A laboratory study was carried out to directly measure the turbulence properties
in a benthic boundary layer (BBL) above a uniformly sloping bottom where the
BBL is energized by internal waves. The ambient fluid was continuously stratified
and the steadily forced incoming wave field consisted of a confined beam, restricting
the turbulent activity to a finite region along the bottom slope. Measurements of
dissipation showed some variation over the wave phase, but cycle-averaged values
indicated that the dissipation was nearly constant with height within the BBL.
Dissipation levels were up to three orders of magnitude larger than background
laminar values and the thickness of the BBL could be defined in terms of the
observed dissipation variation with height. Assuming that most of the incoming wave
energy was dissipated within the BBL, predicted levels of dissipation were in good
agreement with the observations.
Measurements were also made of density and two orthogonal components of the
velocity fluctuations at discrete heights above the bottom. Cospectral estimates of
density and velocity fluctuations showed that the major contributions to both the
vertical density flux and the momentum flux resulted from frequencies near the
wave forcing frequency, rather than super-buoyancy frequencies, suggesting a strong
nonlinear interaction between the incident and reflected waves close to the bottom.
Within the turbulent BBL, time-averaged density fluxes were significant and negative
near the wave frequencies but negligible at frequencies greater than the buoyancy
frequency N. While dissipation rates were high compared to background laminar
values, they were low compared to the value of
εtr ≈ 15vN2, the transition value
often used to assess the capacity of a stratified flow to produce mixing. Existing
models relating mixing to dissipation rate rely on the existence of a positive-definite
density flux at frequencies greater than N as a signature of fluid mixing and therefore
cannot apply to these experiments. We therefore introduce a simple model, based
on the concept of diascalar fluxes, to interpret the mixing in the stratified fluid in
the BBL and suggest that this may have wider application than to the particular
configuration studied here.