THE NEAR-SURFACE LAYER OF THE OCEAN
al., 1999). In near-surface flows with significant vorticity, there is a thin
transitional layer inside which the values of the two horizontal vorticity
components and the vertical derivative of the vertical vorticity component
change from their bulk (isotropic) values to the much smaller values
imposed by the zero-stress conditions at the free surface.
In the absence of surface stress but in the presence of internal sources of
turbulence (for instance in the post wave-breaking interval or in a ship’s
wake), the surface layer developing near a free surface is a region of
decreased kinetic energy dissipation and increased enstrophy dissipation
(enstrophy is the total squared vorticity). When surface stress is imposed, the
properties of the near surface turbulent eddies are still determined by the slip
conditions at the free surface; as a result, the mean shear flow near a free
surface is different from that near a rigid wall. In particular, the thickness of
molecular sublayers at a free surface appears to be smaller than at a rigid
wall.
According to the direct numeric simulation (DNS) by Shen et al. (1999),
a vertically aligned vortex can attach to the ocean surface and experience
significantly slower decay. The second order Stokes drift of the waves tilts
and stretches such vortices in the horizontal direction presumably leading to
the generation of Langmuir circulations (see Chapter 5). At low wind speeds,
when wind waves cease, the wind-induced shear current could carry out the
same function as the Stokes drift tilting and stretching the turbulent vortices.
Remarkably, there are numerous reports of wind streaks on the ocean surface
even before waves develop, which resemble miniature Langmuir
circulations.
The Shen et al. (1999) concept of a free-surface turbulent boundary layer
is essential for understanding free-surface turbulence. Important practical
applications for this information include using surface sensing to deduce
characteristics of the underlying flow (Swean et al., 1991; Handler et al.,
1993). This concept has not yet been applied to situations with finite
amplitude surface waves.
The presence of surface waves alters the hydrodynamics of near-surface
flows. Borue et al. (1995) found that the interaction of surface waves and
free-surface turbulence is weak. Their DNS, however, was confined to
infinitesimally small waves. High-resolution simulation of finite-amplitude
surface waves interacting with fully developed turbulence are not yet
feasible. Large eddy simulation (LES) schemes may not work well for fully
three-dimensional boundary layer flows because there is insufficient spectral
separation between small and large eddies. In particular, LES does not seem
to produce realistic results in the case of Langmuir circulations (see
discussion in Section 5.7).
148
al., 1999). In near-surface flows with significant vorticity, there is a thin
transitional layer inside which the values of the two horizontal vorticity
components and the vertical derivative of the vertical vorticity component
change from their bulk (isotropic) values to the much smaller values
imposed by the zero-stress conditions at the free surface.
In the absence of surface stress but in the presence of internal sources of
turbulence (for instance in the post wave-breaking interval or in a ship’s
wake), the surface layer developing near a free surface is a region of
decreased kinetic energy dissipation and increased enstrophy dissipation
(enstrophy is the total squared vorticity). When surface stress is imposed, the
properties of the near surface turbulent eddies are still determined by the slip
conditions at the free surface; as a result, the mean shear flow near a free
surface is different from that near a rigid wall. In particular, the thickness of
molecular sublayers at a free surface appears to be smaller than at a rigid
wall.
According to the direct numeric simulation (DNS) by Shen et al. (1999),
a vertically aligned vortex can attach to the ocean surface and experience
significantly slower decay. The second order Stokes drift of the waves tilts
and stretches such vortices in the horizontal direction presumably leading to
the generation of Langmuir circulations (see Chapter 5). At low wind speeds,
when wind waves cease, the wind-induced shear current could carry out the
same function as the Stokes drift tilting and stretching the turbulent vortices.
Remarkably, there are numerous reports of wind streaks on the ocean surface
even before waves develop, which resemble miniature Langmuir
circulations.
The Shen et al. (1999) concept of a free-surface turbulent boundary layer
is essential for understanding free-surface turbulence. Important practical
applications for this information include using surface sensing to deduce
characteristics of the underlying flow (Swean et al., 1991; Handler et al.,
1993). This concept has not yet been applied to situations with finite
amplitude surface waves.
The presence of surface waves alters the hydrodynamics of near-surface
flows. Borue et al. (1995) found that the interaction of surface waves and
free-surface turbulence is weak. Their DNS, however, was confined to
infinitesimally small waves. High-resolution simulation of finite-amplitude
surface waves interacting with fully developed turbulence are not yet
feasible. Large eddy simulation (LES) schemes may not work well for fully
three-dimensional boundary layer flows because there is insufficient spectral
separation between small and large eddies. In particular, LES does not seem
to produce realistic results in the case of Langmuir circulations (see
discussion in Section 5.7).
148