Stably stratified turbulence in the presence of large-scale forcing
We perform two high resolution direct numerical simulations of stratified turbulence for Reynolds number equal to Re~25000 and Froude number respectively of Fr~0.1 and Fr~0.03. The flows are forced at large scale and discretized on an isotropic grid of 2048^3 points. Stratification makes the flow an...
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Zusammenfassung: | We perform two high resolution direct numerical simulations of stratified
turbulence for Reynolds number equal to Re~25000 and Froude number respectively
of Fr~0.1 and Fr~0.03. The flows are forced at large scale and discretized on
an isotropic grid of 2048^3 points. Stratification makes the flow anisotropic
and introduces two extra characteristic scales with respect to homogeneous
isotropic turbulence: the buoyancy scale, L_B, and the Ozmidov scale, l_{oz}.
The former is related to the number of layers that the flow develops in the
direction of gravity, the latter is regarded as the scale at which isotropy is
recovered. The values of L_B and l_{oz} depend on the Froude number and their
absolute and relative size affect the repartition of energy among Fourier
modes. By contrasting the behavior of the two simulated flows we identify some
surprising similarities: after an initial transient the two flows evolve
towards comparable values of the kinetic and potential enstrophy, and energy
dissipation rate. Further similarities emerge at large scales: the same ratio
between potential and total energy (~0.1) is spontaneously selected by the
flows, and slow modes grow monotonically in both regimes causing a slow
increase of the total energy in time. The axisymmetric total energy spectrum
shows a wide variety of spectral slopes as a function of the angle between the
imposed stratification and the wave vector. One-dimensional energy spectra
computed in the direction parallel to gravity are flat from the forcing up to
buoyancy scale. At intermediate scales a ~ k^{-3} parallel spectrum develops
for the Fr ~ 0.03 run, whereas for weaker stratification, the saturation
spectrum does not have enough scales to develop and instead one observes a
power law compatible with Kolmogorov scaling. Finally, the spectrum of helicity
is flat until L_B, as observed in the nocturnal planetary boundary layer. |
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DOI: | 10.48550/arxiv.1412.1307 |