Turbulence and Near-Surface Coherent Structures in Laboratory-Scale Wind-Wave Simulations

Near-surface coherent structures and “streaks” associated with Langmuir circulation are well-known features of the upper ocean and can contribute significantly to turbulence and mixing near the air-sea interface. We investigate near-surface turbulence and circulation in a numerical wind-wave tank set up to emulate the large SUrge-STructure-Atmosphere INteraction (SUSTAIN) laboratory at the University of Miami, where companion laboratory experiments were performed. The computational fluid dynamics (CFD) model implemented as large-eddy simulation (LES) explicitly resolves the waves and the air-sea interface using a volume-of-fluid (VOF) multiphase formulation. Wind-wave dynamics are investigated and found to compare well to the laboratory experiments. In particular, the model resolves the multi-scale pattern of streaks at the water surface which organize into circulation cells that scale with the Langmuir number and are consistent with divergence/convergence zones found in the laboratory dye experiments. Turbulent kinetic energy at the sea surface calculated from model velocity fields is in overall agreement with laboratory estimates. To further investigate the role of Langmuir-type cells in air-sea interface dynamics, we compare the multiphase model, which explicitly resolves the waves, to a model with a flat surface, where wind-wave forcing is parameterized through Craik-Leibovich (CL) vortex force terms added as a momentum source. The model with the flat surface resolves less of the smaller-scale near-surface dynamics, but captures well the circulation cells that are responsible for the bulk of the downward entrainment and vertical mixing in the laboratory dye experiments. We discuss the model results and implications for the study of near-surface wind-wave dynamics and associated turbulence. The impact of different wave frequencies representative of oceanic swell conditions is also addressed.