
