The Acoustal and Physical Characterization of Bubble Plumes

IMT Lab investigators conducted studies of the the role of bubble production and distribution within high air void fraction whitecaps and sub-surface bubble plumes on acoustical scattering at the ocean surface and the generation of noise by bubbles. An additional research focus was to relate bubble production and plume properties such as penetration depth and acoustical scattering strength to the state of the ocean surface wave field.

Experiment

  • Relate the acoustical scattering from high void fraction bubble plumes to measured plume properties.
  • Determine the relationship between specific, measured plume properties, such as penetration depth, plume volume, plume void fraction and bubble size distributions to surface wave conditions.
  • Relate bubble production within plumes to the generation of ambient noise.

Two different strategies were undertaken to achieve successful results in this research project. The first was to conduct a series of wave tank studies at the Scripps Institution of Oceanography’s hydraulic laboratory glass flume facility. The laboratory experiments were designed to study the basic physics of air entrainment and bubble production within plunging breakers.

The second strategy was to conduct open-ocean measurements of whitecaps in the Woods Hole Oceanographic Institution Underwater Acoustics Observatory of Martha’s Vineyard during October and November of 2002. The experiment (called Surface Processes and Acoustic Communications 2002, or SPACE02) was hosted by Dr. James Preisig at WHOI. Dr. David Farmer from the University of Rhode Island and Dr. Svein Vagle from the Institute of Ocean Sciences, British Colombia also participated.

Underwater acoustic sources and hydrophone arrays were deployed to study surface, bottom and volume acoustic channel properties simultaneously with our whitecap measurements. During the experiment surface processes, but at length and time scales an order of magnitude longer than our measurements. Our whitecap measurements taken together with the extensive marine boundary layer characterizations of Farmer and Vagle provided a complete surface characterization.

The core sensor system for the whitecap studies was the Advanced Plume Imaging System (APEX), developed and funded by a DURIP award through ONR. APEX was deployed with acoustic wave field meters and a blimp-mounted camera to characterize the 2D surface wave spectrum and the distribution and rates of wave breaking.

APEX, a multi-sensor package designed to probe the structure of dense bubble plumes directly beneath ocean whitecaps on several simultaneous scales, consists of two surface-following platforms deployable in high sea states to measure internal and macroscopic plume properties. The instruments on both frames include an optical bubble counter, an array of conductivity sensors, a conductivity/temperature sensor, an acoustic Doppler velocity profiler, an acoustic system for measuring plume scattering cross-section, and an underwater video camera. These sensors simultaneously measure bubble size distribution and void fraction of air within whitecap plumes, the size of the plumes and scaling of plume size with sea state, the acoustical roughness scales of the plume boundaries and the noise radiated during plume formation.

Using the data from the laboratory wave-tank experiments and our these prior open-ocean whitecap studies, we have made a fundamental advance in our understanding of air entrainment and bubble formation in breaking waves. We found that there are two distinct mechanisms controlling the size distribution of bubbles, depending on bubble size. Turbulent fragmentation determines the size distribution for bubbles larger than about 1 mm. radius, resulting in a bubble density proportional to bubble radius to the power of –10/3. Smaller bubbles are created by jet and drop impact on the wave face with a –3/2 power scaling law. The length scale that separates these processes is the scale at which the forces of turbulent fragmentation are balanced by bubble surface tension, also known as the Hinze scale.

Three important quantative findings supported our findings:

  1. The transition between bubbles stabilized by surface tension and bubbles subject to fragmentation by turbulence results in a change in the power law scaling of bubble size from –3/2 to –10/3. The transition between the scaling laws occurs at bubble radius equal to the Hinze scale, which varies from about 0.5 mm. to 1.5 mm. for open-ocean whitecaps. We observed evidence of the Hinze scale in bubble size spectra taken from breaking waves in the laboratory, the open-ocean, and the surf zone, implying a common mechanism that operates within waves occurring in these three very different environments. In addition, the turbulent dissipation rate within the laboratory waves implied by the observed Hinze scale is consistent with measurements of the dissipation rate within breaking wave crests made by Melville and his co-workers.
  2. We observed a significant result in bubbles in the process of fragmenting. We were able to estimate the Webber number, Reynolds number and probability density of bubble eccentricity for a number of individual fragmenting bubbles within a breaking wave crest. Theoretical predictions of these variables based on prior work in the literature are in good agreement with our observations, which supports the idea that the population of bubbles larger than the Hinze scale is controlled by turbulent fragmentation.
  3. Our results were in excellent agreement between the bubble size distribution scaling law predicted by Garrett, Li and Farmer (radius to the power of –10/3) for bubbles fragmented by turbulence and our observed distributions.

There are a number of significant impacts arising from the bubble creation work and our discovery of simple scaling laws for bubble number density separated by a length scale that depends only on the turbulent dissipation rate. One of them is an opportunity to understand and model the fundamental physical processes leading to the –5/3 power law scaling of the wind-driven, oceanic ambient noise level with frequency. This property of oceanic noise, known for over 50 years as the Knudsen spectrum, is not well understood.

Since the noise radiated by whitecaps is driven by bubble creation rates, our new understanding of bubble formation processes in whitecaps will provide new insight into the origin of the Knudsen spectrum. The results also have important application to modeling the bubble-mediated transfer of greenhouse gases and aerosol production, both important for global climate change.

Figures

Figure 1. Logarithmic timeline of bubble plume evolution.

Figure 2. Three high-speed video images of a breaking wave crest taken during the acoustically active phase of the wave crest.

Figure 3. Spectrogram of wave noise calculated from an average of 17 breaking events.

Figure 4. The average bubble size spectrum estimated from 14 breaking events during their acoustic phase.

Figure 5. Some bubble fragmentation metrics.

Figure 6. Oceanic bubble size distributions observed 30 cm below whitecaps during the plume quiescent phase.

Figures

Figure 1. Logarithmic timeline of bubble plume evolution.

Figure 2. Three high-speed video images of a breaking wave crest taken during the acoustically active phase of the wave crest.

Figure 3. Spectrogram of wave noise calculated from an average of 17 breaking events.

Figure 4. The average bubble size spectrum estimated from 14 breaking events during their acoustic phase.

Figure 5. Some bubble fragmentation metrics.

Figure 6. Oceanic bubble size distributions observed 30 cm below whitecaps during the plume quiescent phase.

Publications

Scale dependence of bubble creation mechanisms in breaking waves

Deane, GB, Stokes MD.  2002.  Scale dependence of bubble creation mechanisms in breaking waves. Nature. 418:839-844.

Date Published: Aug 2002

Abstract: Breaking ocean waves entrain air bubbles that enhance air-sea gas flux, produce aerosols, generate ambient noise and scavenge biological surfactants. The size distribution of the entrained bubbles is the most important factor in controlling these processes, but little is known about bubble properties and formation mechanisms inside whitecaps. We have measured bubble size distributions inside breaking waves in the laboratory and in the open ocean, and provide a quantitative description of bubble formation mechanisms in the laboratory. We find two distinct mechanisms controlling the size distribution, depending on bubble size. For bubbles larger than about 1 mm, turbulent fragmentation determines bubble size distribution, resulting in a bubble density proportional to the bubble radius to the power of -10/3. Smaller bubbles are created by jet and drop impact on the wave face, with a -3/2 power-law scaling. The length scale separating these processes is the scale where turbulent fragmentation ceases, also known as the Hinze scale. Our results will have important implications for the study of air-sea gas transfer.

A robust single-cable sensor array for oceanographic use

Date Published: July 2002

Abstract: Simple, inexpensive and easy to deploy, pressure and temperature arrays have been constructed and tested in the near shore. A method has been devised whereby low data-rate sensors can be powered and send data over a two-conductor cable using time division multiplexing. Using this technology it is possible to rapidly deploy large numbers of sensors in environments that are impractical to instrument with individually cabled or autonomous sensors. Pressure and temperature data collected using two arrays in the surf zone are shown to illustrate the practicality of the deployment method and feasibility of the technology.

We would like to give thanks to:

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