Direct numerical simulations (DNS) of stably stratified homogeneous
turbulence, with and without a background shear, have been performed to
investigate two characteristic types of structures in these flows:
concentrated vortices and density overturns. This presentation will focus
on our study of overturns.
In stratified flows, density overturns, i.e., regions where heavy fluid
overlies light in the same fluid column, are viewed as an indicator of the
ability of turbulence to overcome the stablilizing effect of the
stratification and are thus considered active sites for stirring and
mixing. Field observations of overturns, e.g., in the ocean, are limited to
single time, 1-D vertical profile data and have thereby led to significant
controversy over their interpretation. The objective of this study is to
develop a more complete description of overturns and provide insight for
1-D measurements. We employ DNS together with a 3-D feature identification
and tracking algorithm to investigate the spatial structure, dynamical
evolution and energetic significance of overturns. Here, we define a 3-D
overturn patch as a contiguous volume of non-zero Thorpe
displacement. Patch characteristics based on the conventional 1-D
representation are also determined for comparison. Both individual patches
as well as their entire population are studied. The effect of varying
degree of stratification is also considered in the
analysis. Visualizations show the 3-D spatial structure and demonstrate
the effects of background shear. Results show that the patch evolution
consists of three distinct phases: an inertially-driven growth phase,
buoyancy-dominated collapse phase and final phase of viscous-diffusive
buoyant balance. The contribution of the overturns to the overall flow
energetics varies for each phase. In general, the majority of diapycnal
mixing in the flow is concentrated in the peripheral boundary zone of the
overturn patches. Non-overturning motions may be associated with
significant available potential energy and buoyancy flux but turbulent
overturning is a necessary condition for any significant diapycnal mixing
to occur. Aspects of fossil turbulence theory are examined through tracking
of individual patches. Finally, the Reynolds number dependence of our
results is discussed along with implications for overturn behavior in the
ocean.