Bubbles in the open ocean play a important role in a number of small-scale, upper-ocean physical processes. Physical effects affected by the presence of bubbles include enhancement of air-sea gas transfer of greenhouse gasses at high wind speeds, marine aerosol production, optical scattering, and the generation of underwater ambient noise. Also, the process of air entrainment by laminar and turbulent jets is an important topic in the field of chemical engineering, and has been the subject of numerous studies.

Bubbles are known to be created in the upper ocean through a variety of mechanisms, such as the action of breaking gravity and capillary waves, rain drop impact on the ocean surface, and melting snow. However, experiments have shown that under moderate wind conditions, most bubbles near the ocean surface are caused by breaking waves. The study of air entrainment mechanisms has an important role in small-scale, upper ocean physics and is currently an active area of research.

The bubbles entrained by breaking waves are initially packed into plumes which extend beneath and behind the wave crest. Once created, a bubble plume begins to dissipate through the processes of dissolution, diffusion and degassing. Large variations in the temporal and spatial distributions of the bubbles occur during this cycle. Void fractions of air in newly created bubble plumes can exceed 50% but the background void fraction between plumes may be many orders of magnitude smaller.

The time-scales inherent to the different phases of a bubble plume's life cycle also vary greatly, from 100's of milliseconds during bubble creation to 100's of seconds for bubble dissolution and degassing. Because of the wide variation in void fractions and time scales associated with the various plume phases, the processes occuring during different phases tend to be treated as separate problems.

 

RESEARCH OBJECTIVE

The IMT Laboratory research of small-scale air entrainment is based upon the idea that taking a rapid sequence of pictures of the small-scale structure of the air-water mixture in breaking waves will lead to an understanding of the bubble formation mechanisms which occur. The image sequence will allow the dynamical processes leading to bubble formation to be visualized. It is anticipated that images formed with millimeter length-scale resolution and millisecond time scales will have sufficient spatial and temporal resolution to visualize the formation processes generating bubbles larger than a millimeter.

A high-speed (1000 frames/second) optical imaging device (BubbleCam) was deployed to examine the air-water mixture formed within the intererior of breaking waves on sub-millimeter length scales and millisecond time scales.

 

A schematic of the bubble imaging instrument. The diagram is approximately to scale and the diameter of the pressure housing is 11.4 cm.

 

The physics underlying the operation of BubbleCam is the large-angle scattering of light by a bubble. The instrument functions by creating a sheet of light, approximately 3.5 mm thick and 40 mm wide, a few millimeters in front of an optical face plate. The optical face plate is held perpendicular to the direction of the wave motion and waves pass by the face plate from left to right. The light sheet is flashed on every third video frame, resulting in a 90 millisecond time interval between successive images. The electronic shutter speed of the camera is set to 1/1000 second, which is sufficient to freeze the motion of the bubbles. The images span 4 cm and resolve features on a spatial scale of around 1mm.

Bubbles suspended in water within the beam scatter light out of the sheet and through the optical plate. Bubbles larger than a millimeter or so in radius appear as bright rings when viewed through the face plate. Bubbles smaller than this are imaged as bright dots.

The BubbleCam's optical system and digital control circuitry is documented in the Technology section.

FIELD DEPLOYMENT TESTS

Field deployments utilizing Bubblecam were conducted in the surf zone off La Jolla Shores beach in San Diego, California. BubbleCam, an underwater video camera, a pressure sensor and a hydrophone were deployed under breaking waves in the surf zone near Scripps Pier in about 20 cm of water at low tide. A surface video recorder and a meteorlogical station were mounted on the pier to record the prevailing environmental conditions.

The surf zone frame was placed near Scripps Pier in about 20 cm of water at low tide. The bubble formation measurements were gathered while waves broke over the frame. The instrumentation was mounted between the incident waves and the frame to minimize the effect of the frame on the measurements. A broad-band hydrophone was mounted near the bubble imaging instrument to make simultaneous acoustic measurements.

Since air entrainment is accompanied by bursts of sound, the acoustic measurements allowed the time of greatest bubble formation activity to be identified. An underwater video camera was also deployed to provide semi-quantitative information on the sizes and locations of plumes entrained by the breaking waves, and a large-scale view of activity around the bubble imaging instrument and hydrophone. A pressure sensor to provide wave height data will be mounted on the surf frame approximately 100 cm below the mean water surface.

ANALYSIS OF IMAGES FROM BUBBLECAM

Two methods for interpreting the images were conducted:

Using the above techniques provided first estimates of bubble-size distributions within the high-void fraction interior of waves in the surf zone and open ocean were made during storm events. Evidence showed that the bubble size spectra generated by both open ocean whitecaps and inshore shoaling waves are similar. This suggests using the surf zone as a more convenient natural laboratory for studying some air-sea gas exchange processes rather than in the open ocean where breaking events are statistically rare. The IMT Laboratory has shown that bubble size distributions measured in previous laboratory flume studies underestimate the numbers of small bubbles (<300µm radius) by at least an order of magnitude. The new bubble size distribution estimates, when incorporated into models of soluble air-sea gas exchange, suggest CO2 flux up to 4 times greater than previous estimates, increasing with wind speed.

In addition, the scale dependency of bubble creation mechanisms in breaking waves was elucidated. The creation of bubbles larger than the Hinze scale within a whitecap is controlled by a cascade turbulent fragmentation events. Fluid turbulence acts to cleave larger bubbles into smaller ones until the stabilizing forces of surface tension balance the turbulent pressure fluctuations. These results have provided a new insight into the transient flow phenomena controlling bubble production within open ocean whitecaps