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SUMMARY:Spectra and distribution functions of stably stratified turbulence
  - Herring\, J (N.C.A.R.)
DTSTART:20081211T140000Z
DTEND:20081211T143000Z
UID:TALK15612@talks.cam.ac.uk
CONTACT:Mustapha Amrani
DESCRIPTION:We consider homogeneous stably stratified turbulence both deca
 ying and randomly forced cases. Our tools include direct numerical simulat
 ions (DNS) and elements of statistical theory as expressed by two-point cl
 osures. Our DNS--at $1024^3$--permits a large scales Taylor micro scale $R
 _{lambda} im 300$ The size distribution of such large scales is closely re
 lated to conservation principles\, such as angular momentum\, energy\, and
  scalar variance\; and we relate these principles to our DNS results. Stra
 tified turbulence decays more slowly than isotropic turbulence with the sa
 me initial conditions. We offer a simple explanation in terms of the dimin
 ution of energy transfer to small scales resulting from phase-mixing of gr
 avity waves (Kaneda 1998). Enstrophy structures in stratified flows (scatt
 ered pancakes) are distinctly different from those found from isotropic tu
 rbulence (vortex tubes)\, and we show examples of the transition from isot
 ropic turbulence enstrophy structures to those of strongly stratified turb
 ulence. We discuss briefly changes in the probability distribution functio
 ns for velocity and vorticity for stratified turbulence concluding that st
 ratification induces a return towards Gaussianity for these quantities. Fo
 r the forced case\, we examine the modification of the inertial range indu
 ced by strong stratification ($k_{perp}^{-5/3}\nightarrow  im k_{perp}^{-2
 }$) for the wave component\, and ($k_{perp}^{-5/3}\nightarrow im k_{perp}^
 {-3}$) for the vortical component. Here\, $k_{perp}$ is the horizontal wav
 e number. These DNS findings are discussed from the perspective of two-poi
 nt closure.
LOCATION:Seminar Room 1\, Newton Institute
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