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Monday, July 25, 2011
3:00 PM - 4:00 PM
CNLS Conference Room (TA-3, Bldg 1690)

Colloquium

Rotating Turbulence, Self-Similarity and the Return to Isotropy

Annick Pouquet
University of Colorado, Boulder

Rotating flows prevail in the universe, be it only because of the conservation of angular momentum as molecular clouds contract. Hence, one has to deal with a highly turbulent flow in which inertial waves interact with nonlinear eddies and coherent structures.

In this context, results are described, obtained using large-scale numerical simulations of rotating helical turbulence on grids of up to 30723 points [1]; for the higher resolution run, the Reynolds number is 27,000 and the Rossby number is 0.07. The code is pseudo-spectral with a hybrid MPI/Open-MP parallelization algorithm that scales linearly up to ~ 18,000 processors with 89% efficiency.

Intermittency, or the lack thereof, is analyzed on the smaller computation that barely resolves the Zeman scale, that is the scale at which the eddy turn-over time and the inertial wave time become equal. Both zero helicity (Taylor-Green) and maximal helicity (Beltrami, ABC) forcing are considered and contrasted.

A surprising result is that the self-similarity of the rotating turbulence differs whether or not the forcing is helical, i.e. with or without velocity-vorticity correlations. This result is present as well in experimental configurations, without attribution to helicity though; in all cases, scaling that differs from the Kolmogorov law is found.

With the larger run, just completed, it is shown that at scales smaller than the Zeman scale, isotropy recovers together with a Kolmogorov spectrum whereas at larger scales, the wave-mediated spectrum is present again. The helicity spectrum clearly breaks down at the Zeman scale, although the energy and helicity fluxes remain constant throughout the inertial ranges. (Note that at scales larger than the forcing itself, it is known that an inverse energy cascade takes place but its scaling laws are not studied in this work, due to poor resolution in that range.)

The intermittency property in that new configuration will be studied in a future work; in particular, the question arises as to whether the self-similarity observed in the large scales persist at smaller scales or not. These findings may have implications on how to model and parametrize unresolved small-scales in weather and climate simulations: should anisotropy be taken into consideration or not in the case of rotating turbulence?

[1] Mininni, Rosenberg & Pouquet, arXiv:1104.5519, and references therein.

Host: Robert Ecke, CNLS, 667-1444, ecke@lanl.gov