Chapter 6: HIGH WIND SPEED REGIME
Breaking waves entrain air and create bubble plumes, which are highly
transient and localized phenomena. Based on laboratory experiments of
Leifer and De Leeuw (2001), the lifetime of wave-generated bubbles can be
Senescence. The first phase is observed to occur during the first 0.1 s or less.
During the second phase, the plume rapidly descends, initially at roughly a
30
o angle, then tilting towards the vertical. The second (injection) phase ends
at the maximum penetration depth; it is followed by the third phase, when
the mass of bubbles rises towards the surface. The Injection and rise phases
last roughly the same time. Once the bubble creation process ceases, a newly
formed bubble plume becomes acoustically quiescent and evolves under the
influence of turbulent diffusion, advection, buoyant degassing and
dissolution. This is the fourth, senescence, phase, which corresponds to
typical bubble observations (that is the background distribution). During the
senescence phase the plume consists of the smaller, mostly r < 0.2 mm
bubbles.
Size-dependent bubble rise velocity mainly determines the residence time
of larger bubbles, but the residence time of smaller bubbles may also be
affected by the turbulent flow in the near-surface layer. Bubble plumes
consisting of small bubbles can extend well into the mixed layer. As a result,
the smaller bubbles act as tracers and map out the surface signatures of
Langmuir cells (Farmer and Li, 1995), or the edges of current fronts and rips
(Marmorino and Trump, 1996).
6.1.2 Bubble rise velocity
Bubble hydrodynamics depend on bubble size, temperature, and the
presence of surfactants (Leifer et al., 2000a). Depending on size, the bubble
form varies from spherical to spheroid for small bubbles (r < 0.5 mm) to
ellipsoid for larger bubbles (r > 0.5 mm). The radius of transition between
the spheroidal and ellipsoidal form depends on the temperature and on the
presence of surfactants. Small bubbles are nearly perfect spheres because
surface tension dominates over the drag stress that acts upon the rising
bubble. The surface tension force, however, decreases inversely proportional
to the bubble radius. At the same time, the drag force increases since larger
bubbles rise faster and have larger effective cross sectional area. Bubbles
with radius larger than approximately 0.7 mm (at T = 20
o C) can oscillate, in
both path and shape, affecting the rise velocity. The trajectory oscillations
(zig-zag or helical) are important for bubbles that just start oscillating and
reduce their speed. For large bubbles (r > 3.5 mm) the shape or deformation
oscillations are more important. The latter results in the reduction of the drag
coefficient and thus an increase of rise speed.
397
divided into four phases: 1) Formation; 2) Injection, 3) Rise, and 4)
Breaking waves entrain air and create bubble plumes, which are highly
transient and localized phenomena. Based on laboratory experiments of
Leifer and De Leeuw (2001), the lifetime of wave-generated bubbles can be
Senescence. The first phase is observed to occur during the first 0.1 s or less.
During the second phase, the plume rapidly descends, initially at roughly a
30
o angle, then tilting towards the vertical. The second (injection) phase ends
at the maximum penetration depth; it is followed by the third phase, when
the mass of bubbles rises towards the surface. The Injection and rise phases
last roughly the same time. Once the bubble creation process ceases, a newly
formed bubble plume becomes acoustically quiescent and evolves under the
influence of turbulent diffusion, advection, buoyant degassing and
dissolution. This is the fourth, senescence, phase, which corresponds to
typical bubble observations (that is the background distribution). During the
senescence phase the plume consists of the smaller, mostly r < 0.2 mm
bubbles.
Size-dependent bubble rise velocity mainly determines the residence time
of larger bubbles, but the residence time of smaller bubbles may also be
affected by the turbulent flow in the near-surface layer. Bubble plumes
consisting of small bubbles can extend well into the mixed layer. As a result,
the smaller bubbles act as tracers and map out the surface signatures of
Langmuir cells (Farmer and Li, 1995), or the edges of current fronts and rips
(Marmorino and Trump, 1996).
6.1.2 Bubble rise velocity
Bubble hydrodynamics depend on bubble size, temperature, and the
presence of surfactants (Leifer et al., 2000a). Depending on size, the bubble
form varies from spherical to spheroid for small bubbles (r < 0.5 mm) to
ellipsoid for larger bubbles (r > 0.5 mm). The radius of transition between
the spheroidal and ellipsoidal form depends on the temperature and on the
presence of surfactants. Small bubbles are nearly perfect spheres because
surface tension dominates over the drag stress that acts upon the rising
bubble. The surface tension force, however, decreases inversely proportional
to the bubble radius. At the same time, the drag force increases since larger
bubbles rise faster and have larger effective cross sectional area. Bubbles
with radius larger than approximately 0.7 mm (at T = 20
o C) can oscillate, in
both path and shape, affecting the rise velocity. The trajectory oscillations
(zig-zag or helical) are important for bubbles that just start oscillating and
reduce their speed. For large bubbles (r > 3.5 mm) the shape or deformation
oscillations are more important. The latter results in the reduction of the drag
coefficient and thus an increase of rise speed.
397
divided into four phases: 1) Formation; 2) Injection, 3) Rise, and 4)