Elsewhere in this chapter, we consider k-Htype models, which may
perform better than LES in a three-dimensional boundary layer flow. The kHmodels, however, depend on semi-empirical functions and coefficients;
some of them may not be well known in the case of free-surface turbulent
boundary layer.
There are also other deviations of the free-surface boundary layer from
the wall layer analogy associated with surface waves. In particular, breaking
waves distinguish the free-surface boundary layer from the wall layer. This
is a very powerful source of turbulent energy. The process of wave breaking
is associated with the entrainment of air and the production of bubbles.
Conversely, bubbles have a significant effect on the turbulence dynamics in
wave breakers.
In the case of near-surface stratification (low wind speed conditions),
resonant interaction between two surface waves and an internal wave
developing on a near-surface pycnocline may result in the energy transfer
from surface to internal modes. Such interaction is obviously impossible
near a rigid wall. Surface waves also modulate strain and shear (thus the
gradient Richardson number), which may result in flow instability,
turbulence, and microstructure when the Richardson number drops below its
critical value.
3.1.4 Structure of the upper ocean turbulent boundary layer below
breaking surface waves
Surface waves produce turbulence due to wave breaking and vortex
instability. Wave breaking is a powerful mechanism producing significant
energy flux to small-scale turbulence and momentum flux to the mean
surface current. The local vorticity production due to vortex instability of the
surface waves should also be taken into account in the balance of momentum
and turbulent kinetic energy below breaking waves.
Mean shear flow is a principal source of small-scale turbulence in the
ocean. In this respect, upper ocean turbulence should be similar to the
classical shear turbulence when the mean shear energy production
dominates. The effect of stratification on the near-surface mean shear flow
can be substantial, especially for low winds. In contrast, under high wind
speed conditions when waves break, the influence of stratification on the
wave-turbulent layer is negligible (except when wind squalls are
accompanied by strong rainfalls). This means that when waves start breaking
there is a near-surface layer where the effect of the Coriolis and buoyancy
forces can be neglected.
At no stratification effects, energy from the mean shear flow, wave
motion and wave breaking contribute to the turbulent energy balance. The
dominant source will control the turbulent kinetic energy and dissipation rate
Chapter 3: NEAR-SURFACE TURBULENCE
149
perform better than LES in a three-dimensional boundary layer flow. The kHmodels, however, depend on semi-empirical functions and coefficients;
some of them may not be well known in the case of free-surface turbulent
boundary layer.
There are also other deviations of the free-surface boundary layer from
the wall layer analogy associated with surface waves. In particular, breaking
waves distinguish the free-surface boundary layer from the wall layer. This
is a very powerful source of turbulent energy. The process of wave breaking
is associated with the entrainment of air and the production of bubbles.
Conversely, bubbles have a significant effect on the turbulence dynamics in
wave breakers.
In the case of near-surface stratification (low wind speed conditions),
resonant interaction between two surface waves and an internal wave
developing on a near-surface pycnocline may result in the energy transfer
from surface to internal modes. Such interaction is obviously impossible
near a rigid wall. Surface waves also modulate strain and shear (thus the
gradient Richardson number), which may result in flow instability,
turbulence, and microstructure when the Richardson number drops below its
critical value.
3.1.4 Structure of the upper ocean turbulent boundary layer below
breaking surface waves
Surface waves produce turbulence due to wave breaking and vortex
instability. Wave breaking is a powerful mechanism producing significant
energy flux to small-scale turbulence and momentum flux to the mean
surface current. The local vorticity production due to vortex instability of the
surface waves should also be taken into account in the balance of momentum
and turbulent kinetic energy below breaking waves.
Mean shear flow is a principal source of small-scale turbulence in the
ocean. In this respect, upper ocean turbulence should be similar to the
classical shear turbulence when the mean shear energy production
dominates. The effect of stratification on the near-surface mean shear flow
can be substantial, especially for low winds. In contrast, under high wind
speed conditions when waves break, the influence of stratification on the
wave-turbulent layer is negligible (except when wind squalls are
accompanied by strong rainfalls). This means that when waves start breaking
there is a near-surface layer where the effect of the Coriolis and buoyancy
forces can be neglected.
At no stratification effects, energy from the mean shear flow, wave
motion and wave breaking contribute to the turbulent energy balance. The
dominant source will control the turbulent kinetic energy and dissipation rate
Chapter 3: NEAR-SURFACE TURBULENCE
149