Exploiting Cooling Whitecap Foam to Quantify Wave Breaking Dissipation
Abstract
Wave breaking plays an important role in transfer of mass, momentum, and energy between the atmosphere and the ocean. The energy transferred from the atmosphere to the ocean through wind-wave generation is ultimately dissipated by wave breaking. Large-scale breaking waves entrain air and generate bubbles. After a breaking event, a bubbly turbulent wake is left behind where the bubbles eventually rise to the surface and produce patches of residual foam. Previous studies have suggested that the visible decay time for the bubble plume of a breaker is correlated with the energy dissipation and can provide a measure of energy dissipation. In clean water, the plume decay time also correlates with the visible foam decay time for a breaker; therefore, it is possible to quantify the energy dissipation by measuring the decay time of visible foam for individual breaking waves. But, the relationship between plume and foam decay times becomes more complicated when surfactants are present. Thus, inferring information about energy dissipation using visible imagery techniques requires a method to account for the effects of surfactants on stabilizing the foam and prolonging the foam decay time.
In this study, we present a novel approach for inferring energy dissipation using infrared imagery of the cooling residual foam. It has been observed that after a breaking event, the residual foam rapidly cools due to the enhanced evaporative cooling of the upper layer of the foam bubbles. Preliminary experiments suggest that the onset of cooling is not affected by surfactants and the residual foam starts to cool only after the bubble plume has decayed and no more foam is generated. Here, we present preliminary results from extensive laboratory experiments on the effect of surfactant on the onset of cooling of the residual foam produced by breaking waves. The experiments are performed in a wave tank that is 12 m long and 0.9 m wide, with a water depth of 0.75 m. Breaking waves in the range from spilling to plunging are systematically generated using a focusing wave packet technique. Experiments are performed in both fresh water and salt water, and with varying concentration of surfactants. Visible and infrared cameras are used to measure the behavior of the bubble plume and the surface foam.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMEP54B..28M
- Keywords:
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- 4217 Coastal processes;
- OCEANOGRAPHY: GENERALDE: 4235 Estuarine processes;
- OCEANOGRAPHY: GENERALDE: 4275 Remote sensing and electromagnetic processes;
- OCEANOGRAPHY: GENERALDE: 4546 Nearshore processes;
- OCEANOGRAPHY: PHYSICAL