Multi-Scale Dynamics of Near-Surface Turbulence in a Numerical Wind-Wave Tank

Near-surface turbulence plays an integral role in vertical and horizontal gas and nutrient transport, as well as in the formation of the oceanic surface mixed layer. An important contributor to near-surface turbulence, Langmuir turbulence, has been observed in both ocean and laboratory and can be characterized by the Langmuir number. We investigate near-surface turbulence, including Langmuir-like turbulence, in a numerical wind-wave tank designed to emulate the state-of-the-art SUSTAIN facility at the University of Miami. Langmuir turbulence is commonly modeled through the addition of vortex force, so-called “Craik-Leibovich”, terms to the model equations. Our numerical simulations are aimed at resolving Langmuir-type circulations directly, without the addition of these terms, instead using high spatial and temporal resolution to capture the underlying dynamics. The numerical model is implemented using a large-eddy simulation approach and a Volume-of-Fluid multiphase formulation within a computational fluid dynamics code. The simulations exhibit a pattern of coherent structures in the near-surface layer composed of convergence and divergence zones that form surface streaks. On larger temporal and spatial scales, a multi-scale pattern emerges. The formation of such streak-like features in the near-surface layer has previously been attributed to a number of mechanisms, generally related to surface shear. The simulations investigate the multi-scale interaction of surface streak-like patterns and larger scale features, consistent with Langmuir circulation, as well as the underlying dynamics.