Large Eddy Simulation of Momentum and Scalar Transport in Turbulent Flow Over Resolved Laboratory Breakers

In recent work [1], we developed a large eddy simulation model for the full marine boundary layer with varying geostrophic winds and surface heat flux that imposes a spectrum of moving waves with random phase at its lower boundary. The LES model uses a co-located surface fitted grid that evolves in time to track the motion of the underlying surface waves. In the present effort, we use this LES code to examine turbulent flow over incipient and active breaking waves as observed in the laboratory wind-wave experiments reported by Banner (1990) [2]. In these experiments the waves are strongly forced with wave age c/u_* ~ O(1). Our simulations focus on the airflow near the water surface in the so-called wave boundary layer region [3], and also include a passive scalar. The measured surface wave fields, which contain the essential features of breaking, are externally prescribed. The LES data are analyzed extensively to generate flow visualization, bulk mean flow and turbulence statistics and surface drag, and are compared with the available observed experimental results. Fine mesh LES is in good agreement with the measured mean wind profiles and predicts that the contribution from pressure form stress to the total drag increases from 45 to nearly 80 percent with active breaking. Positive pressure-waveslope correlations from flow reattachment on the upwind face of a downstream wave enhance the form stress. Intermittent flow separation is readily observed for incipient and active breaking in both the wind and scalar fields. Flow visualization of spanwise vorticity shows an elevated separating shear layer populated with vortices above the wave crests. Momentum and scalar budgets illustrate the balance between form stress, resolved turbulence and subgrid-scale contributions. In the constant flux layer, there is a smooth transition between the pressure, turbulent and subgrid-scale contributions to the total stress with increasing distance from the surface. These budgets highlight the fundamental differences between momentum and scalar transport over a rough surface. In flow over active breakers where the form stress is large and dominant the effective total surface roughness increases while for the same flow the total scalar roughness decreases since only turbulence and subgrid-scale motions contribute to the scalar budget.

References:

[1] Sullivan, P.P., J. C. McWilliams, & E. G. Patton, 2014: Large-eddy simulation of marine atmospheric boundary layers above a spectrum of moving waves. Journal of the Atmospheric Sciences, 71, 4001 4027.

[2] Banner, M. L., 1990: The influence of wave breaking on the surface pressure distribution in wind-wave interaction. Journal of Fluid Mechanics, 211, 463-495.

[3] Hara, T. and P. P. Sullivan, 2015: Wave boundary layer turbulence over surface waves in a strongly forced condition. Journal of Physical Oceanography, 45, 868-883.