Under many conditions the strongest sink of surface gravity wave energy in the shoaling wave energy budget occurs through turbulent dissipation in the bottom boundary layer at the sea bed. We present results from LES (Large Eddy Simulation) and DNS (Direct Numerical Simulation) experiments of the wave-induced turbulent bottom boundary layer for combinations of nonlinear waves, wave groups, and mean flows. Several important results have been observed from the numerical experiments including:
1) the bottom drag experienced by a mean flow is reduced by the presence of the oscillatory wave induced boundary layer.
2) turbulent dissipation rates in the boundary layer can be accurately estimated from vertical profiles of u(z,t), v(z,t), and w(z,t).
3) turbulent events for purely wave driven flows are strongly intermittent with instabilities that develop rapidly during periods of flow deceleration and with turbulent bursts that are strongly suppressed during phases of flow acceleration.
Canonical ideas have held that vertical transport across the boundary layer should be strongest during wave phases with the strongest onshore and offshore flow. We have observed, however, that the strongest vertical transport and associated sediment suspension events occur during the transition from onshore to offshore flow when the currents in the boundary layer are the weakest and there is an inflectional shear instability present in the mean velocity profile.
When the boundary layer contains both a mean flow and an oscillatory wave component, the flow behavior depends on several parameters, including the frequency of the wave, the ratio of the free-stream velocity of the mean current to the peak velocity of the oscillatory wave induced current, and the directional orientation between the mean flow and the wave. For situations when the wave dominates the mean flow it is possible to intermittently suppress the turbulence in the boundary layer. When the mean current is dominant, the flow remains turbulent throughout the wave period and the direction of the flow meanders through oblique angles about the mean flow direction.
We compare our results to previous laboratory measurements and demonstrate the capabilities of simple eddy-viscosity models for combined wave and mean flows (e.g., Grant and Madsen, 1979) to approximate features of the boundary layer dynamics. We plan to communicate the richness of the flow behavior within the boundary layer utilizing compelling video flow visualizations of the time dependent turbulence in the boundary layer.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics