A Parametric Study of the Link between Energy Dissipation and Bubble Creation in Laboratory Breaking Waves

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ABSTRACTOCE-0450974The objective of this research is to study the link between energy dissipation and bubble creation in laboratory breaking waves. Energy dissipation by breaking waves is key to a number of important upper-ocean processes, including momentum transfer and bubble-mediated air-sea gas transport. This study builds on the results of an earlier NSF grant to identify bubble creation mechanisms within breaking wave crests. The earlier study identified the scale dependence of bubble creation processes within plunging breakers: bubbles larger than a length scale determined by the ratio of the fragmenting forces of turbulent pressure fluctuations and the stabilizing force of surface tension (the Hinze scale) are subject to a cascade of fragmentation events. Bubbles smaller than the Hinze scale are stabilized against turbulent fragmentation. The two main questions to be addressed in this study are: 1) how does the turbulent energy dissipated in the breaking wave crest vary with variations in wave energy and wave slope and 2) does the Hinze scale vary with intrinsic wave energy dissipation as predicted? The research plan consists of measuring of energy dissipation within breaking wave crests for various wave energies and slopes, and estimating the Hinze scale for turbulent bubble fragmentation as a function of energy dissipation. Estimates of energy dissipation will be obtained by studying breaking wave packets in a glass-walled flume for a range of wave energies and slopes, varied by changing the spectral composition of the packets. The energy dissipation rate within the breaking crest region will be independently estimated with two methods: quantitative analysis of the light emission from bioluminescent dinoflagellates, and conservation of energy. Using bioluminescence to quantify turbulence is a recent technique, but has proven effective for transitory, two phase flows. The bubble size distribution and void fraction of air during breaking will be obtained using optical techniques and a conductivity cell. The bubble Hinze scale corresponds to a distinctive change in slope of the bubble size distribution, which can be estimated from the optical measurements. The total energy lost during breaking will be estimated from changes in wave packet shape measured upstream and downstream of breaking, and the residual energy in coherent and turbulent fluid motions at the end of breaking will be measured with acoustic Doppler velocimeters and ensemble averaging of multiple events. Broader Impact. The project will enhance the understanding of atmospheric momentum transport to the ocean, the small-scale physical processes controlling gas transport, and turbulence and bubble production in the marine boundary layer and coastal regions. All of these processes have a broad impact on atmosphere-ocean coupling dynamics, global climate modeling and coastal oceanography. The experiment and data analysis will include the participation of a graduate student and undergraduate interns from UCSD and the SIO summer research fellowship program. These programs sponsor students from around the country to gain hands-on experience in oceanography. Working with the California Center for Ocean Science Education Excellence, the research team is also developing an outreach plan with the Ocean Institute (Sea Bubbles!) so as to increase public understanding of ocean-atmospheric interactions.