bubbles (r > 1 mm), whose mean shape is elliptical and is subject to
oscillations. The optical methods based on the analysis of video or photo
images are able to measure at high bubble concentrations and over a wide
range of bubble sizes. The optical methods are, however, invasive in general
and may disturb the measurement area.
The initial bubble formation, breakup, coalescence, dissolution, vertical
motion caused by buoyancy forces, and turbulent mixing are the processes
that determine the bubble size distribution in the ocean. Air is initially
entrained into relatively large bubbles as a wave breaks. These bubbles
rapidly break into smaller bubbles. During this active phase the bubble
fragmentation process determines the bubble size distribution. Breaking
waves directly inject air into the wave-stirred layer (see Benilov’s
classification of the upper ocean turbulence given in Figure 3.1). As soon as
the wave breaker spends its energy, the air entrainment ceases and the
bubble creation process stops. The bubble size distribution then evolves
rapidly because larger bubbles leave the area and surface more quickly than
smaller ones.
The bubbles with radius r >0.7 mm have rise velocities of the order of
0.2 m s
-1 - 0.4 m s
-1 (Figure 6-2); after the end of the wave breaking process
the remaining turbulence has little effect on their dynamics. The large
bubbles immediately return to the surface, rarely penetrating below the
wave-stirred layer. Note that the wave-stirred layer depth is typically less
than one significant wave height (see Chapter 3, Figure 3.20). Smaller
bubbles (r ~ 0.1 mm) have rise velocities of the order of 0.01 m s
-1 . The
background turbulence as well as organized structures can transport these
small bubbles to greater depths (see Chapter 5).
The bubble size spectra can be separated into the regions within the
wave-stirred layer and below the wave-stirred layer. The spectra within the
wave-stirred layer can additionally be separated into those taken during
wave-breaking events and those in between wave-breaking events.
Figure 6-3 shows averaged bubble spectra measured at 0.6 m depth
including and excluding wave-breaking events. These observations are
presumably in the wave-stirred layer. The density of bubbles, especially of
large bubbles, is significantly higher during wave-breaking events. In this
example, the slope of the spectrum for large bubbles changes in logarithmic
coordinates from approximately -2.9 during wave-breaking events to
approximately -4.3 between the wave breaking events.
Two mechanisms for large bubble fragmentation are possible in the
wave breaker: 1) Turbulent fragmentation represented by the turbulent
kinetic energy dissipation rate H; 2) Rising-bubble fragmentation,
characterized by the rising velocity w b and the collective behavior of rising
bubbles.
Chapter 6: HIGH WIND SPEED REGIME
401
oscillations. The optical methods based on the analysis of video or photo
images are able to measure at high bubble concentrations and over a wide
range of bubble sizes. The optical methods are, however, invasive in general
and may disturb the measurement area.
The initial bubble formation, breakup, coalescence, dissolution, vertical
motion caused by buoyancy forces, and turbulent mixing are the processes
that determine the bubble size distribution in the ocean. Air is initially
entrained into relatively large bubbles as a wave breaks. These bubbles
rapidly break into smaller bubbles. During this active phase the bubble
fragmentation process determines the bubble size distribution. Breaking
waves directly inject air into the wave-stirred layer (see Benilov’s
classification of the upper ocean turbulence given in Figure 3.1). As soon as
the wave breaker spends its energy, the air entrainment ceases and the
bubble creation process stops. The bubble size distribution then evolves
rapidly because larger bubbles leave the area and surface more quickly than
smaller ones.
The bubbles with radius r >0.7 mm have rise velocities of the order of
0.2 m s
-1 - 0.4 m s
-1 (Figure 6-2); after the end of the wave breaking process
the remaining turbulence has little effect on their dynamics. The large
bubbles immediately return to the surface, rarely penetrating below the
wave-stirred layer. Note that the wave-stirred layer depth is typically less
than one significant wave height (see Chapter 3, Figure 3.20). Smaller
bubbles (r ~ 0.1 mm) have rise velocities of the order of 0.01 m s
-1 . The
background turbulence as well as organized structures can transport these
small bubbles to greater depths (see Chapter 5).
The bubble size spectra can be separated into the regions within the
wave-stirred layer and below the wave-stirred layer. The spectra within the
wave-stirred layer can additionally be separated into those taken during
wave-breaking events and those in between wave-breaking events.
Figure 6-3 shows averaged bubble spectra measured at 0.6 m depth
including and excluding wave-breaking events. These observations are
presumably in the wave-stirred layer. The density of bubbles, especially of
large bubbles, is significantly higher during wave-breaking events. In this
example, the slope of the spectrum for large bubbles changes in logarithmic
coordinates from approximately -2.9 during wave-breaking events to
approximately -4.3 between the wave breaking events.
Two mechanisms for large bubble fragmentation are possible in the
wave breaker: 1) Turbulent fragmentation represented by the turbulent
kinetic energy dissipation rate H; 2) Rising-bubble fragmentation,
characterized by the rising velocity w b and the collective behavior of rising
bubbles.
Chapter 6: HIGH WIND SPEED REGIME
401