Chapter 7: APPLICATIONS
The results of laboratory tests of the Terrill and Melville (2000) method
are shown in Figure 7-12. Bubbles with radii in the range of 60–90 Pm
contribute most to the total void fraction. This bubble size range corresponds
to a peak in the acoustic attenuation at frequencies of 36–54 kHz. Similar
numbers of bubbles exist at the smaller radii; significant differences in the
number of larger bubbles are, however, observed. The rise velocity of the
bubbles is proportional to the square of the radius; as a result, the larger
bubbles reach the surface at a significantly faster rate. The terminal rise
Pm radius is 0.0008 m s
-1
,
while the rise velocity for an 800 Pm air bubble is approximately 0.3 m s -1 ,
an increase of over 350 times. This provides a plausible explanation of the
difference for large bubbles.
7.4.5 Ambient noise produced by bubbles
The fundamental problem of identifying an underwater sound source by
its acoustical signature requires knowledge of the ambient noise spectra
under different hydrometeorological conditions. The first studies of ambient
noise were undertaken in relation to the development of underwater
acoustics after World War II, revealing the dependence of ambient noise on
the wave height and wind velocity (Knudsen et al., 1948). Since that time
many efforts have been made to develop qualitative and quantitative
descriptions of the phenomenon.
Wind is indirectly responsible for the ambient noise generation, namely
via breaking wind waves (Medwin and Beaky, 1989). The fraction of
dissipated wave energy radiated as sound waves is of the order 10
-8 (Loewen
and Melville, 1991a). Two approaches to understanding the nature of the
sound radiated by oscillating air-bubbles in a bubble cloud under breaking
waves have been formulated. Medwin and Daniel (1990) and Loewen and
Melville (1991b) assume that individual bubbles radiate sound, while Carey
and Bradley (1985) and Prosperetti (1985) focus on collective oscillations.
The bubbles with radii from 30 Pm to 1000 Pm, which are less than the
spectrum of bubbles produced by breaking waves. According to (7.7) a
bubble with a radius within this size interval must have a natural frequency
within 3-100 kHz. The available data, however, indicates that the ambient
noise produced by breaking waves has somewhat lower frequencies. The
collective oscillation mode can explain the observed shifting of the ambient
noise to lower frequencies compared to the noise that could be produced by
individual bubbles. Tkalich and Chan (2002) developed a theoretical model
for the prediction of ambient noise level due to collective oscillations of air
bubbles under breaking wind waves. The model predicts the sound intensity
469
velocityof an air bubble (see Section 6.1.2) with a 30
Hinze scale (see Section 6.1.3 for definition), are believed to dominate the
The results of laboratory tests of the Terrill and Melville (2000) method
are shown in Figure 7-12. Bubbles with radii in the range of 60–90 Pm
contribute most to the total void fraction. This bubble size range corresponds
to a peak in the acoustic attenuation at frequencies of 36–54 kHz. Similar
numbers of bubbles exist at the smaller radii; significant differences in the
number of larger bubbles are, however, observed. The rise velocity of the
bubbles is proportional to the square of the radius; as a result, the larger
bubbles reach the surface at a significantly faster rate. The terminal rise
Pm radius is 0.0008 m s
-1
,
while the rise velocity for an 800 Pm air bubble is approximately 0.3 m s -1 ,
an increase of over 350 times. This provides a plausible explanation of the
difference for large bubbles.
7.4.5 Ambient noise produced by bubbles
The fundamental problem of identifying an underwater sound source by
its acoustical signature requires knowledge of the ambient noise spectra
under different hydrometeorological conditions. The first studies of ambient
noise were undertaken in relation to the development of underwater
acoustics after World War II, revealing the dependence of ambient noise on
the wave height and wind velocity (Knudsen et al., 1948). Since that time
many efforts have been made to develop qualitative and quantitative
descriptions of the phenomenon.
Wind is indirectly responsible for the ambient noise generation, namely
via breaking wind waves (Medwin and Beaky, 1989). The fraction of
dissipated wave energy radiated as sound waves is of the order 10
-8 (Loewen
and Melville, 1991a). Two approaches to understanding the nature of the
sound radiated by oscillating air-bubbles in a bubble cloud under breaking
waves have been formulated. Medwin and Daniel (1990) and Loewen and
Melville (1991b) assume that individual bubbles radiate sound, while Carey
and Bradley (1985) and Prosperetti (1985) focus on collective oscillations.
The bubbles with radii from 30 Pm to 1000 Pm, which are less than the
spectrum of bubbles produced by breaking waves. According to (7.7) a
bubble with a radius within this size interval must have a natural frequency
within 3-100 kHz. The available data, however, indicates that the ambient
noise produced by breaking waves has somewhat lower frequencies. The
collective oscillation mode can explain the observed shifting of the ambient
noise to lower frequencies compared to the noise that could be produced by
individual bubbles. Tkalich and Chan (2002) developed a theoretical model
for the prediction of ambient noise level due to collective oscillations of air
bubbles under breaking wind waves. The model predicts the sound intensity
469
velocityof an air bubble (see Section 6.1.2) with a 30
Hinze scale (see Section 6.1.3 for definition), are believed to dominate the