Collaborative Research: Experimental and Numerical Studies of the Effects of Wind, Wave Scale, and Salinity on Bubble Entrainment by Breaking Waves

This is a study of the role of wave scale, wave slope, wind forcing, and salinity on the generation and dispersal of bubbles in breaking ocean waves through scale model laboratory experiments and computational fluid dynamics (CFD) simulations. The work is motivated by the critical role played by bubbles in climate-related air-sea exchanges, including CO2 transfer, aerosol production, and ocean albedo. Identification of the physical processes controlling bubble entrainment in whitecaps and their dependence on wave scale, wave slope, wind forcing, and salinity will fundamentally alter our ability to model air-sea interactions. At present, there are only three oceanic data sets of bubbles in whitecaps, which is obviously insufficient to determine any fundamental physical connection between bubble entrainment and environmental forcing. CFD simulations have become a powerful tool to study wave breaking but achievable length scales are still orders of magnitude smaller than oceanic whitecaps and important physical effects, including salinity effects on bubble coalescence, the role of wind-forcing, and the effects of wave scale, are not fully explored. Validating CFD simulations of wave breaking with a realistic ocean proxy will facilitate their use to study ocean-scale exchange processes while simultaneously identifying where the greatest effort should be directed to scale the numerical models to relevant oceanic processes. Moreover, data from the experiments will help improve existing models of bubble-mediated, air-sea exchange processes. Additional activities are included to broaden the impact of the project. The participating laboratories have a strong commitment to supporting the integration of research and education at various academic levels across a broad spectrum of society. SIO PIs will work with educational professionals at the Birch Aquarium at Scripps (BAS) to increase engagement in STEM and broaden participation in STEM careers. Staff at BAS and the CREATE STEM Success Initiative at UC San Diego will co-design, evaluate, and implement experiential learning programs using the resources of the Scripps Ocean Atmosphere Research Simulator (SOARS). To address inequities in access to STEM learning opportunities and under-representation of BIPOC students in the geosciences, the “Exploring Ocean STEM Careers” program will engage middle and high school students from historically marginalized groups and work with partners to build on students’ interest and excitement with services that include academic counseling and college application support. In addition, funding is included to support two summer undergraduate interns to further their education in marine science. PIs Deane and Stokes will participate in the BAS outreach vehicle: Perspectives on Ocean Science lecture series. PI Shen will lead the development of a portal of computer modeling of water waves for climate and the environment applications as teaching materials to illustrate Geosciences to students at tribal colleges and universities, community colleges, and minority serving institutions. The portal will be disseminated through the Geoscience Alliance, a national alliance for broadening participation of Native Americans in GeosciencesData from experiments in three wave channels of increasing scale and environmental complexity will be compared with state-of-the-art CFD computations to study the fluid dynamics of bubble entrainment and its dependence on environmental parameters. The experiments will leverage the recently commissioned Scripps Ocean-Atmosphere Research Simulator, which provides a realistic yet controllable proxy for the wind-driven sea surface. The CFD simulations will take advantage of recent algorithmic advances, including high-fidelity gas-liquid interface capturing algorithms using the coupled level-set and volume-of-fluid methods with consistent mass and momentum advection schemes, an optimal network bubble identification and tracking algorithm, and adaptive mesh refinement, to provide new insights into air entrainment processes and bubble behavior within breaking wave crests. Reconciliation of the experiments and simulations will enable verification of the CFD models, motivation for further algorithmic and modeling advances, and insights into bubble-mediated exchange processes across the wind-driven air-sea interface.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.