Collaborative Research: Ocean Wave Dissipation through Breaking and Bubble Generation

University Of Washington, Seattle WA

Investigators

Abstract

This project concerns the breaking of waves at the ocean surface and the bubbles generated during that process. Wave breaking is an important source of turbulence and mixing at the ocean surface, including the turbulent exchange of gasses and heat between the ocean and the atmosphere. Wave breaking is also a hazard to navigation and safe passage for vessels at sea. The key question is "how much energy is lost when waves break?". This lost energy is both an important parameter for accurate wave forecasts and for prescribing ocean-atmosphere exchange. Recent work has suggested that the quantity and size of bubbles generated during wave breaking are the key controls on the energy dissipation and thus the strength of breaking. This project will use field observations and computer models to verify this dependence, with application to improve wave forecasting and ocean-atmosphere coupling. The proposed work will support a postdoctoral researcher and contribute to public outreach with specific contributions to the annual Discovery Days event at the University of Washington, that reaches 10,000 students from regional K-12 schools. The technical approach of the project is to measure wave breaking in the open ocean with freely drifting buoys that are equipped with motion sensors, turbulence profilers, and cameras for bubble recording. The proposed work builds on recent results using the Surface Wave Instrument Float with Tracking (SWIFT) drifters to measure the turbulent dissipation rate beneath breaking waves and a VOF-based (volume of fluid) Eulerian-Eulerian polydisperse two-fluid model to simulate the two-phase bubbly flow. Previous results suggest that dissipation by bubbles may be an increasingly large fraction of the total wave breaking dissipation during high sea states. The measurements to be carried in this project will be analyzed to determine the time evolution of the bubbles, as well as the turbulence and motion associated with individual breaking waves. The data will be used to test a recent formulation that scales the total dissipation with the time integral of the bubble plume volume. Further, a related formulation that scales bubble plume depth with the area of foam patches visible at the ocean surface will be evaluated. Improvement of the latter formulation will allow estimates of plume volume and infer dissipation remotely. The formulation derived will be compared with other estimates for dissipation, which include: the Phillips breaking crest distribution, equilibrium wind input, and in situ measurements of turbulent dissipation. The proposed work will also examine the effects of intermittence and the consequences of sparsely sampled field observations. These measurements will be compared with the high resolution, two-fluid model simulations of the breaking process. This approach will enable a more complete investigation of breaking wave conditions and an assessment of the inherent sparseness of the observations. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.