AbstractTurbulent flow over strongly forced steep steady and unsteady waves is simulated using large-eddy simulation (LES) with the wave height h(x, t) imposed as a lower boundary condition. With steady waves, h(x, t) is based on measurements of incipient and active breaking waves collected in a wind-wave flume, while a numerical wave code is used to generate an unsteady evolving wave packet (group). Highly intermittent airflow separation is found in the simulations and the results suggest separation near a wave crest occurs prior to the onset of wave breaking. The form (pressure) drag is most sensitive to waveslope and the form drag can contribute as much as 74% to the total stress. Wind and scalar profiles from the LES display log-linear variations above the wave surface; the LES wind profiles are in good agreement with the measurements. The momentum roughness increases as the water surface changes from wind ripples, to incipient breaking, to active breaking. However, the scalar roughness decreases as the wave surface becomes rougher. This highlights major differences in momentum and scalar transport over a rough wavy surface. For a rapidly evolving, strongly forced wave group, the form drag is highly correlated with the waveslope, and intermittent separation is found early in the packet evolution when the local waveslope −∂h/∂x(x, t) ≥ 0.22. The packet root-mean-square waveslope is 0.084, but the form drag fraction is 2.4 times larger than a comparably forced steady wave. Thus a passing wave group can induce unsteadiness in the wind stress.
Journal of Physical Oceanography – American Meteorological Society
Published: Oct 17, 2017
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