The structural and statistical aspects of stably stratified turbulence is studied by way of Direct Numerical Simulations (DNS) of the Navier-Stokes equations under the Boussinesq approximation. It is well-known that the structures generic to homogeneous isotropic turbulence are ″vortex tubes″ while ″vortex pancakes″ which are randomly scattered throughout the flow, are often observed for stably stratified turbulence. An issue here is whether vortex tubes are somehow part of the stably stratified flow or not. This problem is concerned with the universality of the production mechanism of small-scales in turbulence which is an essential assumption of most Large Eddy Simulations (LES) in meteorology.
We shall report the results of DNS with 5123 grid points for both decaying and randomly forced stratified turbulence. For the decaying case, it is observed that stratified turbulence leads to a near self-similar decay. It is also noticed that the decay rate of energy in large scales differ from the unstratified turbulence. For the forced case, two issues which are related with the aspect of energy transfer for stratified turbulence will be discussed; (1) a spectrum close to k-2, steeper than k-5/3 for homogeneous isotropic turbulence, is observed, and (2) large scale horizontal layers are formed rather suddenly at a later stage of the flow development. |
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